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Mark Weber
GEOG. 500, Fall 2015
Research Paper Scoping Document
I will be researching the changes global warming will have on the weather and rainfall in regards
to the longitude aspect around the world. What changes are to be expected in strength of storms and
droughts and if they will have any trend in moving North or South from their average locations. Also go
over the changes in rainfall, discussing why dry regions will get dryer and wider, while wet regions will
get wetter and thinner.
Climate Science Basics: Why Will Dry Areas Get Drier & Wet Areas Get Wetter
http://www.treehugger.com/climate-change/climate-science-basics-why-will-dry-areas-get-drierwet-areas-get-wetter-video.html By Mat McDermott (@matmcdermott)
Science / Climate Change, September 6, 2012
The wet gets wetter, the dry dryer, thanks to climate change
http://www.smh.com.au/national/the-wet-gets-wetter-the-dry-dryer-thanks-to-climatechange-20120426-1xo0f.html, By Deborah Smith, April 27, 2012
In a Warming World, Storms May Be Fewer but Stronger
http://earthobservatory.nasa.gov/Features/ClimateStorms/ By Adam Voiland Design by Robert
Simmon, March 5, 2013
What is the link between hurricanes and global warming?
http://www.skepticalscience.com/hurricanes-global-warming-intermediate.htm By John Cook,
2009.
Global Warming and Hurricanes
http://www.gfdl.noaa.gov/global-warming-and-hurricanes Geophysical Fluid Dynamics
Laboratory/NOAA, Revised Sept. 30, 2015
An Overview of Current Research Results
1. Has Global Warming Affected Hurricane or Tropical Cyclone Activity?
A. Summary Statement
Two frequently asked questions on global warming and hurricanes are the following:


Have humans already caused a detectable increase in Atlantic hurricane activity or global
tropical cyclone activity?
What changes in hurricane activity are expected for the late 21st century, given the pronounced
global warming scenarios from current IPCC models?
In this review, we address these questions in the context of published research findings. We will first present the
main conclusions and then follow with some background discussion of the research that leads to these conclusions.
The main conclusions are:




It is premature to conclude that human activities--and particularly greenhouse gas emissions
that cause global warming--have already had a detectable impact on Atlantic hurricane or
global tropical cyclone activity. That said, human activities may have already caused changes
that are not yet detectable due to the small magnitude of the changes or observational
limitations, or are model-estimated changes with considerable uncertainty (e.g., aerosol
effects).
Anthropogenic warming by the end of the 21st century will likely cause tropical cyclones
globally to be more intense on average (by 2 to 11% according to model projections for an
IPCC A1B scenario). This change would imply an even larger percentage increase in the
destructive potential per storm, assuming no reduction in storm size.
There are better than even odds that anthropogenic warming over the next century will lead
to an increase in the occurrence of very intense tropical cyclone in some basins—an increase
that would be substantially larger in percentage terms than the 2-11% increase in the average
storm intensity. This increase in intense storm occurrence is projected despite a likely
decrease (or little change) in the global numbers of all tropical cyclones.
Anthropogenic warming by the end of the 21st century will likely cause tropical cyclones to
have substantially higher rainfall rates than present-day ones, with a model-projected
increase of about 10-15% for rainfall rates averaged within about 100 km of the storm center.
B. Statistical relationships between SSTs and Atlantic hurricanes
Likelihood Statements
The terminology here for likelihood statements generally follows the conventions used in the IPCC AR4, i.e., for the
assessed likelihood of an outcome or result:



Very Likely: > 90%,
Likely: > 66%
More Likely Than Not (or Better Than Even Odds) > 50%
Observed records of Atlantic hurricane activity show a correlation, on multi-year time-scales, between local tropical
Atlantic sea surface temperatures (SSTs) and the Power Dissipation Index (PDI) --see for example Fig. 3 on this
EPA Climate Indicators site. PDI is an aggregate measure of Atlantic hurricane activity, combining frequency,
intensity, and duration of hurricanes in a single index. Both Atlantic SSTs and PDI have risen sharply since the
1970s, and there is some evidence that PDI levels in recent years are higher than in the previous active Atlantic
hurricane era in the 1950s and 60s.
Model-based climate change detection/attribution studies have linked increasing tropical Atlantic SSTs to increasing
greenhouse gases, but the link between increasing greenhouse gases and hurricane PDI or frequency has been based
on statistical correlations. The statistical linkage of Atlantic hurricane PDI to and Atlantic SST suggests at least the
possibility of a large anthropogenic influence on Atlantic hurricanes. If the correlation between tropical Atlantic
SSTs and hurricane activity is used to infer future changes in Atlantic hurricane activity, the implications are
sobering: the large increases in tropical Atlantic SSTs projected for the late 21st century would imply very
substantial increases in hurricane destructive potential--roughly a 300% increase in the PDI by 2100 (Figure 1 a).
Figure 1. Two different statistical models and projections of Atlantic hurricane activity. Adapted from Vecchi et al.
2008). (more)
On the other hand, Swanson (2008) and others have noted that Atlantic hurricane power dissipation is also wellcorrelated with other SST indices besides tropical Atlantic SST alone, and in particular with indices of Atlantic SST
relative to tropical mean SST (e.g., Figure 1b from Vecchi et al. 2008). This is in fact a crucial distinction, because
the statistical relationship between Atlantic hurricanes and local Atlantic SST shown in the upper panel of Figure 1
would imply a very large increases in Atlantic hurricane activity (PDI) due to 21st century greenhouse warming,
while the statistical relationship between the PDI and the alternative relative SST measure shown in the lower panel
of Figure 1 would imply only modest changes of Atlantic hurricane activity (PDI) with greenhouse warming. In the
latter case, the alternative relative SST measure in the lower panel does not change very much over the 21st century
in global warming projections from climate models, because the warming projected for the tropical Atlantic in the
models is not very different from that projected for the tropics as a whole.
A key question then is: Which of the two future Atlantic hurricane scenarios inferred from the statistical
relations in Figure 1 is more likely? To try to gain insight on this question, we have first attempted to go beyond
the ~50 year historical record of Atlantic hurricanes and SST to examine even longer records of Atlantic tropical
storm activity and second to examine dynamical models of Atlantic hurricane activity under global warming
conditions. These separate approaches are discussed below.
C. Analysis of century-scale Atlantic tropical storm and hurricane records
Figure 2 (more)
To gain more insight on this problem, we have attempted to analyze much longer (> 100 yr) records of Atlantic
hurricane activity. If greenhouse warming causes a substantial increase in Atlantic hurricane activity, then the
century scale increase in tropical Atlantic SSTs since the late 1800s should have produced a long-term rise in
measures of Atlantic hurricanes activity.
Existing records of past Atlantic tropical storm or hurricane numbers (1878 to present) in fact do show a pronounced
upward trend, which is also correlated with rising SSTs (e.g., see blue curve in Fig. 4 or Vecchi and Knutson 2008).
However, the density of reporting ship traffic over the Atlantic was relatively sparse during the early decades of this
record, such that if storms from the modern era (post 1965) had hypothetically occurred during those earlier decades,
a substantial number would likely not have been directly observed by the ship-based "observing network of
opportunity." We find that, after adjusting for such an estimated number of missing storms, there is a small
nominally positive upward trend in tropical storm occurrence from 1878-2006. But statistical tests reveal that this
trend is so small, relative to the variability in the series, that it is not significantly distinguishable from zero
(Figure 2). In addition, Landsea et al. (2010) note that the rising trend in Atlantic tropical storm counts is almost
entirely due to increases in short-duration (<2 day) storms alone. Such short-lived storms were particularly likely to
have been overlooked in the earlier parts of the record, as they would have had less opportunity for chance
encounters with ship traffic.
Figure 3 (more)
Figure 4 (more)
If we instead consider Atlantic basin hurricanes, rather than all Atlantic tropical storms, the result is similar: the
reported numbers of hurricanes were sufficiently high during the 1860s-1880s that again there is no significant
positive trend in numbers beginning from that era (Figure 3, black curve, from CCSP 3.3 (2008) ). This is without
any adjustment for "missing hurricanes".
The evidence for an upward trend is even weaker if we look at U.S. landfalling hurricanes, which even show a slight
negative trend beginning from 1900 or from the late 1800s (Figure 3, blue curve). Hurricane landfalling frequency is
much less common than basin-wide occurrence, meaning that the U.S. landfalling hurricane record, while more
reliable than the basin-wide record, suffers from degraded signal-to-noise characteristics for assessing trends.
While major hurricanes (Figure 3, red curve) show more evidence of a rising trend from the late 1800s, the major
hurricane data are considered even less reliable than the other two records in the early parts of the record. Category
4-5 hurricanes show a pronounced increase since the mid-1940s (Bender et al., 2010) but again, we consider that
these data need to be carefully assessed for data inhomogeneity problems before such trends can be accepted as
reliable.
The situation for Atlantic hurricane long-term records is summarized in Figure 4. While global mean temperature
and tropical Atlantic SSTs show pronounced and statistically significant warming trends (green curves), the U.S.
landfalling hurricane record (orange curve) shows no significant increase or decrease. The unadjusted hurricane
count record (blue curve) shows a significant increase in Atlantic hurricanes since the early 1900s. However, when
adjusted with an estimate of storms that stayed at sea and were likely “missed” in the pre-satellite era, there is no
significant increase in Atlantic hurricanes since the late 1800s (red curve). While there have been increases in U.S.
landfalling hurricanes and basin-wide hurricane counts since the since the early 1970s, Figure 5 shows that these
increases are not representative of the behavior seen in the century long records. In short, the historical Atlantic
hurricane record does not provide compelling evidence for a substantial greenhouse warming induced longterm increase.
D. Model simulations of greenhouse warming influence on Atlantic hurricanes
Direct model simulations of hurricane activity under climate change scenarios offer another perspective on the
problem. We have developed a regional dynamical downscaling model for Atlantic hurricanes and tested it by
comparing with observed hurricane activity since 1980. This model, when forced with observed sea surface
temperatures and atmospheric conditions, can reproduce the observed rise in hurricane counts between 1980 and
2012, along with much of the interannual variability (Figure 5). Animations showing the development and evolution
of hurricane activity in the model are available here.
Figure 5 (more)
Turning to future climate projections, current climate models suggest that tropical Atlantic SSTs will warm
dramatically during the 21st century, and that upper tropospheric temperatures will warm even more than SSTs.
Furthermore, most of the models project increasing levels of vertical wind shear over parts of the western tropical
Atlantic (see Vecchi and Soden 2007). Both the increased warming of the upper troposphere relative to the surface
and the increased vertical wind shear are detrimental factors for hurricane development and intensification, while
warmer SSTs favor development and intensitification. To explore which effect of these effects might "win out", we
can run experiments with our regional downscaling model.
Our regional model projects that Atlantic hurricane and tropical storms are substantially reduced in number, for
the average 21st century climate change projected by current models, but have higher rainfall rates, particularly
near the storm center. The average intensity of the storms that do occur increases by a few percent (Figure 6), in
general agreement with previous studies using other relatively high resolution models, as well as with hurricane
potential intensity theory (Emanuel 1987).
Figure 6 (more)
Earlier, Knutson and Tuleya (2004) estimated the rough order of magnitude of the hurricane sensitivity to be about
4% per deg C SST warming for maximum intensities and about 12% per deg C for near-storm (100 km radius)
rainfall rates (see also Knutson and Tuleya (2008) abstract here). These sensitivity estimates have considerable
uncertainty, as CCSP 3.3 (2008) , gives an estimated range of 1-8% per deg C SST warming for hurricane intensity,
and 6-18% per deg C for near-storm rainfall rates.
A review of existing studies, including the ones cited above, lead us to conclude that it is likely that greenhouse
warming will cause hurricanes in the coming century to be more intense globally and have higher rainfall
rates than present-day hurricanes.
Turning now to the important question of the frequency of very intense hurricanes, the regional model of Knutson et
al. (2008) has an important limitation in that it does not simulate such very intense hurricanes. For example, the
maximum surface wind in the simulated hurricanes from that model is less than 50 m/s (which is borderline category
3 hurricane intensity). Furthermore, the idealized study of Knutson and Tuleya (2004) assumed the existence of
hurricanes and then simulated how intense they would become. Thus, that study could not address the important
question of the frequency of intense hurricanes.
In our latest Atlantic basin dynamical downscaling studies (Bender et al. 2010; Knutson et al. 2013), we have tried
to address both of these limitations by letting the Atlantic basin regional model of Knutson et al. (2008) provide the
overall storm frequency information, and then downscaling each individual storm from the regional model study
into the GFDL hurricane prediction system. The GFDL hurricane model (with a grid spacing as fine as 9 km) is able
to simulate the frequency, intensity, and structure of the more intense hurricanes, such as category 3-5 storms, much
more realistically than the regional (18 km grid) model.
Using this additional downscaling step, the new GFDL hurricane model study is able to reproduce some important
historical characteristics of very intense Atlantic hurricanes, including the wind speed distribution and the change of
this distribution between active and inactive decadal periods of hurricane activity (Fig. 1 of Bender et al. 2010). The
model also supports the notion of a substantial decrease (~25%) in the overall number of Atlantic hurricanes and
tropical storms with projected 21st century climate warming. However, using the CMIP3 and CMIP5 multi-model
climate projections, the hurricane model also projects that the lifetime maximum intensity of Atlantic
hurricanes will increase by about 5% during the 21st century in general agreement with previous studies. The
hurricane model further projects a significant increase (+90%) in the frequency of very intense (category 4 and 5)
hurricanes using the CMIP3/A1B 18-model average climate change projection (Fig. 7). Downscaled projections
using CMIP5 multi-model scenarios (RCP4.5) as input (Knutson et al. 2013) still show increases in category 4 and 5
storm frequency, but these are only marginally significant for the early 21st century (+45%) or the late 21st century
(+40%) CMIP5 scenarios. Downscaling individual CMIP3 model projections instead of the multi-model ensemble,
we find that three of ten models produced a significant increase in category 4 and 5 storms, while the other seven
produced no significant change. While multi-model ensemble results are probably more reliable than individual
model results, each of the individual model results can be viewed as at least plausible at this time.
Figure 7 (more)
Returning to the issue of future projections of aggregate activity (PDI, as in Fig. 1), while there remains a lack of
consensus among various studies on how Atlantic hurricane PDI will change, no model we have analyzed shows a
sensitivity of Atlantic hurricane PDI to greenhouse warming as large as that implied by the observed Atlantic
PDI/local SST relationship shown in Figures 1 (top panel). In other words, there is little evidence from current
dynamical models that 21st century climate warming will lead to large (~300%) increases in tropical storm
numbers, hurricane numbers, or PDI in the Atlantic. As noted above, there is some indication from high
resolution models of substantial increases in the numbers of the most intense hurricanes even if the overall number
of tropical storms or hurricanes decreases. In the Bender et al. 2010 study, we estimate that the effect of increasing
category 4-5 storms outweighs the reduction in overall hurricane numbers such that we project (very roughly) a
30% increase in potential damage in the Atlantic basin by 2100. This estimate does not include the influence of
future sea level rise or other important factors such as coastal development or changes in building practices.
Finally, one can ask whether the change in Category 4-5 hurricanes projected by our model is already detectable in
the Atlantic hurricane records. Owing to the large interannual to decadal variability of SST and hurricane activity in
the basin, Bender et al (2010) estimate that detection of this projected anthropogenic influence on hurricanes
should not be expected for a number of decades. While there is a large rising trend since the mid 1940's in
category 4-5 numbers in the Atlantic, our view is that these data are not reliable for trend calculations, until they
have been further assessed for data homogeneity problems, such as those due to changing observing practices.
E. Other possible human influences on Atlantic hurricane climate
Apart from greenhouse warming, other human influences conceivably could have contributed to recent observed
increases in Atlantic hurricanes. For example, Mann and Emanuel (2006) hypothesize that a reduction in aerosolinduced cooling over the Atlantic in recent decades may have contributed to the enhanced warming of the tropical
North Atlantic, relative to global mean temperature. However, the cause or causes of the recent enhanced warming
of the Atlantic, relative to other tropical basins, and its effect on Atlantic tropical cyclones, remains highly uncertain
(e.g., Booth et al. 2012; Zhang et al. 2013; Dunstone et al. 2013; Villarini and Vecchi 2013). A number of
anthropogenic and natural factors (e.g., aerosols, greenhouse gases, volcanic activity, solar variability, and internal
climate variability) must be considered as potential contributors, and the science remains highly uncertain in these
areas. Finally recent work (Kossin et al. 2014; see GFDL Research Highlight) indicates that the latitude at which
the maximum intensity of tropical cyclones occurs has expanded poleward globally in recent decades, although
the causes for this have not been firmly established and a significant change was not seen in the Atlantic basin
statistics.
Sea level rise must also be considered as a way in which human-caused climate change can impact Atlantic
hurricane climate--or at least the impacts of the hurricanes at the coast. The vulnerability of coastal regions to stormsurge flooding is expected to increase with future sea-level rise and coastal development, although this vulnerability
will also depend upon future storm characteristics, as discussed above. There are large ranges in the 21st Century
projections for both Atlantic hurricane characteristics and for the magnitude of regional sea level rise along the U.S.
coastlines. However, according to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change,
the average rate of global sea level rise over the 21st Century will very likely exceed that observed during 19612003 for a range of future emission scenarios.
F. Synthesis and Summary for Atlantic Hurricanes and Global Warming
In summary, neither our model projections for the 21st century nor our analyses of trends in Atlantic hurricane and
tropical storm counts over the past 120+ yr support the notion that greenhouse gas-induced warming leads to large
increases in either tropical storm or overall hurricane numbers in the Atlantic. A new modeling study projects a large
(~100%) increase in Atlantic category 4-5 hurricanes over the 21st century, but we estimate that this increase may
not be detectable until the latter half of the century.
Therefore, we conclude that despite statistical correlations between SST and Atlantic hurricane activity in recent
decades, it is premature to conclude that human activity--and particularly greenhouse warming--has already caused a
detectable change in Atlantic hurricane activity. ("Detectable" here means the change is large enough to be
distinguishable from the variability due to natural causes.) However, human activity may have already caused some
some changes that are not yet detectable due to the small magnitude of the changes or observation limitations, or are
not yet properly modeled (e.g., aerosol effects on regional climate).
We also conclude that it is likely that climate warming will cause hurricanes in the coming century to be more
intense globally and to have higher rainfall rates than present-day hurricanes. In our view, there are better than even
odds that the numbers of very intense (category 4 and 5) hurricanes will increase by a substantial fraction in some
basins, while it is likely that the annual number of tropical storms globally will either decrease or remain essentially
unchanged. These assessment statements are intended to apply to climate warming of the type projected for the 21st
century by IPCC AR4 scenarios, such as A1B.
The relatively conservative confidence levels attached to these projections, and the lack of a claim of detectable
anthropogenic influence at this time contrasts with the situation for other climate metrics, such as global mean
temperature. In the case of global mean surface temperature, the IPCC 5th Assessment Report (2013) presents a
strong body of scientific evidence that most of the global warming observed over the past half century is very likely
due to human-caused greenhouse gas emissions.
2. Global Warming and Tropical Cyclone Activity Around the Globe
Figure 8 (more)
The main focus of this web page is on Atlantic hurricane activity and global warming. However, an important
question concerns whether global warming has or will substantially affect tropical cyclone activity in other basins.
In terms of historical tropical cyclone activity, a 2010 WMO assessment of tropical cyclones and climate change
concluded that "it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability
expected from natural causes." This conclusion applied to all basins around the globe.
For future projections, GFDL atmospheric modelers have developed global models capable of simulating many
aspects of the seasonal and year-to-year variability of tropical cyclone frequency in a number of basins, using only
historical sea surface temperatures as input. Examples of the performance of this model on historical data are
provided on this web page.
Our latest study examines the impact of 21st-century projected climate changes (CMIP5, RCP4.5 scenario) on a
number of tropical cyclone metrics, using the GFDL hurricane model to downscale storms from a lower resolution
global atmospheric model. Key findings from these experiments include: fewer tropical cyclones globally in a
warmer late-twenty-first-century climate (Figure 8), but also an increase in average cyclone intensity, the number
and occurrence days of very intense category 4 and 5 storms (Figure 9) and in tropical cyclone precipitation rates
(Figure 10). These global changes are similar to the consensus findings from a review of earlier studies in the 2010
WMO assessment. These changes do not necessarily occur in all basins. For example, there is a projected increase in
tropical storm frequency in the Northeast Pacific and near Hawaii, and a projected decreases in category 4-5 storm
days over much of the southern hemisphere basins and parts of the northwest Pacific basin. These differences in
responses between basins seem to be linked to how much SSTs increase in a given region compared to the tropical
mean increase in SST. Basins that warm more than the tropical average tend to show larger increases in tropical
cyclone activity for a number of metrics. Our simulations show little projected change in the median size of tropical
cyclone projected globally; the model framework does show some skill at simulating the differences in average
storm size between various basins in the present-day climate.
Figure 9 (more)
Figure 10 (more)
http://www.treehugger.com/clean-technology/global-warmingatms-effect-on-precipitation-patternscould-mean-even-bigger-change-in-groundwater.html
Global Warming's Effect on Precipitation Patterns Could
Mean Even Bigger Change in Groundwater
Mat McDermott (@matmcdermott)
Technology / Clean Technology
December 18, 2008
We’ve all heard by now that global warming will bring about changes in precipitation patterns, with some areas
seeing increases with other areas drying out. A new piece of research from MIT delves into this area and finds that
the changes in groundwater levels could be much greater than the changes in precipitation itself.
While the researchers acknowledge that a wide array of factors will influence the effect at a given location, this the
broad stroke: 20% Change in Rainfall = Double or More Change in Groundwater
In areas where rainfall increases 20% groundwater levels might actually rise 40%; while in areas seeing a similar
decrease in rainfall there could be as much as a 70% decrease in groundwater.
But It’s Not That Simple
In describing the types of variables which could influence the effect at a given location, Science Codex summed it
up: Among the most important factors, the team found, is the timing and duration of the precipitation. For example,
it makes a big difference whether it comes in a few large rainstorms or many smaller ones, and whether most of the
rainfall occurs in winter or summer.
If most of the rain falls while plants are growing, much of the water may be absorbed by the vegetation and released
back into the atmosphere through transpiration, so very little percolates down to the aquifer. Similarly, it makes a
big difference whether an overall increase in rainfall comes in the form of harder rainfalls, or more frequent small
rainfalls. More frequent small rainstorms may be mostly soaked up by plants, whereas a few more intense events
may be more likely to saturate the soil and increase the recharging effect.