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Download 1 Changes in Hurricane Climatology in Recent Decades Anais Orsi
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Changes in Hurricane Climatology in Recent Decades Anais Orsi Dian Putrasahan Ha Joon Song James Means Scripps Institution of Oceanography Abstract Hurricanes can inflict catastrophic property damage and human life loss. Thus, it is important to determine how the character of these powerful storms could change with global warming. In this paper we examine different ways of measuring hurricanes, and look at how time series of various hurricane metrics have been analyzed in recent scientific literature. We conclude that the expected connections between hurricane activity and global warming are still equivocal. Introduction and Background A hurricane is an intense tropical cyclone that has sustained winds of greater than 33 meters sec-1. Hurricanes form in the world’s tropical oceans at latitudes from about 8° from the equator to about 20° from the equator—their structure is dependent on the balance between the Coriolis and pressure gradient forces and hence cannot occur too near to the equator, where the Coriolis force vanishes. They are known to occur in the North Atlantic, western North Pacific, eastern North Pacific, Southwest Pacific, and Indian oceans, as well as their associated marginal seas. Hurricanes are classified according to the Saffir-Simpson intensity scale, which grades hurricanes according to their maximum sustained surface wind speed. The SaffirSimpson Scale is outlined in the table below. An important thing to understand about hurricane intensity and the corresponding Saffir-Simpson classification is that there is often no direct measurement of storm intensity. If information is available from “Hurricane Hunter” flights it is the preferred method of assigning intensity, although 1 even in this case the wind speed at the surface is inferred from the flight level winds so it is also an indirect measurement. Saffir-Simpson Category Sustained Wind Speed (ms-1) 1 33–42 2 43–49 3 50–58 4 59–69 5 ≥ 70 If flight information is unavailable then hurricane intensity is usually assigned by the Dvorak classification. In this scheme, the presentation of the hurricane in satellite imagery is compared to standard images of tropical cyclones and maximum wind speed is inferred from comparison with the standards. Allowance is made for storm “spin-up” and “spin-down” so that the intensity classification is not allowed to change too quickly. The original classification flowchart as described by Dvorak [1] is shown in Figure 1, and although the procedure has been updated [2], the present technique remains very similar. Before the Dvorak classification surface winds were estimated from central pressure and sea state estimated from visual observation from aircraft. Figure 1.Template and chart taken from Dvorak's paper showing how to classify hurricanes by their banding and central features in order to arrive at T number. 2 Hurricanes are heat engines, and derive their intense energy from the latent heat of vaporization of water. To sustain a hurricane requires an environment with a high latent heat content and low friction. It has been empirically determined that these requirements are only filled over ocean basins with water temperature exceeding 26°C. Hurricanes quickly diminish in intensity when they move over cooler waters; when they make landfall; when they encounter dry or stable air; or when they move into an environment of vertical wind shear. It is the dependence on sea surface temperature that has led to speculation that global warming—specifically warming of the tropical ocean basins— could lead to changes in the frequency, distribution and intensity of hurricanes. Figure 2. Adopted from Webster et al [3], this plot shows trends in sea surface temperature for different ocean basins. However, it should be noted the other factors mentioned above may be just as important to hurricane climatology, but those factors are not as simply connected with global warming. 3 Evidence for Changes in Hurricane Climatology Both the 2004 and 2005 hurricane seasons have been exceptional in the number of hurricanes affecting the United States and the damage caused by them, which as led to much speculation in both the print and broadcast media that there has been an increase in hurricane occurrence due to global warming. However, it is debatable whether such a trend exists. For example, Figure 3 is a plot of National Hurricane Center data of US hurricane strike by decade, with the data split into all hurricanes (categories 1 through 5) and major hurricanes (categories 3, 4, and 5). It is not at all clear from examining this plot that there is any trend whatsoever, either for all hurricanes or major hurricanes. This is a very limited data set, however, and it is advisable to examine more detailed studies of hurricane frequency and intensity, and especially studies that look at tropical basins other than the Atlantic/Caribbean/Gulf of Mexico. U.S. Hurricane Strikes by Decade 30 Number of Hurricanes 25 20 Total 15 Major 10 5 0 18511860 18611870 18711880 18811890 18911900 19011910 19111920 19211930 19311940 19411950 19511960 19611970 19711980 19811990 19912000 20012004 Decade Figure 3. U.S. Hurricane strikes by decade, for all hurricanes and just major hurricanes. Figure 4 shows another measure of hurricane climatology, the Accumulated Cyclone Energy (ACE) index. The ACE is a statistic that the National Oceanic and Atmospheric Administration (NOAA) uses to measure the total seasonal tropical cyclone activity. It is 4 calculated by summing the squared values of a storm’s maximum wind speed (taken every six hours) for all Atlantic storms for a season. 300 250 200 150 100 50 19 50 19 52 19 54 19 56 19 58 19 60 19 62 19 64 19 66 19 68 19 70 19 72 19 74 19 76 19 78 19 80 19 82 19 84 19 86 19 88 19 90 19 92 19 94 19 96 19 98 20 00 20 02 20 04 0 Figure 4. ACE index for the North Atlantic since 1950, showing values above average for most of the last decade. Webster et al [3] look at trends in tropical cyclone number, duration, and intensity for various tropical basins over the period 1970–2004. They find no global trend for a change in either the frequency or duration of tropical cyclones worldwide. They do find a statistically significant increase in the number and duration of North Atlantic hurricanes over the same period, but do not attribute this to ocean warming because the same trends are not seen in other oceans. This is shown in Figure 5. Although we can see the decadal oscillations in both the number of hurricanes and the number of hurricane days, the 35 year span doesn’t shown an overall increasing trend except over the North Atlantic. The North Atlantic shows an increasing trend both in the number of hurricanes and the number of hurricane days. This follows the trend of increasing summer sea surface temperature, but considering that all the other basins also have had increasing sea surface temperature without a concomitant increase in the number of hurricanes, it is hard to conclude that the global warming leads to an increase in the number of hurricanes. 5 Figure 5. Plot from Webster et al of the number of hurricanes and hurricane days by tropical ocean basin. They also look at hurricane intensity by breaking the frequency histogram into Category 1, Categories 2 and 3, and Categories 4 and 5. When examined this way they find a statistically significant positive trend in the frequency of the Category 4 and 5 storms. This is apparent in Figure 6, taken from Webster et al’s paper. Gray [4] does not believe that firm evidence for an increase in intense tropical cyclones exists. Rather, he believes that the data as used by Webster et al is biased because the observational techniques used to classify storms by their Saffir-Simpson category are inadequate to distinguish between Category 3 and the Category 4 and 5 storms examined in the Webster et al paper. He shows evidence that if the Categories 3 to 5 storms are combined, there has been essentially no change in their global frequency outside of the Atlantic over the last ten years. There has been a substantial increase in their frequency in the Atlantic, which he attributes to multi-decadal changes in the Atlantic thermohaline circulation. 6 Figure 6. Graphs from paper by Webster et al, showing an apparent increase in the fraction of intense hurricanes over the past two decades. In another recent paper [5], Kerry Emanuel looks at several measures of the energy dissipated by a storm. The first one he examines is called the power dissipation, or PD: τ r0 PD=2π ∫ ∫ CD ρ V rdrdt 3 0 0 However, it is difficult to calculate the PD for most storms. It requires a knowledge of both the pressure and wind speed field as well as the surface drag coefficient. As a more tractable storm metric he defines the power dissipation index, or PDI. The definition of PDI is τ PDI ≡ ∫ V 3max dt 0 Here τ is the lifetime of the storm, and Vmax is the maximum sustained wind speed at any point in time. The PDI is very similar to the ACE used by NOAA for evaluating Atlantic hurricane seasons, but it puts a higher emphasis on the most destructive storms by using the maximum wind speed cubed rather than squared. Emanuel uses the National Hurricane Center and the U.S. Navy Joint Typhoon Warning Centers’ [6, 7] best tracks databases to calculate the PDI for all tropical cyclones from about 1930 to the present. These databases provide re-analyzed track and intensity data for all identified tropical 7 cyclones. These databases provide maximum wind (rounded to nearest 5 knots) every six hours during the cyclone’s life. He plots the summed annual PDI for different ocean basins, both separate and combined, on the same graph as basin sea surface temperature. Appropriate scaling is used so that the plots are of the same order. The sea surface temperature and PDI show a marked similarity on these plots and the results are very suggestive of a relationship between the two. Figure 7. Plot from Emanuel’s paper showing PDI and sea surface temperature for the combined North Atlantic and West Pacific basins. There are, however, criticisms to be made of Emanuel’s work. The plots show no error bars for either the PDI or the sea surface temperature. A simple calculation shows that the PDI should have a minimum relative uncertainty given by ΔPDI min 3ΔVmax ∝ PDI Vmax 8 where ΔPDI min represents the minimum uncertainty in the PDI and ΔVmax is the uncertainty in Vmax . This represents the minimum uncertainty in the PDI because it only takes into account the uncertainty due to uncertainties in the maximum wind speed, and not due to other factors, such as an inaccurate specification of the storm lifetime. This failure to account for the uncertainties in PDI due to the uncertainties in the maximum wind speed is a major deficiency of the paper, as pointed out by Gray [8] among others. Major changes have been made in the way that maximum wind speed was estimated over the time span of the databases used by Emanuel. Emanuel has tried to account for some of these changes by developing new algorithms to adjust older wind speeds. Emanuel’s new algorithms lower some of the older values by as much as 10 meters per second. This adjustment is in addition to the rounding to the nearest 5 knots of best track wind speeds, and other errors in wind speed estimation. Considering that a typical hurricane may have maximum winds on the order of 50 meters per second, a relative uncertainty of the maximum wind speed of up to 20% might be seen. This would lead to an uncertainty of the PDI of 60%, which will totally swamp the correlation implied by the graph. It is very difficult to compare historical wind speed data computed by different techniques, and cubing those wind speeds has the effect of tripling any uncertainties in those wind speeds. Two other recent papers by Trenberth [9], and Pielke et al [10] also look at changes in hurricane climatology due to global warming, and come to the conclusion that there is no solid basis for implicating global warming in changes to hurricane climatology. This does not mean that they do not believe that such changes may be occurring, but rather that there are other factors (wind shear, temperature of the entire tropospheric column, etc.) that must be examined when looking for causal changes to hurricane intensity. Indeed, higher sea surface temperatures are associated with a higher water content of the lower troposphere. Since 1998, the amount of total column water vapor over the ocean has increased by 1.3% per decade [9]. Higher available latent heat would favor the development of tropical storms. However, the convective available potential energy is also affected by large scale subsiding air that increases the stability and dryness of the 9 atmosphere, and is often associated with wind shear through the troposphere. Analysis of the 250-850 hPa wind shear for the Atlantic shows a downward trend of 0.3 m/s per decade over the period 1949-2003, which is too small to have much effect [5]. In addition, Pielke et al believe that the destructiveness of future hurricanes will be much more dependent on sociological factors, such as increased population and building in hurricane prone areas, than on any increase in the severity of storms [10]. Conclusion There are theoretical reasons to expect that there might be a connection between global warming, higher sea surface temperatures, and increased intensity of tropical cyclones. Recent research has looked into this possibility and the results are suggestive of increased tropical cyclone intensity over the past several decades. However, the results at this time are far from conclusive and further observational and theoretical study is needed to elucidate any relationship between global warming and the climatology of tropical storms. References 1. Dvorak, V. F. Tropical cyclone intensity analysis and forecasting from satellite imagery. Mon. Wea. Rev. 103, 420-430. (1975). 2. Dvorak, V.F., 1984: Tropical cyclone intensity analysis using satellite data. NOAATechnical Report NESDIS 11, 45 pp. 3. Webster, P.J., et al., 2005: Changes in tropical cyclone number, duration, and intensity in a warming environment. Science, 309, 1844-1846. 4. Gray, W.M., 2005: Comments on Webster, et al., Science, 309, 1844-1846. Submitted to Science. 5. Emanuel, K., 2005: Increasing destructiveness of tropical cyclones over the past 30 years. Nature, 436, 686-688. 6. Joint Typhoon Warning Center Best Tracks for the Southern Hemisphere, Pacific and Northern Indian Oceans. http://www.npmoc.navy.mil/jtwc/best_tracks/ 7. National Hurricane Center Atlantic and Eastern North Pacific Best Tracks. http://www.nhc.noaa.gov/pastall.shtml 10 8. Gray, W.M., 2005: Comments on: “Increasing destructiveness of tropical cyclones over the past 30 years” by Kerry Emanuel, Nature, 31 July 2005, Vol. 436, pp. 686688. Submitted to Nature 9. Trenberth, K., 2005: Uncertainty in Hurricanes and Global Warming. Science, 308, 1753-1754. 10. Pielke, Jr., R. A., C. Landsea, M. Mayfield, J. Laver and R. Pasch, in press, 2005. December. Hurricanes and global warming, Bulletin of the American Meteorological Society. 11 Reviews of Atmospheric Science Topics November 2005 Progress in Understanding Chlorofluorocarbons in the Lower Stratosphere L. Elmegreen, L. Nahid, and D. Richter Scripps Institution of Oceanography, UCSD Abstract Chlorofluorocarbons (CFCs) have been under scrutiny since the 1970’s due to their ability to act both as greenhouse gases (GHGs) and as ozone depleting substances (ODSs). Current efforts are being made to determine CFC concentrations in the stratosphere using gas chromatography and infrared absorption and emission techniques. Satellites have been crucial for determining concentrations over large areas. Recent studies show that CFC mixing ratios decrease with latitude and altitude. Studies also show that fluorine concentrations in the stratosphere can be used to determine CFC mixing ratios, and the change in the mixing ratio over time. Although CFC production has decreased, CFC mixing ratios in the stratosphere are likely to increase over time due to stockpiles (or banks) of CFCs. Introduction History of Use Chlorofluorocarbons (CFCs) have been in use since the 1950’s in applications such as refrigeration, air conditioning, foams, aerosols, fire protection and solvents. Global concentrations of CFCs increased largely from the 1970s to the 1990s [IPCC, 2005]. Today, most emissions originate from manufacture, unintended byproduct releases, intentionally emissive applications, and evaporation. However, the largest contributions to the atmosphere come from banks. Banks are the total amount of substances contained in existing equipment, chemical stockpiles, foams and other products not yet released to the atmosphere [IPCC, 2005]. Ozone Depletion Stratospheric ozone depletion has been observed since 1970 and is caused primarily by increases in concentrations of reactive chlorine and bromine compounds in the atmosphere. They are produced by degradation of anthropogenic Ozone Depleting Substances (ODSs), including halons, CFCs, hydrochlorofluorocarbons (HCFCs), methyl chloroform, carbon tetrachloride, and methyl bromide [IPCC, 2005]. CFCs were first implicated in ozone destruction in 1974 by Molina and Rowland [Schauffler et al., 2003]. To address the growing hole in the ozone layer, the production of CFCs, HCFCs, halons, methyl chloroform, and carbon tetrachloride began to be regulated in developed countries by the Montreal Protocol and subsequent amendments [United Nations Environmental Programe (UNEP), 1987, 1992, 1997]. The protocol applies only to developed countries, and does not regulate the emissions from banks. Even so, it has been very successful in curbing ODS contributions to the atmosphere, as recent data indicate that stratospheric chlorine levels have approximately stabilized and may have already started to decline [IPCC, 2005]. CFC replacements, such as hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs), have been identified as potential long-term replacements for ODSs because they contain neither bromine nor chlorine, which are ultimately responsible for Elmegreen et al., 2 ozone depletion. Ozone recovery is expected to follow decreases in chlorine and bromine loading in the stratosphere as ODS concentrations decline. However, emissions of other greenhouse gases such as CO2, methane and nitrous oxide can affect both tropospheric and stratospheric chemistry, and will have some effect on ozone recovery [IPCC, 2005]. Global warming CFCs are also greenhouse gases, as are many of their replacements. The relative future warming and cooling effects from emissions of CFCs, HCFCs, HFCs, PFCs and halons vary with gas lifetimes, chemical properties and time of emission. The atmospheric lifetime for most CFCs is decades to centuries [IPCC, 2005]. Figure 1. The atmospheric window, a region characterized by low IR absorption from non-CFC greenhouse gases [IPCC 2005]. Halocarbons, including CFCs, absorb Earth’s outgoing infrared radiation in a spectral range where energy is not removed by CO2 or water vapor. This gap filled by halocarbons is sometimes referred to as the atmospheric window (Figure 1). As a result, a halocarbon molecule may be many thousand times more efficient at absorbing radiant energy emitted from the Earth than a molecule of CO2. While the primary radiative effect of CO2 and water vapor is to warm the surface climate but cool the stratosphere, the direct radiative effect of halocarbons is to warm both the troposphere and stratosphere [IPCC, 2005]. The amount of direct radiative forcing generated by a gas is given by the product of its mixing ratio in ppb and its radiative efficiency in W m-2 [IPCC, 2005]. Between 1970 and 2000, 23% of the increase in greenhouse forcing from well-mixed GHGs was due to increases in halocarbons. In contrast, from 1750 to 2000, halocarbon increases accounted for only 13% of the increase in greenhouse forcing. However, CFC replacements generally have lower global warming potentials, and total halocarbon emissions have decreased in recent years, so the combined CO2-equivalent emission of halocarbons has been reduced. The CO2-eq per year of greenhouse forcing by CFC emissions is projected to decrease from 1.7 gigatons in 2002 to 0.3 gigatons CO2-eq in 2015 [IPCC, 2005]. CFCs also have an indirect cooling effect though their degradation of the ozone layer. Ozone is a GHG, and its formation releases latent heat to the atmosphere. CFCs reduce the heating effect from both these phenomena, resulting in indirect cooling. A depleted ozone layer transmits more solar radiation, causing warming, but this warming effect is small compared to the cooling effects. However, the indirect cooling effect of CFCs is very likely less than their direct warming effect [IPCC, 2005]. CFC Reactivity Chlorofluorocarbons are ideal as refrigerants and blowing agents because of their extremely stable nature in the local atmosphere. However, it is because of their non-reactivity that they pose a threat as both Elmegreen et al., 3 an ozone destroying species and as a greenhouse gas. Our understanding of the sources and sinks, as well as the lifetime of CFCs, is essential for monitoring and predicting their environmental impact. The lifetime of a compound is defined as the time it takes for the concentration of that compound to decay to 1/e (Eq. 1). For any given compound, the lifetime can be written as: "X = # Burden # Loss Rate atm (1) The burden of CFCs can be calculated from observations combined with emission data. Calculating the CFC loss rate is more ! complex because there are several processes that may contribute to the loss of CFCs from the atmosphere. Some of those processes are uptake into the oceans, wet and dry deposition, and reactions (including photolysis). The lifetimes of many common CFCs have been quantified using this approach, as well as a mass balance technique. Mass balance involves extrapolating the concentration of CFCs measured in one region to a total atmospheric mass. This mass must balance the emissions into and losses of CFCs from the atmosphere. From emissions data banks, the loss can be calculated. Research has shown that CFCs are unable to photolyse with wavelengths above 290 nm. The ultraviolet radiation that is filtered out in the upper stratosphere makes ! CFCs stable enough to reach the lower stratosphere. In addition, CFCs are not very soluble in water, making wet deposition and ocean uptake very small contributors to the loss of CFCs. Because of these qualities, CFCs have particularly long lifetimes. For example, CFC-11 has a global lifetime of 4080 years, and CFC-12 lasts twice as long [Finlayson-Pitts et al., 2000]. Reactions involving CFCs have been widely studied over the past few decades. An example of CFC photolysis is shown below: CF2Cl2 + hv( λ < 240 nm) ⇒ CF2Cl. + Cl. Cl. + O3 ⇒ ClO. + O2 ClO . + O ⇒ Cl. + O2 The major sink of CFCs is therefore stratospheric photolysis. The chlorine radical acts as a catalyst in ozone destruction. Subsequent reactions of the chlorine produce temporary reservoirs of chlorine such as HCl and ClONO2. These reservoirs release chlorine radicals in high concentrations upon the start of Antarctic spring. For this reason, CFCs have been implicated in the appearance of an ozone hole over the southern pole. The ability of any given CFC to destroy ozone depends directly on its formula and structure. To rank CFCs in terms of their ability to destroy ozone, scientists have assigned Ozone Depleting Potentials (ODPs) to each CFC. This is similar to the Global Warming Potential (GWP) assigned to all greenhouse gases. The ODP is defined as the ratio of the global loss of ozone from that compound at steady state per unit mass emitted relative to the loss of ozone due to the emission of a unit mass of a reference compound (CFC-11, CFCl3). Eq. 2 gives the full definition of ODP; see Finlayson-Pitts et al., [2000] for more details. t % ts e"(t"ts ) /$ X dt FX M CFC"11 n X (2) ODP(t) = # FCFC"11 M X 3 % t e"( t"t s ) / $ CFC-11 dt t s Recent experiments have been conducted to determine how the concentration of stratospheric CFCs has changed since the initiation of the Montreal Protocol and subsequent international agreements, and to determine the present distribution of CFCs in the stratosphere. Methods Wavelengths that cause photolysis of CFCs are filtered out by the time radiation reaches the troposphere. As a result, CFCs have long lifetimes in the troposphere, so are well mixed. Therefore, air can be collected at ground level to measure the tropospheric Elmegreen et al., 4 concentration of CFCs. This enables relatively easy measurement of CFC mixing ratios in the troposphere. In fact, NOAA scientists have done this since 1977. At 9 sites around the world, flasks of air have been collected and analyzed weekly. This has given a lot of information about how tropospheric CFC concentrations have changed over time [NOAA website]. Sampling air in the stratosphere to measure CFC mixing ratios is less straightforward. A few analytical techniques are used to measure the CFC concentrations in the stratosphere, including gas chromatography and infrared absorption or emission. These techniques can be used on airplanes or balloons, and infrared techniques can also be used on satellites. These techniques are discussed below in the context of actual scientific experiments. Many of these experiments were interested in determining CFC concentrations not out of interest in the chemistry of the CFCs, but \ because CFCs make very good tracers due to their long lifetimes. Therefore, analyzing the distribution of CFCs can give information about atmospheric circulation. Gas Chromatography In gas chromatography, gas is pumped through a column with nitrogen (the carrier gas), and the rate of flow varies with the chemical depending on its chemical properties. Therefore, gases elute at different times, and become separated. As the gases flow from the column, CFC concentration is measured with an electron capture detector (ECD). One gas chromatograph (GC) used to measure CFC concentrations is the Airborne Chromatograph for Atmospheric Trace Species (ACATS-IV). This GC has been used in several airplane-based experiments such as Stratospheric Tracers of Atmospheric Transport (STRAT) in 1996 and Photochemistry of Ozone Loss in Arctic Regions in Summer (POLARIS) in 1997. A balloon-based experiment that used a similar GC was the Lightweight Airborne Chromatograph Experiment (LACE). ACATS-IV measures CFC-11 and CFC-113 every 140 seconds, and CFC-12 and halon1211 every 70 seconds [Romashkin et al., 2001]. The LACE GC measures CFC-11, CFC-113, CFC-12 and halon-1211 every 70 seconds [Moore et al. 2003]. Both instruments also measure other trace gases. Calibrations are made during the experiments by periodically injecting samples with trace gases of known concentration. In LACE, the vertical resolution was 300 m. Samples were taken at 7°S, 35°N, 65°N, and in the arctic vortex. Measurements had a 1-4% error, and showed good correlation with measurements using ACATS-IV. Infrared Absorption and Emission Each CFC absorbs radiation in specific areas of the infrared (IR) region. As mentioned above, their unique absorbances in the atmospheric window make them particularly potent greenhouse gases, but also allow their concentrations to be measured based on the amount of absorbance in a wavelength region. Infrared absorption instruments are useful on satellites, because satellites allow a large area to be measured for a relatively long period of time. Also, unlike gas chromatography, infrared absorption does not require the physical collection of air samples. One study that used the IR absorption technique to measure CFC-12 was the Improved Limb Atmospheric Spectrometer (ILAS) on the Advanced Earth Observing Satellite (ADEOS) [Khosrawi et al., 2004]. ILAS measured CFC-12 concentrations using absorptions of 850-950 cm-1 and 1050-1200 cm-1. Measurements were taken from November 1, 1996 to June 30, 1997, from 57°N to 72°N and 64°S to 89°S. Comparisons with balloon measurements that Elmegreen et al., 5 used various types of CFC measurement techniques showed that the ILAS measurements are most valid up to 20-25 km altitude. Above this altitude, CFC-12 concentrations are too small to be measured accurately. If a molecule can absorb light in a given region, it can also emit light in that region. For this reason, another method of measuring CFCs uses their IR emissions. The CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA 2) experiment measured CFC-11 based on its emission at 11.7 µm, which corresponds to the wavenumber of 854 cm-1 used in ILAS [Kuell et al., 2005]. CRISTA measured CFC mixing ratios from 8 km up through the stratosphere and into the mesosphere. The accuracy of these measurements is 6-8% from 12-18 km, and the precision is 1.8% at 12 km. Error assessment As with any experiment, it is important to have an idea of the error of a measurement. One way that these experiments assessed the validity of their data was by comparing measured CFC concentrations with those of another Figure 2. Comparison of data from ILAS (lighter points) and balloon-based infrared absorption (darker points) (left), and the percent difference between the two data sets (right) [Khosrawi et al., 2004]. experiment. The ILAS experiment compared its measurements with those from various balloon-based measurements to determine that ILAS results were valid up to 20-25 km, but at higher altitudes had results that differed greatly (up to 200%) from the balloon-based results (Figure 2). Some of these experiments determined the error of their data by comparing the ratios between concentrations of CFCs and a reference gas with the ratios from another experiment. Similar experimental methods are required for this type of comparison. The idea is that there should be a relationship between the concentration of CFC and of another gas with a long lifetime, such as O3 or N2O. Because O3 and N2O have much higher concentrations than CFCs (ppb compared to ppt), O3 and N2O concentrations should be easier to measure than CFCs, and the measurements should be more precise. Therefore, if two experiments show the same relationship between CFCs and O3 or N2O, then the data are consistent. The LACE experiment compared its O3 to CFC relationships with those from ACATS-IV data (Figure 3). The ILAS experiment compared CFC and N2O relationships between ILAS data and balloon-based data. Both experiments showed consistency between the data sets. Figure 3. Comparison of LACE (red) and ACATS (blue) data using the relationship between ozone and CFC-12 (left) and ozone and CFC-11 (right) [Moore et al. 2005]. A third way to assess data is by comparing it with results from a computer model. The CRISTA experiment compared Elmegreen et al., 6 its data with results from a model called EURAD. Unlike the other methods, this comparison showed problems in the model rather than in the measured data. Although this is not helpful for assessing the validity of a measured data set, it is essential to use measured data to determine the legitimacy of assumptions made in a model. Results Vertical distribution Data from LACE, ILAS, and CRISTA show that the concentration of CFCs decreases with increasing altitude. For a given latitude, concentrations do not change much up to the tropopause, because there are no CFC sinks in the troposphere and the lifetime of CFCs is long compared to the mixing rate of the troposphere (Figure 4). In the stratosphere, concentrations decrease rapidly with height (Figure 4). This rapid decrease is due to photolytic reactions. As altitude increases, there is more radiation, particularly more UV radiation that can react with CFCs. This means there is a larger sink of CFCs at higher altitudes, so the concentrations of CFCs decrease [Moore et al., 2003]. Also, because CFCs move into the stratosphere from the troposphere, it makes sense that concentrations would be higher closer to the troposphere. Figure 4. Mixing ratios of halon-1211, CFC-11, CFC-113, and CFC-12 versus altitude, for various latitudes. Data from LACE [Moore et al. 2003]. The concentration varies depending on the particular CFC. Among CFC-12, CFC-11 and CFC-113, CFC-12 has the highest mixing ratio, at around 500 pptv at the tropopause [Khosrawi et al., 2004, Moore et al., 2003]. CFC-11 has a mixing ratio of around 250 pptv, and CFC-113 has a mixing ratio of around 80 pptv at the tropopause [Kuell et al., 2005, Moore et al., 2003]. Horizontal distribution In the troposphere, there is low variation in CFC concentrations with latitude. However, at higher altitudes, the variation with latitude becomes much greater. In the stratosphere, the concentrations are highest closest to the equator, and decrease with latitude (Figure 5) [Kuell et al., 2005, Moore et al., 2003]. One reason for this trend is that air enters the stratosphere from the troposphere in the tropics [Kuell et al., 2005]. Not surprisingly, mixing ratios are greatest closest to the source. Also, as mentioned above, photolysis begins to break up CFCs once they are in the stratosphere. As CFCs are in the stratosphere longer, their concentration decreases because there has been more time for them to react. At higher Elmegreen et al., 7 latitudes, CFCs have been in the stratosphere longer because they travel from the tropics. Therefore, their concentrations are lower. There is good agreement between the data from LACE and CRISTA. At 16 km, the mixing ratio of CFC-11 is around 225 pptv from 20-40°N, 190 pptv from 40-60°N, and 150pptv and less in the polar region. Figure 5. Mixing ratio of CFC-11 from 20°N to 80°N and 80°W to 60°E. Data from CRISTA experiment [Kuell et al. 2005]. Changes over time: The Halogen Occultation Experiment The Halogen Occultation Experiment (HALOE) was a continuous eight-year, globally ranging experiment [Anderson et al., 2000]. The experiment involved satellite measurements that were weighted by latitude and globally averaged. The goal of HALOE was to monitor CFC concentrations in the stratosphere, to compare these measurements with the UNEP scenarios, and to understand the effect of the new regulations on the CFC levels in the atmosphere. HALOE measured species include HCl, HF, O3, CH4, H2O, NO and N2O. Previous studies have shown that CFCs dominate the stratospheric chlorine budget, and that there is no natural source of HF in the atmosphere. Therefore HF, a product of CFC photolysis, is an excellent proxy for CFC concentration in the stratosphere. From October of 1991 to May of 1999 these species were monitored at 55km by solar occultation. The results of this experiment agree with previous studies in that there exists a 5.3year time lag from the average tropospheric altitude to 55km (the stratopause). In addition, the results also agree with previous tropospheric measurements of chlorine concentration. Figure 6b shows the HALOE chlorine mixing ratio (parts per billion by volume) compared to several models and 5.3year projected concentrations. The concentrations fall directly on top of the 5.3year projected concentrations. These results illustrate the degree to which scientists understand the processes that control CFC lifetime, transport, and sinks. Figure 6a is from a study by Montzaka et al. [1999]. The peak in methyl chloroform concentration in the troposphere occurs between 1992 and 1993, almost exactly five years before the peak in Figure 6b. Figure 6c is a time series plot of the UNEP best-case scenarios for the emission of CFCs. The scenarios progress alphabetically with each one containing one more species than the previous scenario. Scenario G contains methyl chloroform, and is the only scenario that matches observations. From these two plots the authors conclude that methyl chloroform is a critical source of atmospheric chlorine. By a similar method, the stratospheric fluorine concentration was found to be dominated by the emission of CFC-11 and CFC-12. Figures 6d and 6e show the time series measurements and models of HF. The measured HF concentrations match Scenario E in Figure 6e, in addition to showing the approximate 5 year time lag in Figure 4. From this experiment it is evident that chlorine concentrations in the stratosphere are presently decreasing, while fluorine concentrations are increasing. In order to more accurately predict future concentrations, we must consider future emissions and Elmegreen et al., 8 therefore current compounds. stockpiles of CFC Future Emissions The emission due to these stockpiles, or banks, of CFCs has been modeled by comparing industry and regulatory data to the Refrigerant Inventory and Emission Previsions (RIEP) database. Ashford et al. [2004] have used the RIEP database as well as observations to predict future emissions to the year 2015 based on three different scenarios: (1) business as usual, (2) emission reduction 1st tier, and (3) partial phase-out of HFCs. a b c d e Figure 1. (a) Methyl chloroform concentrations in the troposphere, peaking in 1992, (b) HALOE inorganic chlorine measurements compared to models and earlier tropospheric measurements. Darker symbols on the right are the HALOE 55km measurements, (c) UNEP scenarios over the last twenty years, Scenario G includes methyl chloroform, (d) HALOE inorganic fluorine measurements compared with tropospheric fluorine measurements and models, (e) UNEP scenarios for fluorine emission, Scenario E includes CFC-12 [Anderson et al., 2000]. Elmegreen et al., 9 Figure 7. Observations and future emission scenarios for HCFC-22, a major compound used in refrigeration. The scenarios are based on future regulations and known bank sizes [Ashford et al., 2004]. Figure 8. Observations and future emission scenarios for HFC-134a, a common CFC replacement. Again the scenarios are based on future regulations and known bank sizes [Ashford et al., 2004]. Based on the business as usual scenario, the level of refrigerant-related emissions would reach 845,000 metric tons, with the most increase coming from HFCs. The best-case scenario predicts only 400,000 metric tons of emissions. From these models we can predict not only future environmental problems associated with CFCs but we can also determine the best possible way to reduce those emissions. For example, Ashford et al. [2004] have determined that short-term emissions come from aerosols, solvents, and open cell foams whereas longer-term emissions come from refrigerants and closed cell foams. They have also shown that the best way to prevent future emissions from blowing agent banks is to reform end-of-life procedures. Refrigerant banks of CFCs, HCFCs, and HFCs totaled 2.58 million tons in 2002. For the same year, emissions of those compounds were larger than 480,000 metric tons. The release of halons can occur during manufacturing, servicing, and at the end-oflife stage. The release of these compounds from blowing agents can occur during manufacturing and installation, use, and at the end-of-life stage. Knowing how and when CFCs and related compounds are released by refrigerants and blowing agents is crucial for reducing emissions beyond the present level. Ashford et al. [2004] have also considered the effect CFCs have as greenhouse gases. The predicted emission levels are converted into Global Warming Potentials (GWPs) by the relative warming they cause compared to CO2. From this they estimate that the business as usual scenario would be equivalent to the release of 1.53 billion tons of CO2 from refrigeration and air conditioning alone. The greatest source of uncertainty in these models stems from the slow release rate of blowing agents. Those compounds produced primarily by blowing agents show less agreement with models than do those produced by refrigeration. However, HCFC22 and HFC-134a show a high level of agreement to models thereby giving confidence to the 2015 predictions (Figures 7 and 8). Elmegreen et al., 10 Conclusion The best understanding of CFCs in the stratosphere will be gained by combining new technology, such as satellite measurements and atmospheric circulation models, with comprehensive data sets of all CFC sources. If we know where the most damaging CFC compounds are being produced and stored, proper disposal will be much easier to manage. Recent years have shown a great deal of progress in this field, and the outlook for eliminating CFCs from the atmosphere is promising. References Anderson, J., J.M. Russell III, S. Solomon, and L.E. Deaver, Halogen Occultation Experiment confirmation of stratospheric chlorine decreases in accordance with the Montreal Protocol, J. Geophys. Res., 105, D4, 4483-4490, 2000. Ashford, P., D. Clodic, A. McCulloch, and L. Kuijpers, Emission profiles from the foam and refrigeration sectors comparison with atmospheric concentrations. Part 2: results and discussion, Int’l J. Refrigeration, 27, 701-716, 2004. Finlayson-Pitts, B.J., and J.N. Pitts, Jr, Chemistry of the Upper and Lower Atmosphere, Academic Press, San Diego, CA 2000 IPCC and TEAP, 2005: Safeguarding the Ozone Layer and the Global Climate System. Contribution of Working Group I and III to the Special Report to the Intergovernmental Panel on Climate Change [Anderson, S., L. Kuijpers, J. Pons (TEAP) and S. Solomon, O. Davidson, B. Metz (IPCC) (steering committee)] Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 96pp. Khosrawi, F. et al., Validation of CFC-12 measurements from the Improved Limb Atmospheric Spectrometer (ILAS) with the version 6.0 retrieval algorithm, J. Geophys. Res., 109, D06311, doi:10.1029/2003JD004325, 2004. Kuell, V. et al., Tropopause region temperatures and CFC 11 mixing ratios from CRISTA 2, J. Geophys. Res., 110, D16104, doi:10.1029/2004JD005592, 2005. Montzka, S.A. et al., Present and future trends in the atmospheric burden of ozone-depleting halogens, Nature, 398, 690-694, 1999. Moore, F. L. et al., Balloonborne in situ gas chromatograph for measurements in the troposphere and stratosphere, J. Geophys. Res., 108(D5), 8330, doi:10.1029/2001JD000891, 2003. NOAA ESRL, Global Monitoring Division, Hats Flask Sampling Program, http://www.cmdl.noaa.gov/hats/flask/flasks.html. Romashkin, P. A. et al., In situ measurements of longlived trace gases in the lower stratosphere by gas chromatography, J. Atmos. Ocean Tech., 18, 11951204, 2001. Russell III, J.M., M. Luo, R.J. Cicerone, and L.E. Deaver, Satellite confirmation of the dominance of chlorofluorocarbons in the global stratospheric chlorine budget, Nature, 379, 526-529, 1996. Schauffler, S. M., et al., Chlorine budget and partitioning during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE), J. Geophys. Res., 108, 2003. Reviews of Atmospheric Science Topics Group B 1 Progress in Understanding Aerosol-Cloud Interactions: A Review Keywords- Heterogeneous nucleation, aerosol indirect effect, Twomey effect, Semi-direct effect, water clouds, climate forcing Abstract: The impact that aerosols have on climate is an important, yet poorly understood process. The aerosol direct effect, indirect effect, and semi-direct effect are three factors with the potential to significantly influence global climate. This review discusses progress that has been made in recent years towards understanding the Twomey effect, a subset of the aerosol indirect effect, and the aerosol semi-direct effect. The Twomey effect is thought to have a 0.5 to 1.9 W m-2 cooling effect. The semi-direct effect is thought to have a warming effect of 0.1 W m-2, but this result is less certain. Though the understanding of these two effects, and aerosol climate effects in general, has increased since the last IPCC report, published in 1991, there is still much progress to be made in the field of aerosol climate science. 1. Introduction Climate sensitivity is how strongly the earth’s climate system responds to a given perturbation. Models that estimate climate sensitivity need to be as accurate as possible and depend on approximate parameters. The relationship of observed climate change to estimated magnitude of forcing is done through estimates that are uncertain due to incomplete understanding of atmospheric aerosols. Future changes in the balance of climate forcing factors depend strongly on the balance of greenhouse gases and aerosol effects. The climate will become more sensitive to changes as the aerosol concentration is reduced. While there is a very complex relationship of causes and effects in the atmosphere, this paper will focus on the aerosol aspect of climate forcing on liquid water clouds. Effects from such aerosol-cloud interactions can result in modifications to rainfall generation that change thermodynamic processes in the clouds and dynamics of the atmosphere that drive all weather and climate. Aerosols, greenhouse gases, and the carbon cycle form a complex mix of disparate effects, the future balance of which is uncertain (Andreae 2005). There are three ways in which atmospheric aerosols can significantly affect Earth’s climate in a number of ways. First, the aerosol direct effect describes how aerosols at the top of the atmosphere reflect or absorb shortwave radiation causing a cooling effect at Earth’s surface. Secondly, the aerosol indirect effect is how aerosols as cloud condensation nuclei (CCN) and change cloud properties. The semi-direct effect explains how shortwave absorbing aerosols cause a net warming at the surface of the earth. The main focus of this paper is the aerosol indirect effect, as we will not mention the aerosol direct effect in further detail. Additionally, aerosols can effect climate forcing by increasing the number of CCN. This is known as the aerosol indirect effect. Indirect aerosol forcing is defined as the process through which aerosols perturb the earth’s atmospheric radiation balance by modulating cloud albedo and cloud amount (IPCC 2001). This occurs through a series of processes linking intermediate variables such as aerosol mass, CCN, ice nuclei, water phase partitioning, cloud optical depth, and other factors (IPCC 2001). These observations are supported by remote sensing measurements and satellite studies. A summary of these effects can be seen in Table 1. We will go into more detail on these effects in this paper. Reviews of Atmospheric Science Topics Group B 2 TABLE 1. Summary of Aerosol Indirect Effects and range of the radiative budget perturbation at the top-of-the atmosphere (FTOA)[Wm−2], at the surface (FSFC) and the likely sign of the change in global mean surface precipitation (P). (Lohmann and Feichter 2005). The Twomey effect is a type of indirect aerosol effect for clouds based on the assumption of fixed water amounts. The Twomey effect is defined as the increase in cloud albedo due to an increase in cloud droplets (that are smaller), and has a net negative forcing on the climate. The cloud lifetime effect is for clouds with fixed water amounts. The smaller cloud particles decrease the precipitation efficiency of the cloud overall and extend the lifetime of the cloud. The semidirect effect is due to absorption of solar radiation by aerosols within a cloud that cause evaporation of cloud droplet with uncertain effects on climate forcing. This main focus of this review is the Twomey indirect effect and the semi-direct effect. Clouds are an important aspect of the regulation of Earth’s radiation balance, as sixty percent of the earth’s surface is covered by clouds (Lohmann and Feichter 2005). Of the total incoming solar radiation, 48 W/m2 is reflected back by clouds (Lohmann and Feichter 2005). Small changes in the macrophysical (coverage, structure, altitude) and microphysical (droplet size, phase) properties of clouds can have significant effects on climate (Lohmann Feichter 2005). 2. Indirect effect for clouds with fixed water amounts (Cloud Albedo or Twomey effect) 2.1 Introduction The Twomey effect is the change reflection of solar radiation due to more but smaller cloud droplets whose liquid water content remains constant (Lohmann and Feichter 2005). According to the 2001 IPCC report, the Twomey effect of anthropogenic aerosols is between 0 to 2 W/m2, as can be seen by Figure 1. Other studies show between -1.9 to -0.5 W/m2 (Lohmann and Feichter 2005). More, but smaller, cloud droplets reduce precipitation efficiency and enhance cloud lifetime, known as the “second indirect effect,” that we will not discuss. However, the magnitude of the cloud lifetime effect may be comparable to the Twomey effect (Lohmann and Feichter 2005). This process involves feedbacks because cloud lifetime and cloud liquid water content change (Lohmann and Feichter 2005). Reviews of Atmospheric Science Topics Group B 3 FIG. 1. Graphs of Indirect Aerosol Effect in W m-2 from various models globally (a) and the ratio of Southern Hemisphere to Northern Hemisphere (b). Red bars are models for anthropogenic sulfate aerosols, anthropogenic sulfate and organic carbon (blue bars), anthropogenic sulfate and black, and organic carbon (turquoise bars) and the mean plus standard deviation from all simulations (olive bars). (Lohmann and Feichter 2005). Near areas with high emissions of sulfur dioxide, it can be shown that these polluted clouds have a higher average reflectivity than background clouds (IPCC 2001). Satellites retrieve column cloud droplet concentrations in low level clouds, which increase substantially from marine to continental regions (IPCC 2001). Areas of high cloud drop concentrations also occur in tropical areas where biomass burning is prevalent (IPCC 2001). There is a negative correlation between aerosol optical depth and the effect of cloud droplets on radiation (IPCC 2001). There is a positive correlation between aerosol optical depths and cloud optical depths. This can be shown by an increase in cloud albedo and a decrease in droplet size for optically thick clouds. The liquid water path determined by cloud dynamics is associated with the absorption of solar radiation (IPCC 2001). Observations of the indirect effect on changing cloud albedo can be observed by ships tracks. Examples of this are evident in Figure 4. This effect is due to the increase of CCN in polluted clouds. With the same amount of water distributed over a larger number of particles a smaller droplet size results (Lohmann and Feichter 2005). The difference in top of the atmosphere (TOA) radiation budget is due to the anthropogenic aerosol effect and its relationship to the concentration of cloud droplets (Lohmann and Feichter 2005). Warm clouds form precipitation size particles by collision and coalescence, which in global climate models (GCM) can be divided into collisions among cloud droplets and accumulation of rain droplets (Lohmann and Feichter 2005). The automatic conversion rate depends on the size or number of cloud droplets, Twomey and cloud lifetime effect can then be calculated separately (Lohmann and Feichter 2005). The relationship between these effects is uncertain and may depend on background aerosol concentration (Lohmann and Feichter 2005). 2.2 Droplet Size Droplet size is largest over the oceans and smallest over highly polluted areas where more CCN force water content to be spread over a larger amount of particles (Breon 2002). Aerosols acting as CCN cause an increase in droplets thus decreasing the mean radius and increasing cloud albedo. This increase in cloud albedo is proportional to absorption and the cloud optical thickness (Breon 2002). The micrometer difference in droplet sizes over the ocean compared to continental droplets supports the Twomey hypothesis (Breon 2002). Polarization and Reviews of Atmospheric Science Topics Group B 4 directionality of earth’s reflection (POLDER) can be used to asses Twomey globally (Breon 2002). Spatial and temporal resolutions are most sensitive to biomass burning and anthropogenic aerosols (Breon 2002). Figure 5 shows the cloud droplet size and optical thickness over the land and ocean. FIG. 2. Polarization and directionality of the earth reflectances (POLDER) images of (a) Aerosol Index (unitless) and (b) Cloud droplet radius (µm). (Breon 2002). As industrial activity increases, there is a net increase in the number of (CCN) (Barker 2000). A few micrometer decrease in effective radius can change cloud albedo by up to fifty percent, with an error fifteen to thirty percent (Barker 2000). The largest overestimates of these models occur with reductions to effective radius of cloud water droplets (Barker 2000). The homogeneity or inhomogeneity of these clouds due to cloud variability and liquid water paths relates to the transport of solar radiation. 2.3 Discussion A seasonal variation in droplet concentration can cause a seasonable variation in CCN (IPCC 2001). This has a dramatic impact on the cloud precipitation effect (IPCC 2001). This can be seen through increased cloud albedo with ship tracks (IPCC 2001). Changes in cloud microstructure due to aerosols can also cause a change in albedo (IPCC 2001). To quantify the relationship between droplet concentration and CCN, there are currently two methods. The quantity of aerosols can be empirically related to the quantity of cloud droplets, however, the accuracy of this strongly depends on cloud type (IPCC 2001). The other method relates the change in cloud droplet concentration to aerosol concentration through a Reviews of Atmospheric Science Topics Group B 5 parameterization of cloud droplet formation which assumes certain aerosol properties (IPCC 2001). Most models suggest an increase in water with increases in anthropogenic aerosol but new studies may show they have less water (Lohmann and Feichter 2005). These indirect effects may change occurrences and the frequencies of convection and therefore can be responsible for droughts and or floods (Lohmann and Feichter 2005). Factors controlling CCN are their size and response to water (IPCC 2001). How effective they are depends on their hydrophobic tendencies or if they are water soluble with hydrophilic sites, which are dependent on the state of mixing of the aerosols (IPCC 2001). Water soluble aerosols activate at lower relative humidity, which is significant for indirect forcing, however there are widely varying degrees of solubility (IPCC 2001). Composition effects are well known for aerosols of sulfates, sodium chloride and other soluble salts, but are poorly understood for organics, a critical uncertainty in climate forcing by aerosols (IPCC 2001). There is a range in composition of aerosols, how the different compounds are mixed within the aerosol itself (internal or external), therefore observations do not always show a constant uptake of water (IPCC 2001). As a droplet starts to form on an aerosol particle, soluble gases can add to the growing droplet, greater reducing the critical saturation for the droplet, however this effect has not be properly evaluated (IPCC 2001). An increase in albedo occurs due to an increase in reflectivity and a decrease in droplet size (Albrect 1989). In nature there are fewer CCN over the ocean, therefore any increase will have a significant impact on microphysics and climate, which can clearly be seen by ship tracks, as shown in Figure 3 (Albrect 1989). This effect can also be seen by dimethylsulfide (DMS) (Albrect 1989). The impact on climate from cloud processes not very well understood quantitatively (Albrect 1989). An assumption made on the Twomey effect is that the liquid water content remains constant as CCN increase, however this is difficult to quantitate (Albrect 1989). Changes in global albedo due to cloud amount would be largely offset by changes in long wave radiation budget (Albrect 1989). FIG. 3. Ship tracks. (Coakley 1987). 3. The Aerosol Semi-Direct Effect 3.1 Introduction First reported in 1997 by Hansen et al., the aerosol semi-direct effect describes how aerosols that absorb solar radiation influence the climate on both a local and global scale. These absorbing aerosols include dust, organic matter (OM), and black carbon (BC). Dust in the lower Reviews of Atmospheric Science Topics Group B 6 atmosphere comes mainly from deserts and arid regions. The OM and BC in the atmosphere however come from the burning of fossil fuels and biomass. Burning of biomass can include forest fire and other natural processes, but there are also anthropogenic contributions such as slash and burn agriculture in the Amazon rainforest. The primary source of black carbon in the atmosphere is from the burning of fossil fuels (Table 2). TABLE 2. Sources of black carbon and organic carbon in the lower atmosphere, from Lohmann et al. (1999). As BC is much more absorbing than either OM or dust and its sources are mainly anthropogenic, the focus of this paper will be on the climate impacts of the semi-direct effect as it relates to BC. In an area of high BC concentration, the main atmospheric impacts of BC are to decrease low cloud cover and liquid water path (LWP). Absorbing aerosols can decrease cloud cover either by dissipation of inhibition of cloud formation. In the first process, BC in the atmosphere absorbs solar radiation, becoming warmer. As the particles come into equilibrium with their surroundings, they warm the atmosphere. Warmer temperatures are less favorable for the formation of clouds because the saturation vapor pressure for water is higher at higher temperatures. Warming of the atmosphere can also cause existing cloud particles to evaporate, decreasing cloud cover and also LWP (Lohmann and Feichter 2001). Additionally, when solar radiation is absorbed by BC in the atmosphere, less solar radiation reaches the surface of the earth. The result is a decrease in warming at the surface and thus an increase in the static stability of the lower atmosphere. Less warming at the surface causes the temperature differential between the surface and the lower atmosphere to decrease. The increased static stability of the lower atmosphere leads to a reduction in the amount and intensity of convection currents carrying warm air from the surface into the cooler lower layers of the atmosphere. In addition, as convection decreases so too does the amount of water vapor transported from the surface to the lower atmosphere, and thus less moisture is available for cloud formation (Lohmann and Feichter 2001). Via various routes, the overall impact of the aerosol semi-direct effect is to decrease cloud cover. Based on standard climate models, a reduction in cloud cover should lead to a positive climate forcing at the earth’s surface. There is, however, some debate as to the actual direction of the forcing, due to variations in modeling parameters. Nevertheless, it is agreed that the atmosphere has a very high sensitivity to absorbing aerosols. That is to say that a small change in the concentration of absorbing aerosols over a particular region could lead to large changes in the climate. Several previous studies have estimated the climate sensitivity to Reviews of Atmospheric Science Topics Group B 7 absorbing aerosols at 2 to 3 times that of CO2 and other green house gases (Hansen et al. 1997, Cook and Highwood 2004, Jacobson 2002). 3.2 Results As with most atmospheric processes, determining how the semi-direct effect impacts global climate is a difficult task. Atmospheric aerosol effects encompass many different and often competing processes. The effects of one effect are often masked by another, making the exact magnitude and direction of climate forcing due solely to the semi-direct effect difficult to determine. In addition processes in the atmosphere are highly interconnected and may cause chain reactions which mitigate or enhance the effect. Furthermore, BC possesses many variable properties which could lead to different climate forcings based upon their treatment in climate models. Among these properties are height of injection, size, composition, mixing state, concentration, and distribution. The height of injection of the absorbing aerosols can have a significant effect on not only the magnitude of climate forcing from the semi-direct effect, but also the sign (Table 3). Table 3. Modeled semi-direct, direct, and total climate forcing (W m-2) based on aerosol injection height. BL is the boundary layer and Zinv is the height of the tropopause, from Johnson et al. (2004). Using a Large Eddy Model (LEM), Johnson et al. (2004) determined that the climate forcing from the semi-direct effect would be positive if the absorbing aerosols were within the boundary layer, but negative if the absorbing aerosols were above the boundary layer. The composition or mixing state of BC can also change the absorbing properties of the particles and the resultant climate forcing. It is generally thought that BC exists in three separate mixing states – an external mixing state where BC particles are equally mixed with nonabsorbing particles, an internal mixing state where an individual particle is a homogeneous mixture of BC and non-absorbing aerosols, or a BC core surround by layers of non-absorbing aerosols. Most models have assumed that BC is either well mixed externally or internally (Johnson 2000). However, in 2000, Jacobson suggested that these two mixing states were not representative of the physical states of BC in the atmosphere and that a better model would be one with a BC core and a shell made of non-absorbing aerosols. In 1996, Chylek et al. postulated that particles with BC cores and non-absorbing shells could absorb up to 2.5 times the solar radiation of externally mixed BC. This change in absorbance would certainly have an impact on the magnitude of climate forcing caused by the semi-direct effect, but to this point, no studies have been done to show exactly what the change in climate forcing would be. Reviews of Atmospheric Science Topics Group B 8 Absorbing aerosols are not uniformly emitted throughout the globe. The Indian Ocean Experiment (INDOEX) showed that BC absorption was higher over the Indian subcontinent than over the ocean due to increase pollution over the land mass (Figure 4). FIG. 4. Absorption of solar radiation by BC from INDOEX (O’Carroll). The areas of high absorption and thus high BC concentration correspond to the Indian subcontinent, while the absorption is much lower over the ocean. Numerous studies have confirmed that the absorption of solar radiation impacts climate at the local scale, though the exact forcings are uncertain. Most agree that the semi-direct effect will cause a positive climate forcing locally, although the magnitudes vary from 0.1 W m-2 (Lohmann and Feichter, 2001) to 23 W m-2 (Johnson et al. 2004). Penner et al. (2003) used a “relaxed forcing” model that accounted for changes in longwave emissions due to shortwave absorption by BC, and found forcing values that ranged from 0.1 W m-2 to –1.24 W m-2, again showing the uncertainty in the magnitude of the semi-direct effect. Though these studies all prove that the semi-direct effect is significant on the local scale, Lohmann and Feichter (2001) concluded that the semi-direct effect was negligible on a global scale when compared with the indirect effect. Several studies have also shown that the semi-direct effect will decrease LWP over areas of high BC concentration, but whether this translates to a global impact is unknown. In the same study, Lohmann and Feichter determined that LWP would decrease by 0.3 g m-2 in polluted areas, but that LWP increased globally by 10 g m-2 due to the indirect effect. 3.3 Results Because of high climate sensitivity, the impact of absorbing aerosols, especially BC, will become increasingly important as concentrations of BC and other absorbing aerosols in the atmosphere increase. In 2002, Jacobson claimed that controlling BC and OM could be the most effective way to curb global warming. Conversely, Lohmann and Feichter (2001) maintain that the semidirect effect is insignificant in comparison to other aerosol interactions in the atmosphere though their assertions come with the caveats that there are high degrees of uncertainty in both BC emissions and the absorbing properties of BC due to different mixing states. Despite the fact that the semi-direct effect has known impacts on climate forcing over small areas, the global impact Reviews of Atmospheric Science Topics Group B 9 has yet to be characterized. Thus further study on the aerosol semi-direct effect is necessary to ascertain the effects that BC and other absorbing aerosols will have on the global climate. 4. Summary and Conclusion of Current Progress Atmospheric aerosols have the ability to significantly influence the atmosphere. Changes in the way light enters the atmosphere causes significant alterations to global radiative forcing. Several effects contribute to climate forcing by aerosols. Specifically, aerosols are able to alter cloud amount, formation, and reflective properties. The aerosol indirect effect causes a significant cooling or negative forcing estimated at -0.5 to -1.9 Wm-2 (Lohmann and Feichter 2005). The semi-direct effect has largely variant effects on the atmosphere. The semi-direct effect can induce negative or positive forcing based on a number of factors, but the overall effect is assumed to be positive. Thus it can be concluded that the indirect effect is slightly mitigated by the semi-direct effect. There is less uncertainty about the indirect effect, however both effects are not easily quantified. High interconnectivity between the indirect and semi-direct effects and other atmospheric processes make modeling and predicting individual effect forcing difficult. Models are able to provide estimates of the forcing but are calculated based on parameter assumptions. It is perhaps most valuable to consider the effects in tandem rather than trying to separate the aerosol effects individually. Combining both effects leads to empirical observation of a negative forcing as is suggested in Figure 3. FIG. 3. Global mean total indirect aerosol effects over the oceans, land and the ratio ocean/land. Anthropogenic sulfate (red bars) anthropogenic sulfate and black carbon (green bars) anthropogenic sulfate and organic carbon (blue bars) anthropogenic sulfate and black, and organic carbon (turquoise bars), a combination of satellite results (black bars) and the mean plus standard deviation from all simulations (olive bars). (Lohmann and Feichter 2005). Reviews of Atmospheric Science Topics Group B 10 This figure shows several variant observations from multiple sources over both land and ocean. Although observations differ in magnitude, the overall ocean, land, and global averages result in negative forcing. This indicates a higher reliance of forcing on the indirect effect in comparison to the semi-direct. The sources of these aerosols are inferred as anthropogenic. A ratio of ocean to land forcing of less than unity indicates a larger negative forcing over land, supportive of increased aerosol concentration over land than ocean. Overall, the magnitude of these effects is highly uncertain, but generally the direction of climate forcing is known with more confidence. Reviews of Atmospheric Science Topics Group B 11 References Albrect, B.A. Aerosols, 1989: Cloud Microphysics, and Fractional Cloudiness. Science, 1989, 245, 1227-1230. Andreae, M.O.; Jones, C.D.; Cox, P.M. 2005: Strong present-day aerosol cooling implies a hot future. Nature, 435, 1187-1190. Barker, H.W. 2000: Indirect Aerosol Forcing by Homogenous and Inhomogenous Clouds. Journal of Climate, 13, 4042-4049. Breon, F.M; Tanre, D.; Generoso, S. 2002: Aerosol Effect on Cloud Droplet Size Monitored from Satellite. Science, 295, 834-838. Coakley, J.; Bernstein, R.; Durkee, P. 1987: Effect of Ship-Stack Effluents on Cloud Reflectivity Science, 237, 1020-1022. Cook J. and Highwood, E., 2004: Climate response to tropospheric absorbing aerosol in an intermediate general-circulation model. Q. J. R. Meteorol. Soc., 130, 175-191. Curry, J.A.; Webster, P.J. Thermodynamics of Atmospheres and Oceans. San Diego: Acadamic Press, 1999. Chylek, P., Lesins, G., Videen, G., Wong, J., Pinnick, R., Ngo, D., and Klett, J., 1996: Black carbon and absorption of solar radiation by clouds. J. Geophys. Res., 101, 22336523371. Hansen, J., Sato, M., and Ruedy, R., 1997: Radiative forcing and climate response. J. Geophys. Res., 102, 6831-6864. Jacobson, M., 2002: Control of fossil-fuel particulate black carbon and organic matter, possibly the most effective method of slowing global warming. J. Geophys. Res., 107, 4410-4431. Johnson, B., Shine, K., and Forster, P., 2004: The semi-direct aerosol effect: Impact of absorbing aerosols on marine stratocumulus. Q. J. R. Meteorol. Soc., 130, 1407-1422. Lohmann, U.; Feichter, J. 2005: Global indirect aerosol effects: A Review. Atmospheric Chemistry and Physics, 5, 715-737. Lohmann, U., Feichter, J., Chuang, C., and Penner, J., 1999: Prediction of the number of cloud droplets in the ECHAM GCM. J. Geophys. Res., 104, 9169-9198. Lohmann, U. and Feichter, J., 2001: Can the direct and semi-direct aerosol effect compete with the indirect effect on a global scale? Geophys. Res. Lett., 28, 159-161. O’Carroll, C., Aguileira, M., and Clark, C. New NASA Satellite Sensor and Field Experiment Shows Aerosols Cool the Surface but Warm the Atmosphere. http://earthobservatory.nasa.gov//Newsroom/NasaNews/2001/200108135050.html Penner, J., Zhang, S., and Chuang, C., 2003: Soot and smoke aerosol may not warm climate. J. Geophys. Res., 108, 4657-4665. Penner, J.E., M. Andreae, H. Annegarn, L. Barrie, J. Feichter, D. Hegg, A. Jayaraman, R. Leaitch, D. Murphy, J. Nganga, G. Pitari In: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T.,Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp. Current Understanding of Changes in Cloud Amount, its Causes, and Effects Oliver Sun, Gregg Dobrowalski, Adam Orin, Mike Stukel November 17, 2005 1 Introduction Clouds play an important role in our climate. Understanding how clouds interact with the climate, unfortunately, is a very difficult task [2]. As the climate changes from global warming, cloud amount is likely to change, and will have important consequences on the changing climate. In this paper we review our current understanding of cloud change. First we look at various anthropogenic sources of cloudiness change. Next we examine several recent studies on cloud amount change over the twntieth century. Finally, we explore the possible effect of changes in cloud amount. 2 Anthropogenic causes of cloud amount change When trying to understand change in cloud amount it is worthwhile to consider potential causes of such change. Two possible anthropogenic effects are the emission of sulfur and contrails caused by jet aircraft. Parungo [7]. and Myhre [5] will be discuss the topic of sulfur emissions. Part of the IPCC report[1] will discusses aircraft contrails. A key idea to remember when thinking about the following information is that correlation does not necessarily mean causation. Data is presented along with an interpretation but no claim is made that sulfur emissions are solely responsible for the observed increase in cloud cover. Some data suggests that cloud cover is changing most over populated regions. Data from 1952-1981 shows a net percent net increase in the global amount of cloud cover normalized to the 1952 level[7], although we will see there is no consensus on the amount of cloud change. This increase is not split evenly throughout the world; there was a 2.3 percent increase in the Northern Hemisphere compared to a 1.2 percent increase in the Southern Hemisphere. One possible explanation for this is related to population density as well as density of industrial facilities. Such areas emit sulfur particles from power plants, smelting plants and fuels containing sulfur. Latitudes between 30 degrees to 50 degrees North show the greatest increase in cloud cover. This corresponds to the areas of the globe most heavily populated. 1 Figure 1: Trends of Northern Hemisphere daytime altostratus and altocumulus cloud amounts over 10 degree latitude bands. Figure 2: Sulfur emissions as a function of year, for various regions. 2 Sulfur particles are a possible cause of increased cloud cover. The particles act as cloud condensation nuclei (CCN) which affect cloud formation and the stability of clouds[5]. Heterogeneous nucleation requires a lower saturation level compared to homogeneous nucleation to form clouds. The clouds that are formed from CCN are colliodially stable; they have a greater number of smaller droplets which this results in the clouds not producing much rain and therefore lasting longer. Figure 1 shows the amount of sulfur released into the atmosphere. Note the three sub-classifications, all are in the latitudes that saw the greatest increase in cloud cover. Jet aircraft flying in the upper troposphere (∼10 km) can produce contrails which are essentially cirrus clouds. A combination of particulate emission from the exhaust as well as the heat of the exhaust can create cloud forming conditions in the wake of the aircraft[1]. The contrails start small, only a few meters wide and can spread to cover a much larger area. This was seen on September 12, 2001. As the skies were closed to commercial aircraft the few contrails left by six high flying military aircraft outside Washington D.C. could be easily seen. Over the course of a few hours the contrails spread to 20, 000 km2 [8]. If they had not initially been identified as contrails, it would be difficult for satellites to distinguish the clouds from ones formed naturally. This is due to the fact that the algorithms used by the satellites to sort the images rely on the linear nature of contrails to distinguish them from other clouds. After time has passed and the contrails have spread the linear features are blurred thus making the distinction difficult. 3 3.1 Observations on changes in cloud amount Changes in cloudiness over land Several studies have examined changes in cloud cover over land specifically. In this paper we will look at two such studies, one by Sun and others[9], and one by Croke and others[2]. Both papers use human-made observations of cloud amount throughout the twentieth century. Both papers also find an overall increase in cloud cover. Croke uses land based observations from 1900-1987 to estimate the change in annual cloud cover over three regions in the United States: the coastal southwest, the southern plains, and the coastal northwest. The data was collected by meteorological stations throughout the United States, and includes estimates of fractional cloud amount. Although not explicitly defined in the paper, we assume that the “annual fractional cloud cover” for a given region is the fraction of the region covered by clouds, averaged over a year. For all three regions, Croke finds that cloud cover increases over the timespan of the datasets. Figure 3 shows a plot of percent cloud cover over the coastal southwest as a function of year. As can be seen, there is roughly a ten percent increase in cloud cover over this region in the twentieth century. The two other regions also show roughly a ten percent increase in cloud cover over the twentieth century. Croke also finds that the increase in cloud cover is linearly correlated to both the global change in surface temperature, and to changes in several high/low pressure systems. Figure 3 Figure 3: Plot of percent cloud cover, and mean global surface temperature anomaly as a function of year. As can be seen, the percent cloud cover is linearly correlated to the temperature anomaly. 3 also plots the global mean surface temperature anomaly, ∆T , as a function of year. The percent cloud cover is found to be linearly related to ∆T with a linear correlation coefficient of R = .89. Croke also finds that the intensity of the North Pacific High is linearly correlated to the global surface temperature anomaly, with a negative sense. The global surface temperature anomaly, the percent cloud cover, and the intensity of the North Pacific High all seem to be linearly correlated. Although this interesting, it is important to remember that correlation does not imply causation. It is too simple to say that a change in one of the three features causes the others to change. The climate is a complex system of feedbacks, and it is hard to isolate one part of the system. Croke does discuss some possible mechanisms for the North Pacific High to effect cloud cover over the coastal southwest. Sun uses land based observations, similar to those used by Croke, from the past 40-50 years to estimate the change in cloud frequency and type over the former USSR and the United States. Rather than focus on the changes in cloud amount over time, Sun focuses on the changes in the frequency that various cloud types occur. Monitoring changes in the relative amount of various cloud types, and not just the total cloudiness, may help identify causes of cloudiness variations, and deepens out understanding of the influence of clouds on climate change. Figure 4 shows Sun’s conclusions on changes in cloud cover for various seasons, and for various cloud types. Sun defines cloud type frequency as the ratio of number of observations where the given cloud type was observed to the total number of observations made. Sun concludes that there has been a net increase in cloudiness both over the US and USSR over the past 50 years. Neither papers offer a significant discussion or a reference to a discussion of the accuracy of the datasets. Sun does note that an increase in the amount of low level clouds will impair the ability of observers to determine the amount of high level clouds. Thus, an increase in low-level clouds causes an apparent decrease in the amount of high level clouds. Sun does not attempt to estimate the effect of this bias. Instead, if the higher cloud types were obscured by lower clouds, Sun counted this as an absence of the higher clouds, which will under4 Figure 4: A table that summarizes Sun’s results. Sun finds an overall increase in cloud amount over both the US and USSR. report the frequency of the higher clouds. Sun also notes that the differences between cloud classification schemes and data collecting practices of the American and Russian stations made it difficult to compare the datasets. Another issue for Sun’s datasets, and possibly Croke’s, is that 19 cloud types were recorded by the observers. With such a large variety of cloud types, it is difficult for a human to consistently classify a cloud as the proper type. Without any quantitative measures or estimates of the inherent inconsistancies of humanbased observations, it is hard to judge the quality of the datasets from which these authors base their results. We see that Croke and Sun both offer evidence that cloud cover increases during global warm periods. Croke’s datasets are more limited in spacial scope, and the observed change in cloud cover is a roughly ten percent increase. Sun’s datasets span two nations, and his data is broken into various cloud types. Sun estimates a net increase in annual cloud amount but does not provide a quantivitave estimate of the increase. 3.2 Changes in cloud amount – a more recent study The effects of cloud variation on a warming climate are not well known [4]. In light of the uncertainties in cloud simulation, Norris [6] undertook a survey to compare the decadal trends in three data sets: the recently-available satellite-observed cloud cover and satellite-observed outgoing radiation flux, and the surface-observed cloud cover, for which much longer records exist. If these records were to be in close agreement, then it might be feasible to estimate the outgoing radiation flux for many decades prior to the satellite era. Satellite observations were taken from the International Satellite Cloud Climatology Project (ISCCP), available from 1983 forward, and comprising observations from a variety of polar and geosynchronous weather satellites. This data included low-level, mid-level, 5 and high-level cloud cover fractions, with the divisions at cloud top heights of 440 and 680 hPa, respectively. Due to inconsistencies in infrared-only observations, only the daytime results were reported. Long-term surface observations employing a consistent methodology were recorded in the Extended Edited Cloud Report Archive (EECRA). Land observations were provided by numbered World Meteorological Organization stations in the period 1971-1996, and ocean observations from Volunteer Observing Ships were available from 1952-1997, including lowlevel and total cloud cover fractions reported in eighths. A simple model for random occlusion between lower and upper-layer clouds was employed for the purposes of comparison with satellite-observed data. The baseline for radiative flux was established by the Earth Radiation Budget Satellite (ERBS), operational during 1985-1999. Two instruments were referenced: the wide-area nonscanner instrument, and the finer scanner instrument, which was only available during 1985-1989. When comparing the 72-day anomaly in cloud cover from the two observation sets to the radiation flux anomaly, there was a slight decreasing trend over the period when all three observational sets were available; however, the simple linear correlation was clearly not a good statistical model for the system (Norris, 2005). While the upper-layer cloud cover measurements were in good agreement with each other as well as the ERBS-observed flux, inconsistencies in the total cloud cover due to differences in low-level cloud cover were not accounted for. Norris [6] cites secular changes in satellite field makeup (a shift toward high viewing angle, geosynchronous satellites) and errors in lowlevel surface observations of cumulus clouds as possible culprits, but further investigation is needed to extend these observations into reliable radiation flux estimates. Norris then attempted a simple linear correlation model for estimating radiation flux, using the EECRA upper-layer the surface observations to estimate the outgoing long wave (OLW) radiation. Excluding certain areas which were heavily affected by the lower-level cloud cloud discrepancy, these results nevertheless show promise for future longer-term estimates of radiation flux derived from surface observed data. 4 Global effects of changes in cloud amount According to the 2001 IPCC [4], the incredible complexity of cloud dynamics “remains a dominant source of uncertainty” in models of how the global climate will respond to human activities [10]. Changing cloud cover patterns are certain to exhibit a strong effect on the climate and life on Earth. They will effect precipitation patterns as well as the amount of photosynthetically active radiation reaching the earth’s surface, which will in turn alter ecosystems as well as regions that are suitable for human farming. Clouds play a huge role in the earth’s albedo, that varies spatially, as clouds above polar ice caps and snow decrease the albedo, while clouds over vegetation or ocean increase the albedo. The clouds also can reflect or absorb outgoing longwave radiation, thus heating the coupled earth-atmosphere system. The 2001 IPCC Report estimates that the total cloud effect will be a net 10 ∗ 10 −20 W/m2 negative feedback averaged over the globe, yet there is great uncertainty in this figure 6 Figure 5: 72-day anomaly, EECRA (red), ISCCP (blue), ERBS nonscanner (black), ERBS scanner (green). Reference 1985-1989 mean.. 7 Figure 6: 72-day anomaly in LW, SW, and net CCRF. Estimated from EECRA (red) and reported by ERBS (black), and divided into Region A (30o S-30o except tropical Atlantic and eastern tropical Pacific) and B (excluded regions). and in local feedbacks in areas such as the poles, where latitudinal trends in cloud cover will be important [4]. Ongoing research in many different areas is currently attempting to address these unknown parameters of global climate change. The Polar Regions are particularly sensitive to climatic changes, and have an important role in how humanity will feel changes in the climate system, due to the possibility of increasing sea levels as the polar ice caps melt. In the Arctic, cloud formation is likely to have a positive feedback with human induced global warming. In large part, this arises from the strong reflectivity of ice and snow, which has a higher albedo factor than clouds do. Clouds also serve to insulate the region, and reduce wintertime cooling by roughly 40 ∗ 10 −50 W/m2 , but only decrease summer heating by 20 to 30 W/m2 . This radiative forcing, however, is dependent on several different parameters of clouds; cloud coverage, height, thickness, and water content [11]. Vavrus (2004) attempted to estimate the role that cloud feedback will play on the Arctic climate. He used an atmosphere-mixed-layer ocean global climate model (GENESIS2) to model the climate of the region. He found that when CO2 was increased by a factor of two, the model generated more clouds at high latitudes, but lower cloud coverage at low latitudes, thus acting as a positive feedback on global warming at all latitudes. To determine the role that cloud cover changes played in the model he constructed a concurrent model that was similar in all ways, except that cloud coverage was forced to remain at present levels. It was prescribed in the model instead of being linked to climate. He then compared the two models. The results can be found in Figure 7. As the figures show, at all latitudes the tem8 Figure 7: (a) Latitudinal average temperature for a GCM forced by a doubling of CO 2 with a coupled atmosphere-ocean-land system (open dots) or cloud cover fixed at current concentrations (closed dots). (b) Percentage temperature change due to effect of changes in cloud cover. (c) Changes in precipitation with and without cloud changes. [11] 9 Figure 8: Sensitivity of LW radiation to threshold optical thickness values. Top graph is of LW radiation up to the top of the atmosphere. Bottom graph is of LW radiation down to the surface. Surface radiation increases by roughly 10 W/m2 when optically thin clouds are treated as clear sky.[12] perature was higher in the coupled atmosphere-land-ocean model than in the model where cloud cover remains constant. The models also show that at most latitudes and particularly high latitudes, precipitation will increase with the cloud changes forced by increased CO2 concentrations. When cloud-climate models are constructed, it is very important for researchers to ground-truth their models by comparing their predictions for the current climate to actual measurements made in the real world. Precipitation, temperature, cloud cover, and many other parameters can be compared, but cloud coverage can be particularly difficult to determine due to inaccuracies in human estimation of cloud fraction. In particular, thin clouds are frequently missed by observers, who erroneously score them as clear sky. This effect is important in Polar Regions, because during the winter a large fraction of the clouds formed are optically thin. To address this bias in real-world measurements, Wyser and Jones[12] decided to study the effect that optically thin clouds had on the surface heat budget of the of the Arctic. They used data obtained from a Canadian Coast vessel that was frozen in the pack ice for one year. This data showed a strong dependence on the type of instrument used, whether it was cloud lidar, radar, or human observations. To correct for the error, Wyser and Jones, put an optical thickness threshold into their model. They set this threshold as a parameter for their model, and asked the model to predict the level of cloud cover that would be found above this threshold. They found that by using this technique they could get much better agreement between their model and the observed data. Using their model, they determined that long-wave radiation increases by 10 W/m2 when optically thin clouds are treated as clear sky. Figure 8 shows the decrease in light-wave radiation at the surface (and increase above the atmosphere) as the critical threshold for optical thickness increases. This error in long-wave radiation can have strong effects on the climate as illustrated by one example 10 Figure 9: Changes in LW flux and Baltic Sea ice coverage in a model by Doscher, if forced by the increased LW radiation seen when optically thin clouds are included.[12] given by Wyser and Jones. They used their increased long-wave radiation in a coupled model of the Baltic Sea climate created by Doscher et al. Their data produced a significant increase in long-wave flux and a concurrent decrease in ice extent as seen in Figure 9. Kaufmann et al.[3], in contrast, present an example of a negative feedback of cloud cover changes on anthropogenic global warming. They examined the effect that smoke, dust, and pollution aerosols have on lower atmosphere clouds over the Atlantic Ocean, by looking at latitudinal trends in these aerosols. They defined bands where the clouds are dominated by different types of aerosols: Marine aerosols from 30 degrees S to 20 degrees S, smoke from 20 degrees S to 5 degrees N, mineral dust from 5 degrees N to 25 degrees N, and pollution from 30 degrees N to 60 degrees N. They found that all aerosols lead to smaller water droplets, which inhibits precipitation and leads to longer cloud residence time, and hence higher cloud fraction. They then addressed the impact that this effect would have on incoming radiation. They found that incoming long-wave radiation was reduced by 11 W/m2 due to the presence of these aerosols, with a third of it coming directly from reflection by the aerosols and the rest due to the indirect effect of increased cloud cover over the Atlantic. 5 Conclusions Clouds play an important role in climate change, yet more work must be done to fully understand their role. Both sulfur emissions and aircraft contrails are changing cloud amount. Although much work has been done to understand how much cloud amount has changed, there is little consensus amoung the various studies. More quantitative datasets from satellites should improve the ability of researchers to estimate changes in cloud amount. Changes in cloud amount will have several important effects on the climate, with current estimates of a net 10 ∗ 10−20 W/m2 cooling effect on the global energy budget. But this number is still a topic of debate, and likely will remain uncertain until we better understand cloudiness. 11 References [1] Ackerman, S., P. Artaxo, O. Boucher, M.Y. Danilin, B. Krcher, P. Minnis, T. Nakajima, O.B. Toon. Observed Climate Variability and Change. In: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T.,Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp (2001). [2] Croke M., Cess R., Hameed S. Regional Cloud Cover Change Associated with Global Climate Change: Case Studies for Three Regions of the United States. Journal of Climate, 12 (1999), 2128-2134. [3] Kaufmann Y., Koren I., Remer L., Rosenfield D., Rudich Y. The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean. Proceedings of the National Academy of Sciences 102:32 (2005). [4] Moore III B., Gates W., Mata L., Underdal A. Advancing Our Understanding In: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T.,Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp (2001). [5] Myhre, G., Myhre A., Stordal F. Can human activity have led to global cooling? (2001) [6] Norris, J. Multidecadal changes in near global cloud cover and estimated cloud cover radiative forcing. Journal of Geophysical Research, 119 (2005), D08206. [7] Parunngo F., Boatman J., Sievering H., Wilkison S., Hicks B. Trends in Global Marine Cloudiness and Athropogenic Sulfur. Journal of Climate 7 (1993), 434 440. [8] Perkins, S. September’s Science: Shutdown of airlines aided contrail studies. Science News 161:19 (2002), 291. [9] Sun, B., Groisman, P., Mokhov, I. Recent Changes in Cloud Type Frequency and Inferred Increases in Convection over the United States and Former USSR. Journal of Climate, 14 (2001), 1864-1880. [10] Stocker T., Clarke G., Le Treut H., Lindzen R., Meleshko V., Mugara R., Palmer T., Pierrehumbert R., Sellers P., Trenberth K., Willebrand J. Physical Climate Processes and Feedbacks In: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T.,Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp (2001). 12 [11] Vavrus, S. The Impact of Cloud Feedbacks on Arctic Climate under Greenhouse Forcing. Journal of Climate 17:3 (2001), 603-615. [12] Wyser, K., Jones C. Modeled and observed clouds during Surface Heat Budget of the Arctic Ocean (SHEBA). Journal of Geophysical Research 10 (2005). 13