* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chapter 5. CO2 as a Climate Regulator during the Phanerozoic and
Climate change and agriculture wikipedia , lookup
Iron fertilization wikipedia , lookup
Climate change, industry and society wikipedia , lookup
Climate engineering wikipedia , lookup
Fred Singer wikipedia , lookup
Snowball Earth wikipedia , lookup
Effects of global warming on human health wikipedia , lookup
Climate change and poverty wikipedia , lookup
Mitigation of global warming in Australia wikipedia , lookup
Low-carbon economy wikipedia , lookup
Effects of global warming on Australia wikipedia , lookup
Climate sensitivity wikipedia , lookup
Carbon Pollution Reduction Scheme wikipedia , lookup
Climate-friendly gardening wikipedia , lookup
Citizens' Climate Lobby wikipedia , lookup
Global warming wikipedia , lookup
Attribution of recent climate change wikipedia , lookup
Carbon governance in England wikipedia , lookup
General circulation model wikipedia , lookup
Politics of global warming wikipedia , lookup
Physical impacts of climate change wikipedia , lookup
Instrumental temperature record wikipedia , lookup
Business action on climate change wikipedia , lookup
Solar radiation management wikipedia , lookup
IPCC Fourth Assessment Report wikipedia , lookup
Name ______________________________ Chapter 5. CO2 as a Climate Regulator during the Phanerozoic and Today Summary: This chapter explores how the exchange of carbon to and from the atmosphere is a primary factor in regulating climate over timescales of years to millions of years. In Part 5.1 you will make initial observations about the short term global carbon cycle, its reservoirs and the rates of carbon transfer from one reservoir to another. In Part 5.2 you will investigate how CO2 directly and indirectly affects temperature. In Part 5.3 you will examine the recent record of atmospheric CO2 levels, and identify which parts of the carbon cycle are most important at regulating climate over short time scales. In Part 5.4 you will investigate the long-term global carbon cycle, CO2, and Phanerozoic climate history. You will identify Greenhouse and Icehouse times, and place modern climate change in a geologic context. Figure 5.1. (a) Volcanic outgasing and lava. (from: Reuters/Bernardo de Niz, http://www.abc.net.au/science/articles/2007/04/27/1907876.htm) (b) Oil refinery, Alaska, (from http://www.alaska-in-pictures.com/mapco-oil-refinery-3329-pictures.htm) [note copyright license for this photo is $75. maybe find a different one from a gov site] Page 1 of 27 Name ______________________________ CO2 as a Climate Regulator during the Phanerozoic and Today Part 1. The Short Term Global Carbon Cycle Introduction Examine the short term carbon cycle diagram (Fig. 5.2, next page). You can consider this to be a snapshot in time of the major reservoirs in which carbon is stored today, the processes that exchange carbon between those reservoirs, and the rates at which that carbon is exchanged from one reservoir to another in the Earth system. The major carbon reservoirs in the Earth System are the Atmosphere, the Terrestrial Biosphere, Soil, the Ocean (including surface waters, deep waters, and the marine biosphere), and the Lithosphere (including carbon in sediments and rocks, and fossil fuels). 1. Using data from Figure 5.2 compare the relative size of each of the five major carbon reservoirs of the Earth system by listing the reservoirs in rank order from largest to smallest in Column 2 of Table 5.1 (below). From Fig. 5.2, the “Ocean” reservoir includes “dissolved organic carbon”, the “hydrosphere” (or deep ocean), the “surface waters”, and “marine organisms”. The “Lithosphere” reservoir includes “marine sediment and sedimentary rocks”, “sediments”, “oil and gas field”, and “coalfield”. 2. In what form(s) (e.g., organic carbon, CO2 gas, inorganic carbon in mineral compounds of rocks) is carbon stored in each of these reservoirs? Record your ideas in Column 3 of Table 5.1. Table 5.1. Carbon Cycle Reservoirs Column 1 Relative Size 1 (largest) Column 2 Reservoir Name 2 3 4 5 (smallest) Page 2 of 27 Column 3 Form(s) of Carbon in Reservoir Name ______________________________ Figure 5.2. A simplified diagram of the short term carbon cycle showing carbon reservoirs and transfers, expressed in Gigatonnes (1000 million tonnes) of carbon. The arrows are proportionate to the volume of carbon. The figures for the flows express amounts exchanged annually. Modified from UNEP/GRID-Arendal, http://maps.grida.no/go/graphic/the-carbon-cycle1. [need to modify by labeling the unlabeled 40-50 cycle in surface ocean.Or ask Lynn to draft a custom fig. Page 3 of 27 Name ______________________________ 3. Compare the natural (non-anthropogenic) processes and rates of carbon transfer from both land and ocean reservoirs to the atmosphere by examining Figure 5.2 and recording your observations in Table 5.2. List the processes of carbon transfer in Column 2, and categorize the rates of transfer [very fast (>1 yr), fast (1-10 yrs), slow (10-100 yrs), or very slow (>100 yrs)] in Column 3. Table 5.2. Comparison of the Rates of Exchange of Carbon from Land and Ocean Reservoirs to the Atmosphere Reservoir from to Terrestrial Biosphere to Atmosphere Process of Carbon Transfer Relative Rate of Transfer Soil to Atmosphere Ocean to Atmosphere Lithosphere to Atmosphere n/a because the processes are so slow not even on this diagram Another way to consider the rate at which each carbon reservoir is affected by these processes is to calculate the average amount of time that an atom of carbon spends in that reservoir. This is known as the residence time for that reservoir. For a reservoir in which the total amount of carbon is staying the same (or changing very slowly over time), the residence time can be calculated by dividing the total amount of carbon in that reservoir by the rate at which carbon is added or removed each year. 4. Using data from Fig. 5.2 complete Table 5.3 in order to estimate the residence times for carbon in the atmosphere, surface ocean, terrestrial biosphere, and soil due to natural processes only. For these calculations, use the rates at which carbon is added to the reservoir of interest each year. Page 4 of 27 Name ______________________________ Table 5.3 Residence Time of Carbon in Different Reservoirs Reservoir Addition Process Amount Total Total in Added/yr. Added/yr. Reservoir Residence Time (yrs.) Atmosphere Plant Decomposition Soilatmosphere exchange Oceanatmosphere exchange Surface OceanOcean atmosphere exchange Terrestrial Plant growth biosphere Soil Addition from vegetation (calculate this as the difference between “plant growth” and “plant decomposition”) 5. What are the primary anthropogenic processes shown in Figure 5.2 that affect the carbon cycle? Fossil fuel burning, deforestation [affect CO2 exchange with atm, and change in soil use] Agriculture: crops and livestock [affect CH4 exchange with atm, and change in soil use] 6. (a) Calculate the total rate of carbon transfer to the atmosphere from anthropogenic processes, and compare this to the rate of carbon transfer due to natural processes only. (b) If the rates of carbon transfer out of the atmosphere do not change, what effects will anthropogenic activity have on the total amount of carbon in the atmospheric reservoir? Page 5 of 27 Name ______________________________ CO2 as a Climate Regulator during the Phanerozoic and Today Part 2. CO2and Temperature The Greenhouse Effect The exchange of carbon to and from the atmosphere is a primary factor in regulating climate over timescales of years to millions of years. This is because two of the atmospheric greenhouse gases with the greatest influence on Earth’s surface temperature are carbon compounds: carbon dioxide (CO2) and methane (CH4). Other important greenhouse gases include stratospheric water vapor (H2Ov), nitrous oxide (N2O), ozone (O3), and halocarbons (e.g., chlorofluorocarbons, CFCs). 1. Read the box below on the Greenhouse Effect and think about the carbon cycle. If the rate of carbon transfer to the atmosphere is greater than the rate of carbon removal from the atmosphere, how would you expect Earth’s surface temperature to change? Explain. Page 6 of 27 Name ______________________________ The Greenhouse Effect Energy from the Sun is the primary driver of the Earth’s climate system. Energy travels through space as electromagnetic radiation, in the form of electromagnetic waves with a wide range of wavelengths. However, the heat energy that drives Earth’s climate system is only a small part of this broad electromagnetic spectrum; the important radiation for Earth is predominantly visible light, which has a relatively short wavelength. Because Earth has a temperature above absolute zero (-273° C, or 0K), Earth radiates heat to its surroundings. Because of the temperature of Earth, the energy released by Earth to its atmosphere (also known as Earth’s back radiation) has a longer wavelength than the incoming visible radiation from the Sun. This back radiation is also known as longwave radiation. Some gases absorb the longwave radiation emitted by Earth and reradiate that energy, providing additional warming to the atmosphere and Earth’s surface. Gases that behave this way are known as greenhouse gases, and exhibit this behavior because of the details of their composition and molecular structure, as well as their abundance in the atmosphere. The two most abundant gases in Earth’s atmosphere, nitrogen (N2) and oxygen (O2), do not behave as greenhouse gases. Several less abundant gases, however, are important greenhouse gases on Earth. The following table summarizes the estimated contribution from four individual gases to Earth’s total greenhouse gas effect: Gas Chemical Formula Abundance in Earth’s Atmosphere Greenhouse Effect Contribution (%) Water Vapor H2O <1% to 3% 36 – 72 % Carbon Dioxide CO2 ~0.038% 9 – 26 % Methane CH4 ~0.00018% 4–9% Ozone O3 ~0.00006% 3–7% (Data from Kiehl and Trenberth, 1997) Other important greenhouse gases include nitrous oxide (N2O) and halocarbons (e.g., chlorofluorocarbons, CFCs). Page 7 of 27 Name ______________________________ The Direct Effect of CO2 on Temperature: Understanding Radiative Forcing While it may be relatively simple to qualitatively understand the relationships between changing greenhouse gases levels and changing temperatures, quantifying these relationships is more challenging. Complications arise because there are both direct relationships and indirect relationships between each of the greenhouse gases and temperature. The indirect relationships are known as positive and negative Earth system feedbacks that amplify or diminish the temperature change, respectively. Determining the temperature sensitivity (also called “climate sensitivity”) refers to quantifying how much the temperature will change for a given change in radiative forcing (see box below), such as an increase (or decrease) of a particular greenhouse gas. Radiative Forcing (RF) Radiative forcing is a measure of the direct influence that a factor has in altering the balance of incoming and outgoing energy in the Earthatmosphere system. It is used to quantitatively rank the importance of climate change factors (Fig. 5.3). Radiative forcing does not take into account indirect influences on climate (i.e., Earth system feedbacks). The unit for radiative forcing is watts per square meter (W/m2), the same unit used to measure incoming solar radiation and Earth’s back radiation. Positive forcing increases surface temperature, whereas negative forcing cools it. The radiative forcing of CO2 is determined from atmospheric radiative transfer models. It is calculated by taking the natural logarithm of the change in CO2 concentration (ppm) and multiplying by a constant: RF = 5.35 * ln Present concentration of CO2 Initial concentration of CO2 (RF discussion adapted from Forster et al., 2007; RF calculation from http://www.esrl.noaa.gov/gmd/aggi/) Measurements of the trapped atmospheric gas in ice cores indicate that the 1750 (pre-industrial) concentration of atmospheric CO2 was 280 ppm. CO2 concentrations have been directly measured from the atmosphere since 1959 at the NOAA Mauna Loa Observatory in Hawaii; the 2005 concentration of CO2 was 378 ppm. Page 8 of 27 Name ______________________________ 2. Calculate the radiative forcing of CO2 on climate between 1750 and 2005 using the equation in the box above. Show your work. 5.35 ln (378/280) = 1.6 W/m2 3. How does your answer compare to the radiative forcing of CO2 between 1750 and 2005, as reported in Figure 5.3? Should be the same Figure 5.3. Radiative forcing of human and natural factors that affect climate change over a 255-yr window, from 1750 to 2005. Uncertainties are represented by the thin black lines. (from Forster et al., 2007). Page 9 of 27 Name ______________________________ 4. Which factor had the highest radiative forcing of climate between 1750 and 2005, based on the data in Figure 5.3? Atmospheric CO2 5. Based on your understanding of the carbon cycle from Part 1, why might the increase in CO2 be categorized under “human activities” and not “natural processes” for this 255-yr time period (Fig. 5.3)? This question gets at industrialization and scale (timeframe)… human factors affecting carbon cycle outpacing the natural factors for this time period. 6. By listing CO2 under the category of human activity (Fig. 5.3), does this mean that no carbon was transferred to or from the atmosphere during this time period by natural processes? Explain. No – natural carbon cycle still at work, but being outpaced by human activities 7. In climate models, temperature sensitivity is typically quantified on the basis of a doubling of atmospheric CO2 concentrations. What would the radiative forcing of CO2 be if the atmospheric level of CO2 rose to twice its preindustrial level? 5.35 ln (560/280) = 3.7 W/m2 Land Surface Albedo: A Different Story of Radiative Forcing While CO2 has a high radiative forcing, and is therefore an important factor in directly regulating atmospheric temperature, it is not the only factor. For example, land-surface albedo also affects atmospheric temperature. Albedo is a measure of the reflectivity of a surface. Lighter surfaces (e.g., fresh snow and ice) are more reflective (have a higher albedo) than darker surfaces (e.g., road pavement, dark soil, forests). The more reflective the surface, the less incoming solar radiation is absorbed by that surface. Model estimates indicate a decrease in the radiative forcing of the land surface albedo between 1750 and 2005 (Fig. 5.3). In other words, the land surface was absorbing less (and reflecting more) energy in 2005 than in 1750 (i.e., the land surface albedo increased). It is important to understand that changes in snow and ice cover due to Earth system feedback were not factored in this calculation, since such changes would be indirect, and radiative forcing is a measure of only direct affects on energy transfer in the Earth-atmosphere system. Page 10 of 27 Name ______________________________ 8. What direct changes in land use could cause land surface albedo to increase? If possible, provide examples of changing land use in your own region or state that might affect the reflectivity of the local land surface. Changes in snow and ice cover are excluded in most of the RF models used by the IPCC, because the RF is only for direct effects (not indirect effects from changes in snow and ice coverage). Changes in the distribution of agriculture, pasture land, and forests are the primary cause of the increase in land surface albedo, according to the IPCC (Forester et al., 2007). The albedo of crop land can be much higher than that of forested land. In 1750, 6-7% of the total land surface was used for agriculture or pastures. In 1990, this was elevated to 35-39% of the total land surface (Forester et al., 2007 and references therein). Urbanization would presumably also affect albedo, however this was not discussed directly in the IPCC report (although the heat island effect of cities was discussed; Forester et al., 2007). Humans may also have altered the reflective properties of snow and ice because of carbon soot deposits (pollution) on these white surfaces, and because of snow and ice melting. However, it is important to remember that ice melting can change the ice thickness (with little effect on surface albedo), as well as the geographic extent of the ice (affecting surface albedo). Indirect Effects of CO 2 on Temperature: Earth System Feedbacks In general, as the radiative forcing increases, Earth’s average temperature increases. However, the relationship between a change in radiative forcing and the corresponding change in temperature is not simple or straightforward. It is calculated within large-scale computer models of Earth’s atmosphere, called general circulation models or GCMs. In general, GCM calculations indicate that the direct effect of doubling CO2, and thereby increasing the radiative forcing 3.7 W/m2 (see question 7 above), is an increase in global average temperature of ~1.25ºC (Ruddiman, 2008; Rajmstorf 2008). This is not the only way that changing CO2 levels affect temperature. There are also indirect effects on other parts of the Earth system, which in turn either augment (positive feedback) or mute (negative feedback) the direct effect that CO2 has on temperature. These earth system feedbacks include: (a) the water vapor feedback, (b) the surface albedo feedback, and (c) the cloud feedback. To qualitatively determine how these Earth system feedbacks affect temperature, consider the following (Table 5.4): Page 11 of 27 Name ______________________________ Table 5.4 Earth System Feedback Factors Feedback Water Vapor • • Land Surface Albedo • • • • Clouds • • • • • Key Physical Relationships/Conditions A warmer atmosphere can store more water vapor than a colder atmosphere. A warmer atmosphere will evaporate more water from the ocean. Water vapor is a greenhouse gas. Albedo is a measure of the reflectivity of a surface. Lighter surfaces (e.g., fresh snow and ice) are more reflective (have a higher albedo) than darker surfaces (e.g., road pavement, dark soil, forests) The more reflective the surface, the less incoming solar radiation is absorbed by that surface. A warmer atmosphere evaporates more water from the ocean, thereby supplying more moisture to the atmosphere. A warmer atmosphere also can store more water vapor than a cold atmosphere, thereby reducing condensation. Both of these factors affect the abundance, type, and size of clouds. There are many different types of clouds. Cloud thickness increases with temperature. The upper surface of some clouds reflects incoming solar radiation; these have a high albedo. Other clouds are less reflective and therefore have only a moderate albedo. The lower surface of some clouds re-radiates long wavelength radiation from the Earth. 9. With the information from Table 5.4 in mind, complete the blanks in each of the following six feedback scenarios. In each case, predict how each of the feedback factors will change (will it increase or will it decrease?), and how the resulting temperature will change (will it increase or will it decrease?). For each scenario indicate whether it outlines a positive (P) or negative (N) feedback. If you are uncertain about how the system will respond, note that. (a) Atmospheric CO2 increases temperature increases water vapor __________ temperature __________. P or N? (b) Atmospheric CO2 increases temperature increase snow and ice __________ land surface albedo __________ temperature __________. P or N? Page 12 of 27 Name ______________________________ (c) Atmospheric CO2 increases temperature increases clouds __________ temperature __________. P or N? (d) Atmospheric CO2 decreases temperature decreases water vapor __________ temperature __________. P or N? (e) Atmospheric CO2 decreases temperature decreases snow and ice __________ land surface albedo __________ temperature __________. P or N? (f) Atmospheric CO2 decreases temperature decreases clouds __________ temperature __________. P or N? 10. Based on your responses to the six scenarios in Question 8, which of these feedbacks introduces the most uncertainty in predicting its ultimate effect on Earth’s climate? Why might that be? [Clouds] The Combined Effect (Direct and Indirect) of CO2 on Temperature In the same way that you’ve identified uncertainties in your qualitative predictions, each GCM identifies uncertainties and has its own way of dealing with those uncertainties in its calculations. As a result, different GCMs often predict different temperature changes for the same change in the concentration of atmospheric CO2. From this range of GCM results, the “average”, or mid-range, model projections for a doubling of CO2 are: • an increase of ~2.5ºC due to the water vapor feedback, • an increase of ~0.6ºC due to the land surface albedo feedback (including snow and ice albedo changes), and • a decrease of ~1.85ºC due to the cloud feedback (Ruddiman, 2008). 11. What is the projected combined effect of these three climate system feedbacks on temperature, given a doubling of CO2? 2.5 ºC + 0.6 ºC - 1.85 ºC = a warming of 1.25 ºC Page 13 of 27 Name ______________________________ 12. What is the projected total warming due to the direct radiative effect and the climate system feedbacks when CO2 is doubled? 1.25 ºC + 1.25 ºC = 2.5 ºC Page 14 of 27 Name ______________________________ CO2 as a Climate Regulator during the Phanerozoic and Today Part 3. Recent Changes in CO2 Consider the atmospheric CO2 data from air samples collected at Mauna Loa Observatory for the period 1958-2010 (Fig 5.4). Figure 5.4. Atmospheric CO2 concentrations measured at Mauna Loa Observatory, Hawaii (From: Dr. Pieter Tans, NOAA/ESRL (www.esrl.noaa.gov/gmd/ccgg/trends/). 1. Is this carbon dioxide record a direct measure of carbon dioxide concentrations in our atmosphere, or is it a proxy (indirect indicator) of carbon dioxide concentrations? 2. What do you think the solid black curve represents? 3. What do you think the red saw-tooth curve represents? Page 15 of 27 Name ______________________________ 4. How many peaks (or “teeth”) are present in a 5 year – interval on the saw-toothed curve? Based on your observation, what is the length of time between adjacent peaks on the saw-toothed curve? 5. Speculate on the process controlling the saw-toothed nature of the red curve. 6. What reservoirs from the global carbon cycle (Fig 5.2) and what form(s) of carbon (organic or inorganic) might be involved in producing the saw-toothed pattern plotted in Figure 5.4? 7. Go to the web link listed in the caption for Figure 5.4. What is the CO2 concentration today? Today’s Date ___________________ CO2 concentration __________(ppm) 8. From Part 5.2, recall that climate models often focus on predicting when the atmospheric CO2 concentration will have doubled, and what the resulting temperatures will be at that time. The baseline for these models is the 1750 (or pre-industrial) level of 280 ppm CO2. Are the CO2 levels today double the pre-industrial concentration? No, not yet Now consider the atmospheric CO2 data for the last 10,000 years, measured from bubbles of trapped gas in ice cores (Fig. 5.5, below). 9. What were the atmospheric CO2 concentrations 10,000 years ago? Page 16 of 27 Name ______________________________ 10. Describe the trend of CO2 over the last 10,000 years. In particular, identify a time when the slope of the CO2 trend changed noticeably. 10000 5000 Time (before 2005) 0 Figure 5.5. Atmospheric CO2 concentrations over the last 10,000 years (large panel) and since 1750 (inset panel). Measurements are shown from ice cores (symbols with different colors are from different studies) and atmospheric samples (red lines). The corresponding radiative forcing is shown on the right axis of the large panels (modified from IPCC, 2007). 11. Speculate on the transfer processes within the carbon cycle (Fig. 5.2) that controlled the CO2 trend before and after this change in slope. Page 17 of 27 Name ______________________________ 12. What reservoirs in the global carbon cycle (Fig 5.2) and what form(s) of carbon (organic or inorganic) might have been important for producing the older part of the record shown in Figure 5.5? What additional reservoirs and forms of carbon might have been important for producing the younger part of the record? Reservoirs Older part of record Younger part of record Page 18 of 27 Form(s) of Carbon Name ______________________________ CO2 as a Climate Regulator during the Phanerozoic and Today Part 4. The Long-term Global Carbon Cycle, CO2, and Phanerozoic Climate History The Long-Term Carbon Cycle As you learned in Part 5.1, carbon reservoirs include the lithosphere, the atmosphere, the ocean, and the biosphere. Carbon is transferred to and from the atmosphere over short time scales (Fig. 5.2), and also over long time scales (Fig 5.6, below). Figure 5.6. A simplified diagram of the long-term carbon cycle. (From Berner, 1999). 1. There are over 66,000,000 gigatons of carbon stored in the lithosphere (Fig. 5.2). What are the long-term processes (Fig. 5.6) that transfer this geologically-stored carbon to the atmosphere? 2. The rate at which each process moves carbon from one reservoir to another can change over geologic (i.e., “long”) time scales. How could changing rates in seafloor spreading affect the rate of CO2 transfer to the atmosphere? Page 19 of 27 Name ______________________________ 3. Examine the age-distribution map of the seafloor (Figure 5.7). Is there any evidence that the rate of seafloor spreading has varied in the past 180 million years? Explain. [Distance/Age = Rate of Spreading. While it is a bit of a “back of the envelope calculation”, one can measure the width of the green and the width of the dark red to one side of the ridge (e.g., the North Atlantic is a good place), and calculate the spreading rates. The rate during the Cretaceous (green; from 131-67.7 Ma) was ~2X the rate today (dark red, 9.7-0 Ma)] Figure 5.7. Seafloor age in millions of years. Age determined from the Paleomagnetic reveral pattern in basalts (see Chapter 3). From http://pubs.usgs.gov/of/1999/ofr-990132/. Originally made by R.D.Mueller, W.R. Roest, J.-Y. Royer, L.M. Gahagan , and J.G. Sclater. 4. The natural transfer of carbon from the lithosphere to the atmosphere typically takes hundreds of thousands to millions of years (although there are important exceptions; see Chapter 9). How have humans accelerated the transfer of geologically-stored carbon to the atmosphere? [student answers should demonstrate what they learned from earlier parts in this exercise; burning organic carbon (fossil fuels) is of primary importance] Page 20 of 27 Name ______________________________ CO2 and Phanerozoic Climate History The importance of CO2 in Earth’s recent climate history was demonstrated in Parts 5.1-5.3. Here you will examine the results of a model that reconstructs the CO2 contents of Earth’s older atmosphere, as well as proxy data (Fig. 5.8), for the Phanerozoic Eon, which encompasses the last 540 million years of Earth’s history (see geologic time scale in appendix). The goal of this examination is to evaluate the importance of CO2 as a regulator of long-term climate change. Figure 5.8. (A) Paleoatmospheric CO2 proxy and model data for the Phanerozoic; (B) Average value of the combined CO2 proxy data (black line). Standard deviation of the combined data is shown in grey, and represents the uncertainty of the data; (C) Frequency distribution of the CO2 proxy data. (From Royer et al., 2004) Page 21 of 27 Name ______________________________ The paleo-atmospheric CO2 proxy data (Fig. 5.8) come largely from geochemical and paleontological sources (Royer et al., 2004, and references therein). These include the stable carbon-isotope records (δ13C) of minerals in fossil soils (paleosols) and of phytoplankton; the stable boron isotope record (δ11B) from planktonic foraminifera; and the changes in characteristics of fossil plant leaves (stomatal density). The GEOCARB III model also supplies data on the long-term CO2 record. This model is based on estimates of the rate of transfer of carbon by processes in the long-term carbon cycle (Fig. 5.6). 5. For what time period would you be most confident of the CO2 paleoatmospheric proxy data and why? [recent – most proxies, highest sampling frequency, and relatively low standard deviation] 6. How do the records from the different independent paleo-atmospheric CO2 proxies compare to the results from the paleo-atmospheric CO2 geochemical model? [they correlate fairly well, exception is ~170 myr] 7. Estimate the percent of Earth history that had a higher CO2 level than today? Direct evidence for Phanerozoic climate change comes from the rock record. The temporal and spatial distributions of glacial features (e.g., striations) and deposits (tills) can be used to constrain the timing and geographic extent of glaciations of the past (Fig. 5.9). 8. Compare the Phanerozoic records of paleo-atmospheric CO2 and glaciation. Is there any correlation between the two records? Explain. [yes, good correlation: low (<500 ppm) CO2 during periods of long-lived and widespread continental glaciation, and high (>1000 ppm) CO2 during warm periods. However, older glaciations (430 and 520 Ma) do seem to occur when models estimated atmospheric CO2 to have been high. There is little to no CO2 proxy data for older record for which to compare the model results. 9. According to the data in Figure 5.9, what is the maximum atmospheric CO2 level at which widespread ice sheets can exist (i.e., for ice sheets to extend equatorward of 60°)? Page 22 of 27 Name ______________________________ [Make the point (from Royer) that there is a strong CO2-temperature relationship for much of the Phanerozoic. Perhaps 500 ppm is a threshold that must be passed (i.e., CO2 must be lower than 500 ppmv) for large ice sheets to exist.] Figure 5.9. (A) Paleo-atmospheric CO2 from proxies (black line) and the GEOCARB III model (red line), model uncertainty represented by the orange shading, (B) time periods of glaciation (dark blue) or cool climates (light blue), and (C) the latitudinal distribution of glaciation (From Royer et al., 2004). Fix 5.9B to show thin lines for the two oldest ice times Page 23 of 27 Name ______________________________ Greenhouse and Icehouse Times Through approximately the last 2 billion years, Earth’s climate has fluctuated – at timescales of 10s of millions of years to ~100 million years – between relatively cold times, with low atmospheric CO2 levels, relatively cool temperatures, and long-lived large ice sheets, and relatively warm times, with high atmospheric CO2 levels, relatively warm temperatures, and no long-lived large ice sheets. The warm episodes are described as Greenhouse Times, whereas the cold episodes are described as Icehouse Times. The occurrences of Greenhouse Times and Icehouse Times in Earth history appear to have been triggered by one or more of several factors: changes in the latitudinal positions of large continents; opening or closing of oceanic gateways, which affect ocean circulation patterns; and changes in the global carbon cycle, such as by the uplift and chemical weathering of mountain ranges, by changes in the rates of seafloor spreading or other volcanic activity, or by changes in biological cycling of carbon. 10. Read the box above. Was the Cretaceous a greenhouse time or an icehouse time? Greenhouse 11. Do we currently live during a greenhouse time or an icehouse time? Icehouse 12. How can you reconcile your answer to question 11, with the documented global warming that is occurring today (e.g., Part 3)? [Gets at the issue of scale – yes it is getting warmer now, but we are still within an icehouse world.] Page 24 of 27 Name ______________________________ Figure 5.10. Atmospheric CO2 concentrations (parts per million vapor) observed at Mauna Loa (black dashed line, for 1958-2008), and 6 model-projections of atmospheric CO2 concentration (colored lines, for 2008-2100). The colored solid vs. colored dashed lines represent two different carbon cycle models used in each scenario. (From http://www.ipccdata.org/ddc_co2.html). 13. Examine Figure 5.10. What is the range of model-projected atmospheric CO2 concentrations by the year 2100? 550 ppmv to 950 ppmv 14. Compare the data in Figure 5.10 to that in Figure 5.9. In the past, has the Earth ever experienced the atmospheric CO2 levels that are predicted to be reached in the next 100 years? Yes…several times… 15. The data from the field of anthropology indicate that our species (Homo sapiens) evolved ~200,000 yrs ago. Examine Figure 5.11 (below). In the past 200,000 years, have humans ever experienced the atmospheric CO2 levels that are predicted to be reached in the next 100 years? No [add note to instructor that Fig 5.11 displays glacial-interglacial cycles in CO2 and temp, and although this pattern and timescale of change in CO2 and temperature is not a focus of this chapter, it is addressed in Chapter 8.] Page 25 of 27 Name ______________________________ Figure 5.11. Inferred temperature (red line) and measured atmospheric CO2 (yellow) for the past 649,000 years from Antarctic ice cores. The vertical bar represents the increase in atmospheric CO2 over the past two centuries, measured in ice cores and directly at Mauna Loa (From http://www.epa.gov/climatechange/science/pastcc_fig1.html). 16. Based on your answer to Question 15, how would you compare the global average temperature that humans are likely to face in the next 100 years to the global average temperatures faced by humans during the past 200,000 years? Explain your answer. 17. What effects do you think these projected temperatures are likely to have on human populations and societies in the next 100 years? (These may include both direct effects and indirect effects.) Page 26 of 27 Name ______________________________ References Berner, R.A., 1999, A new look at the long-term carbon cycle: GSA Today, v. 9, p. 2-6. IPCC, 2007, Summary for Policymakers, in Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., and Miller, H.L., eds., Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Forster, P., Ramaswamy, V., Artaxo, P., Berntsen, T., Betts, R., Fahey, D.W., Haywood, J., Lean, J., Lowe, D.C., Myhre, G., Nganga, J., Prinn, R., Raga, G., Schulz, M., and Van Dorland, R., 2007, Changes in Atmospheric Constituents and in Radiative Forcing, in Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., and Miller, H.L., eds., Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Kiehl, J. T.; Kevin E. Trenberth, 1997, "Earth’s Annual Global Mean Energy Budget". Bulletin of the American Meteorological Society, v. 78 (2), p. 197–208. Rahmstorf, S., 2008, Anthropogenic Climate Change: Revisiting the Facts, in Zedillo, E., ed., Global Warming: Looking Beyond Kyoto, Brookings Institution Press, p. 34–53. http://www.pikpotsdam.de/~stefan/Publications/Book_chapters/Rahmstorf_Zedillo_2 008.pdf. Royer, D.L., Berner, R.A., Montanz, I.P., Tabor, N.J., and Beerling, D.J., 2004, CO2 as a primary driver of Phanerozoic climate, GSA Today, v. 14, p. 4-10. Ruddiman, 2008, Earth’s Climate Past and Future, W.H. Freeman and Co., New York, NY, USA, 388 p. Page 27 of 27