1.Study of Hydro Logical Cycle

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WASHINGTON ROUNDTABLEON SCIENCE & PUBLIC POLICY

Global Warming and the

Hydrologic Cycle: How arethe Occurrence of Floods,

Droughts, and Storms Likely

to Change?

By David Legates

Washington, D.C.


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The George C. Marshall InstituteThe George C. Marshall Institute, a nonprofit research group founded in1984, is dedicated to fostering and preserving the integrity of science in thepolicy process. The Institute conducts technical assessments of scientificdevelopments with a major impact on public policy and communicates theresults of its analyses to the press, Congress and the public in clear, readilyunderstandable language. The Institute differs from other think tanks in itsexclusive focus on areas of scientific importance, as well as a Board whose

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The Washington Roundtable on Science and Public policy is a program ofthe George C. Marshall Institute. The Roundtable examines scientific ques-tions that have a significant impact on public policy and seeks to enhancethe quality of the debate on the growing number of policy decisions thatlook to science for their resolution.

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Global Warming and the Hydrologic Cycle:How are the Occurrence of Floods, Droughts, and

Storms Likely to Change?

by

David R. Legates

The George Marshall InstituteWashington, D.C.


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David R. Legates, Ph.D., is the Director of the Center for Climatic Re-search and an Associate Professor of Climatology at the University ofDelaware. He participated in the joint USA/USSR Working Meeting onDevelopment of Data Sets for Detecting Climatic Change that led to a pro-tocol for the first climatic data exchange between the USA and the USSR.He also has twice been called to testify before US Senate subcommittees.Dr. Legates has published more than 100 journal articles, monographs,

proceedings papers, and other publications on climate change and precipi-tation, surface water hydrology and hydroclimatology, and the use of statis-tical/numerical methods in climatology.


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Global Warming and the Hydrologic Cycle:

How are the Occurrence of Floods, Droughts,and Storms Likely to Change?

*

David R. Legates

April14, 20041

Jeff Kueter: I am pleased today to have with us Dr. David Legates fromthe University of Delaware as part of our continuing series, the WashingtonRoundtable on Science and Public Policy. The Washington Roundtable isdesigned to bring to scientists to Washington, D.C. to interact with con-gressional staff, members of the think-tank community, industry and what-not on topics of controversy in the policy realm. Today Dr. Legates willtalk about the enhanced hydrological cycle and its relationship to climatechange.

David Legates: Thank you for hosting me and thank all of you for com-ing. Today I want to talk about the impact of global warming on the hydro-logic cycle. Two weeks ago, as you are probably aware, we witnessed ahurricane, or at least we think it might have been a hurricane, in the SouthAtlantic, a phenomenon that we never before have seen. The question is,Is this a harbinger of things to come, or is this maybe just one of thoserare events that occurs once every fifty years or so, and this happens to bethe year in which it occurs?

Now I want you to consider the following: What are the weatherevents that result in the most deaths and incur the biggest economic im-pacts? Are they changes to the mean field the average conditions? No,

they are changes to the extreme conditions. Thus, what we really want tolook at is how these extreme conditions might change; that is, the occur-rence of floods, droughts, mid-latitude storms (including noreasters), torna-dos, thunderstorms, tropical storms, tropical cyclone-hurricanes, heat wavesand cold spells. Today, I am going to focus on the hydrological cycle andchanges in storminess and changes in precipitation, but I wont talk aboutheat waves and cold spells. Since four of these major events are related to

*The views expressed by the author are solely those of the author and may not represent

those of any institution with which he is affiliated.1 Dr. Legates presented this Roundtable on Capitol Hill on April 12 and April 14, 2004.


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the hydrological cycle, we want to see how they might be changing andhow they might change in the future. The question is: How are the occur-rence of floods, droughts and storms likely to change in the future?

On May 28, a movie called The Day After Tomorrow opens intheaters worldwide. The same producer who brought us IndependenceDay, where the earth was destroyed by aliens, brings us a movie in which

the earth is nearly destroyed by natural forces such as tornadoes, massivesnow storms, hail the size of grapefruit, and other extreme climatic events.I mention the aliens in particular because there is an unexpected connec-tion here. But we want to address whether the movie represents, to someextent, what we can expect to see in the future, or whether this is just go-ing to be a Hollywood representation of fantasy,

The Day After Tomorrow has been turned into a novel written byWhitley Strieber, a name that may be familiar to some of you. In fact, thescreenplay itself comes loosely from a book written by Art Bell and WhitleyStrieber entitled The Coming Global Superstorm whose premise is thatwe can expect a superstorm nearly half the size of a continent. This su-

perstorm will create all sorts of havoc, including fifty-foot tidal waves andfifty feet of snow in New York City.

So who are these people? Art Bell is a top-rated talk show host,which hopefully is not his credential for writing about climatology. He usedto have a show that ran from about 2 to 6 a.m. and discussed the super-natural, aliens, UFOs, and so forth. Whitley Strieber is a best-selling au-thor, probably not noted so much for The Coming Global Superstorm asfor his books Communion and Confirmation, which describe his contactswith aliens. So there is a strong alien connection to the movie here. Butmy point is that while the movie may be very entertaining, we cannot use it

as an indication of future conditions. Instead, I want to go back to ouroriginal question: What really is the scientific evidence and what is the sci-ence behind what might be changing our future?

The Intergovernmental Panel on Climate Change (IPCC) Report isan important element in this. Three assessments have been produced and Iam going to talk primarily about the Third Assessment Report, which cameout about three years or so ago. I will also allude to the earlier report of1996 on occasion. Most people dont realize that each IPCC report is nota unitary document, but it is made up, in fact, of two reports the first be-ing the scientific assessment and the second, the Summary for Policymak-


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ers. Most people assume the Summary for Policymakers is just that, asummary that takes the science, distills it, removes some of the scientificjargon, and essentially says, Here is the essence of what the scientificdocument says. That couldnt be farther from the truth. It was created bya completely different process in which each nation has one vote. So ifenough nations decide that the earth is flat, for example, then the docu-ment shall read, The earth is flat, regardless of the science. So we have a

situation where sometimes and I will show you a couple of these casescoming up the scientific document is completely at odds with what we seein the Summary for Policymakers.

Warmer temperatures will lead to a more vigorous hydro-logical cycle; this translates into prospects for more severedroughts and/or floods in some places and less severedroughts and/or floods in other places. Several models indi-cate an increase in precipitation intensity, suggesting a pos-sibility for more extreme rainfall events.

IPCC politics document (1996)

I would like to examine some of the statements in this 1996 IPCCSummary for Policymakers. Warmer temperatures will lead to a morevigorous hydrologic cycle. That concept of a vigorous hydrologic cycle iswhat we refer to as an enhanced hydrologic cycle. There will be pros-pects for more severe droughts and/or floods in some places and lesssevere droughts and/or floods in others. When you stop and think aboutthat statement, it really doesnt say anything. There could be more floods,there could be less, more drought or less. It doesnt necessarily say much.Several models, though, indicate an increase in precipitation intensity,suggesting a possibility for more extreme rainfall events, which, if true,could have serious ramifications. The more recent politics document, the

Summary for Policymakers (2001), indicates

Global warming is likely to lead to greater extremes of dry-ing and heavy rainfall and increase the risk of droughts andfloods that occur in many different regions.

Now there is no longer waffling; this document indicates that floodsand droughts are likely to increase in frequency. Again, if that is true, itmay have dire consequences for human life and future economic impacts.


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Estimates of confidence in observed and projected changesin extreme weather and climate events

Confidence in observed changes(latter half of the 20

thcentury)

Changes in Phenomenon Confidence in projected changes(during the 21

stcentury)

Likely

Very likely

Very likely

Likely, over many areas

Likely, over many NorthernHemisphere mid-to high latitudeareas

Likely, in a few areas

Not observed in the few analy-ses available

Insufficient data for assessment

Higher maximum temperaturesand more hot days over nearlyall land areas

Higher minimum temperatures,fewer cold days and frost overnearly all land areas

Reduced diurnal temperaturerange over most land areas

Increase of heat index over landareas

More intense precipitationevents

Increased summer continentaldrying and associated risk ofdrought

Increase in tropical cyclone peakwind intensities

Increase in tropical cyclonemean and peak precipitationintensities

Very likely

Very likely

Very likely

Very likely, over most areas

Very likely, over many areas

Likely, over most mid-latitudecontinental interiors. (Lack ofconsistent projections in otherareas)

Likely, over some areas

Likely, over some areas

Figure 1

Figure 1, a table from the Summary for Policymakers document,shows the current assumptions and confidence in projected changes overthe next 100 years. Summer drying and drought are likely to be in evi-

dence now over a few areas and likely in the future to be in evidence overmany more regions. For the increase in cyclone peak wind intensities, thatis, will hurricanes and tropical cyclones become more intense, we have notobserved that to have happened yet, but the conclusion is that it is likely tohappen in some areas. And finally, for the increase in cyclone mean andpeak precipitation intensity heavy flooding events occurring with majorrainfall events of a tropical nature there currently is insufficient data for anassessment, but it is likely to occur in the future.


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Figure 2

Now I want to talk a little bit about theory. Based on the theory,what can we say is likely to happen? Figure 2 shows the amount of watervapor in the atmosphere when the atmosphere is at 100% relative humid-ity, i.e., when it is saturated. The curve has an exponential increase; thatmeans that if temperatures increase, then the potential for water vapor inthe atmosphere at saturation increases. Therefore, with a slightly warmeratmosphere, we can get more water vapor into the atmosphere and whenwe get precipitation, we can possibly get more precipitation out. So theo-retically we could say that, in a global sense, we should expect to see anincrease in precipitation.

But I cant necessarily say that is true for any particular location be-cause two things are required on a macro scale to get precipitation: mois-ture in the air and a mechanism to release it. As it has been said, Youcant get blood from a turnip, if there isnt much moisture in the atmos-phere, you arent going to get much water out. But secondly, regardless ofthe moisture content of the atmosphere, we still need a mechanism in placeto release that moisture, and all of those mechanisms result in cooling theair down to saturation. Some of the most humid places, for example inNorth Africa, are associated with desert conditions. There is a lot of mois-ture in the air, but there is nothing to get that moisture to condense.


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Now when we examine precipitation in hydrological studies, we en-counter three big problems. The first is the bias in precipitation gage meas-urement, the second is our perception of what we mean by floods anddroughts, and the third is climate-modeling problems or complexities asso-ciated with large-scale climate models. I will first start with gage measure-ment biases.

I am partial to this issue because it was part of my Ph.D. disserta-tion and I have done a lot of work on it. One of the things I attempted todo with my dissertation was to put together the highest resolution climatol-ogy we had to date. We recognized that when we use a standard raingage,which is little more than a container that catches water, a bias in the meas-urement occurs, and that bias is largely a function of the wind speed andwhether the precipitation falls in the form of rain or snow. In my disserta-tion, I found that the global bias would be about 11%, which is consistentwith a study that was undertaken in the Soviet Union at about the sametime. But the bias is not constant; it is different for different places and inareas where there are higher winds and more snowfall, the bias becomesgreater.

I later examined the effect of precipitation gage measurement biaseson climate change detection. I took a hypothetical station, near Madison,Wisconsin, that had no change in its time series, by definition, over a 100-year period, and I introduced into that record a slight change in wind speedand temperature. I decreased the wind speed by one knot (one nauticalmile) an hour over one century and increased the temperature by one de-gree Celsius, simulating the potential urbanization that had occurred. As-sume, for example that the station was located outside of a small town in1900 A.D., which grew into a large urban metropolis area by 2000 A.D.A one degree Celsius rise and a decrease of wind speed due to urbanization

are not uncommon. But with a simple one-knot decrease in the windspeed and a one degree Celsius increase, the change in the gage measure-ment bias caused an apparent 6.4% increase in the precipitation justfrom the change in the siting characteristics alone. So part of the problemis that when we look at time series of precipitation, we are seeing trends inprecipitation, but we are also seeing the effect of changes in the sitingcharacteristics around the gage. This makes it difficult to separate the localenvironment change from the climate change that we are trying to study.

The second problem is our perception of floods and droughts.Generally we define floods as streamflow above a certain level. Increases in


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flood frequencies can result from increasing rainfall events, but it can alsooccur due to changes in timing or the form of precipitation. If the rain fallswhen the soil is saturated or very dry, on some types of soil, there can bemore runoff than there would be if the soil were in a different moisturecondition. Temperature also is important; if precipitation falls in the formof snow, it will lay there until it melts. If it falls in the form of rain, it will bedealt with immediately, either as runoff or infiltration. Due to urbanization,

we replace grasses and trees with asphalt, macadam and other hard sur-faces. These surfaces do not allow infiltration, so virtually all of the rain-water becomes runoff, which changes the flooding characteristics. Chan-nelization of streams and rivers, the building of levies, dredging activities,reinforcement of banks, for example, also change the streamflow. Thestate climatologist of Louisiana, for example, characterizes the Mississippiin southern Louisiana as a freeway with no on-ramp because the USArmy Corps of Engineers have built levees and channelized the river. Infact, water that is supposed to flow into the Mississippi from the ComiteRiver basin actually flows in the opposite direction; it is no longer allowedto break the levee. We have changed the hydrologic patterns of the regionand, as a result, we have changed in the flooding conditions. So changes

in flooding conditions can result from climate changes in precipitation andalso from changes in the local characteristics.

The same thing happens with drought. Drought is a streamflowbelow a certain level and increases in drought frequencies can be inducedby decreases in rainfall events, but also by urbanization, increased demandfor water, and, in particular, an increase in water-intensive activities, suchas irrigation, residential or industrial activities. Such problems with observa-tions led Tom Wigley to make the famous statement, But the data aredirty. When you start looking at climate change trends as seen in the ob-served record, it is full of all sorts of measurement problems and biases.

What we want to do is clean that up and get a record that does not havethese biases. But where can we get that?

From general circulation models (GCMs), of course mathematicalrepresentations of the climate system. Using GCMs, we can play what-ifscenarios, we can change certain things, see what happens, but more im-portantly, we can hold things constant, so that measurement biases andchanges in the landscape are not an issue.

A wide variety of problems are associated with trying to measureprecipitation with a GCM. I often say, if you really want to look at the


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efficacy of a climate model, dont look at air temperature, look at precipita-tion. Anything you do wrong in a climate model will show up in precipita-tion. If you get the mountains in the wrong place, the atmospheric circula-tion wrong, the amount of moisture going into the atmosphere wrong, thecondensation rate wrong in summary, anything you do wrong shows upin precipitation. And now the kicker: anything wrong in the precipitationaffects everything else. If I have too much precipitation in a region, I re-

lease too much energy from condensing water, which in turn affects theenergy balance. Water is part of the mass balance of the hydrological cycleand part of the energy balance, and as a result, getting precipitation wrongwill necessarily affect everything else you do.

Zonally-Averaged Global Precipitation by 31 GCMs

Figure 3

Figure 3 is from the Atmospheric Model Intercomparison Project byLarry Gates done a couple years ago, which looks at thirty-one GCMs. Ihave seen modelers look at this and say, By golly, thats pretty good. Weare able to see the low latitude tropics, the dry subtropics, increases in pre-cipitation in mid-latitude, and the dry deserts of the poles. My response isthat if a climate model couldnt get the gross characterization of the globelike that, it wouldnt be worth anything at all, so I am not surprised that it


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can do that. My concern is that from model to model, there is considerableuncertainty in the level of precipitation. There is uncertainty in our estima-tion of precipitation, but the differences in Figure 3, I contend, are not triv-ial.

Global Precipitation Estimates

Model January July .GFDL 3.13 in. 3.35 in.GISS 3.89 in. 4.06 in.NCAR 3.76 in. 4.24 in.UKMO 3.54 in. 3.93 in.

Global Precipitation Estimates

Model January July .GFDL 3.13 in. 3.35 in.GISS 3.89 in. 4.06 in.

NCAR 3.76 in. 4.24 in.UKMO 3.54 in. 3.93 in.

Observed 4.37 in. 3.44 in.

Figure 4

Figure 4 is from a study I did several years ago. I examined fourdifferent models to compare summertime versus wintertime precipitation.You can see they give us different numbers for globally averaged monthlyprecipitation; I took data for the given month from the entire globe and av-eraged it to a single average rainfall depth. In this case, all four models

agree, not on the numbers, but they do agree that July is wetter than Janu-ary. There is one hitch. When you compare that with the observations,exactly the opposite pattern occurs. We can argue from theory that thewetter month should be January, since its summer in the southern hemi-sphere. Thats where you have more water, thats where you have warmertemperatures, thats where we should see a more vigorous hydrologic cycle.And we do, in the observations. But we dont see that at all in the modelsand that is a major concern.

Now when we start to look spatially at model simulations of precipi-tation, things get even more complex. Figure 5 shows precipitation over


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summer. This is by Doherty and Mearns; using my global precipitation cli-matology, they compared the two models used in the National Assessment.

Model Minus Observed: Precipitation

Figure 5

The Hadley Centre climate model is at the top and at the bottom isthe model from the Canadian Climate Centre. What we can see here, if wefocus on just the extreme blues and extreme reds and yellows, is that theseare areas where we are off by three, six, nine, fifteen millimeters a day.Now if you are off in a model by one millimeter of precipitation a day, thatequates to thirty millimeters a month, which is more than an inch of rain-

fall. In Oklahoma in the central United States, we get about thirty-sixinches of rain a year. Divided by twelve, that is about three inches amonth. So an error of just one millimeter is an error in precipitation esti-mation by one-third of the monthly total. That is significant. And in factsome of these numbers are physically impossible because it just doesnt rainthat much. But I want to compare this to our model estimates of precipita-tion and the assumed changes under global warming.


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Prognostications for 2030: Precipitation

Figure 6

Figure 6 is a scenario for 2030, twenty-five years in the future andthirty years from when the models were run. The colors represent differ-ences from the control run (2000) and green is in the middle: little to nochange. Figure 6 is predominantly green, showing relatively small changes.But Figure 5 has lots of blues, reds, yellows, which represents lots of uncer-tainty. Thus, the prognostications from Figure 6 represent a small signal in

a large sea of uncertainty. But look at the scale we are using. In figure 6,the maximum was ten, the minimum was minus five. But in figure 5, wehave extremes of plus or minus fifteen, so the scale was stretched on figure6. So, what we see in figure 5 is the noise, the uncertainty in the model,and there is lots of it. The signal that we get out of the model is figure 6and it is much less than the uncertainty in the model. So if the model hasmore uncertainty than the signal it is simulating, it is very difficult to arguethat the signal is significant.


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Figure 7

But there is an even bigger problem. As I said earlier, we are notreally interested in changes in the mean per se, we are interested inchanges in the extreme. Figure 7 is from an article by Soden in the 2000issue of theJournal of Climate. The heavy dark line is observed precipita-tion. The thin line is model ensemble average in year and one modelstandard deviation is shown by these crossed lines. Soden concludes thatGCMs differ with respect to the observations and that they also lack coher-ence among themselves. We saw that in the spatial distributions betweenthe Hadley Centre Model and the Canadian Climate Centre Model. TheClimate Centre Model had lots of drying in the central United States, theHadley Centre Model doesnt. But Soden says, and this is important,Even the extreme models exhibit markedly less precipitation variabilitythan observed

If we want to determine how the extremes might change and ourclimate model doesnt model the extremes, it is impossible to determinefrom that model how the extreme conditions are likely to change. Sodengoes on, if the GCMs are in error, this deficiency would presumably reflecta more fundamental flaw common to all models.

Now, I further argue that those errors that we have seen in themodels are not trivial. For example, if I take one millimeter of rainfall, con-dense it out of the troposphere, compute how much energy is given off bythat millimeter of rainfall, and turn that into a temperature change, we findthat one millimeter of rainfall is almost 0.4 of a degree temperature changein the troposphere (Figure 8). If you prefer English units, a tenth of an inchis 1 degrees Fahrenheit.


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P ( w L g ) / ( Cp p ) = T

1 mm of rainfall 0.39C in air temperatureor

0.1 inch of rainfall 1.77F in air temperaturefor the troposphere

Figure 8

Now when we think about the climate change scenarios and theclimate change prognostications for air temperature, we can see that beingoff by just a little bit in precipitation yield errors that are larger than ourclimate change signal! This is my fundamental argument: if you do thingswrong with precipitation, it shows up everywhere else; in this case, it canquite dramatically induce major problems with temperature. So when cli-mate models are tuned to try to get the air temperature right, we may havea fundamental problem because if the models are not adequately simulatingthe precipitation, the mere process of model calibration and tuning may

introduce considerable biases into the model simulation.

But enough of the theory what is the observational evidence? Inparticular, what do we see and what do we expect to see with regard tochanges in total precipitation, changes in precipitation frequency and inten-sity, changes in flood and drought frequency, changes in tropical storm fre-quencies and intensities, and changes in extra-tropical or mid-latitude stormfrequencies and intensities? The first thing we are going to look at ischanges in precipitation totals.

Precipitation increased by 0.5 to 1% per decade over mid- and high latitudesof the Northern Hemisphere continents increased by 0.2 to 0.3% per decade over the tropical land

areas, but not over the past few decades decreased by about 0.3% per decade over Northern Hemisphere

subtropical land areas no systematic changes over the Southern Hemisphere insufficient data over the worlds oceans.

IPCC politics 2001


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Trends in terrestrial precipitation estimates over the past century. Since 1980,when the most spatially complete assessments of global precipitation became avail-able, few regions show marked trends in mean precipitation. (from New et al.,2001)

Figure 9

Figure 9 shows more recent results by Michael New and colleaguesfrom the University of East Anglia in Norwich, England. At the top is along-term time series of the last century for global precipitation over land;the middle is Northern Hemisphere precipitation over land, and the bottomis Southern Hemisphere precipitation over land. None of these signals hasa statistically significant slope, so none of them shows significant increasesor changes. The conclusion is that since 1980, when the most completeassessment of global precipitation is available, few regions show markedtrends in mean precipitation. We do not see large-scale changes in precipi-

tation, except only in selected areas, and that goes back to my argument: itis not just the moisture in the atmosphere but the concomitant mechanismto release that moisture.

Concerning changes in precipitation frequency and intensity, theIPCC Summary for Policymakers (2001) states

In the mid-and high latitudes of the Northern Hemisphereover the latter half of the 20th century, it is likely that therehas been a 2 to 4% increase in the frequency of heavy pre-cipitation events.


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That is significant and important, if true. Much of that statement comesfrom Karl and Knight (Bulletin of the American Meteorological Society,1998), where they examined precipitation regimes in the United States.They concluded that

The precipitation regimes in the United States are chang-ing disproportionately across the precipitation distribution.

The proportion of total precipitation derived from extremeand heavy events is increasing relative to more moderateevents.

This analysis presents a problem. Their procedure is to create bins,each representing sequentially heavier rainfall events, and compute the fre-quency of precipitation events for each bin. Now assume that precipitationis increasing by the same proportion for each category. Some of theevents in each category would be moved to the next higher category. Inthat case, each bin would lose events (moving to the next higher category)but also would gain events (moving up from the next lower category).Thus, only the bin at the end may exhibit a trend, since there is no bin

higher than it. We may erroneously conclude that only the precipitation inthe highest category is increasing, which clearly is not the case.

The other important point to note is that they focus on the pro-portion of total precipitation. We want to know particularly if it is the fre-quency of the precipitation events that is changing, not the proportion ofthe total.

But Kunkel et al., (Journal of Climate, 1999) actually looked atthat question. For the Midwest, they concluded that heavy event frequen-cies from 1896-1906 were higher for all ten-year periods except 1986-96.

Their conclusion was

Interpretation of the recent trends must account for thepossibility of significant natural forcing There is no im-plication in these results that the upward trends will neces-sarily continue.


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Figure 10

Last year, they produced another study (Geophysical Research Let-ter, 2003). Figure 10 shows the changes from 1895-2000 in one-daystorm events and five-day storm events. The return period simply meansthe event is likely to occur once every year on average, once every fiveyears on average, or once every twenty years on average. We can see al-most a U shape in both of these graphs. The values for one-day events in

the late 1800s and early 1900s are as high as they are now. This is animportant issue to note because the values for the late 1800s and the early1900s must be natural variability; it cannot be anthropogenic change.They concluded

Frequencies at the beginning of the 20th Century werenearly as high as during the late 20th Century suggestingthat natural variability cannot be discounted as an impor-tant contributor to the recent high values.


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So we are not sure that precipitation frequency intensity is changing.

What about changes in floods, droughts, tropical and extra-tropicalstorms? It is the last three that fall into our category of the major eventsthat lead to the most deaths and the largest economic impact. The IPCCSummary for Policymakers (2001) says


This suggests that we can expect more floods and drought events. But ifwe look at the same year, the IPCC science document, the Technical As-sessment by scientists, indicates

Over the 20th century there were relatively small in-creases in global land areas experiencing severe drought orsevere wetness.

Thus, we havent seen these things happen. To continue

Changes are dominated by inter-decadal and multi-decadal climate variability

There are years when it went wet and years when it went dry, years whenwe have had dust bowls, years when we have had floods. There is consid-erable variability and most of the changes are dominated by that variability.They go on

In some regions the frequency and intensity of droughts

have been observed to increase in recent decades.

In some cases, for example, the Sahel region in Africa, we have seen theeffects of desertification. That is likely a feedback from the surface increas-ing drought event, not a change in the climate.

So compare the politics document with the science documentand you can see the two dont match. The Summary for Policymakers,therefore, is not a good summary of the scientific assessment.


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Now lets look at flooding events. Lins and Slack (1999) examinedpercentiles in streamflow and they found

Trends are most prevalent in the annual minimum to me-dian flow categories and least prevalent in the annualmaximum category.

They examined changes in streamflow by locating streams that aredevoid of anthropogenic influences (channelization, urbanization and soforth). They concluded that the low flows are getting higher and the me-dian flows are getting higher which isnt a bad thing but the high flows,the flood events, are in fact, least prevalent and show the least increase.Their conclusion is

Hydrologically, these results indicate that the contermi-nous US is getting wetter, but less extreme.

On the other hand, Groisman, Knight, and Karl did a study in 2001that was very similar to their precipitation analysis. Their conclusion about

changes in stream flow for the continental United States was that they sawsignificant increases in stream flow, particularly for the highest flowevents. So on the one hand we have Lins and Slack saying high flowevents arent changing and on the other hand, Groisman, Knight and Karlsaying they are. How do we reconcile the apparently contradictory state-ments?

It turns out they are not contradictory at all; the two researchgroups are just answering different questions. Lins and Slack asked thequestion: Are trends occurring in stream flow percentiles? Is the 95th per-centile of stream flow getting much larger, that is, is the distribution in an

event that occurs once in twenty years, for example, changing relative toone that occurs once in five years? Groisman, Knight and Karl ask: Of thetotal volume of water that changed, how much of that water came from aspecific percentile? But when we have high flow conditions, we expect tohave lots of water; small percent change to a large number gives us lots ofwater. So it is true that more of the water is coming in high flow condi-tions, but that is not really the question we want to ask. The question is:How are the frequencies of these events changing? Thus, Lins and Slacksquestion is really the one that we want to answer and, therefore, the highflow events really arent increasing as much as the median and lower flows.


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That is consistent with an overall increase in precipitation, but not an in-crease in storminess or the extreme events.

Finally, lets look at changes in storm frequencies, tropical, extra-tropical. The 1996 IPCC scientific document indicates

In the few analyses available, there is little agreement be-

tween models on the changes in storminess that might occurin a warmed world. Conclusions regarding extreme stormevents are obviously even more uncertain.

You cant say anything; it was just not possible at that time. Didthings change, though, over the next five years of research? The IPCC sci-entific technical document (2001) now indicates

There is no compelling evidence to indicate that the char-acteristics of tropical and extratropical storms havechanged. Changes globally in tropical and extra-tropicalstorm intensity and frequency [have] no significant trends

evident over the 20th century. Conflicting analyses make itdifficult to draw definitive conclusions about changes instorm activity, especially in the extra-tropics.

So apparently not much has changed. Two recent researchers, Henderson-Sellers et al. and Goldenberg et al. conclude

In the two regions where reasonably reliable records exist(the North Atlantic and the western North Pacific) thereis no clear evidence of long-term trends.

Henderson-Sellers et al. (1999)

Bulletin of the American Meteorological Society

and

There have been various studies investigating the potentialeffect of long-term global warming on the number andstrength of Atlantic-basin hurricanes. The results are incon-clusive.

Goldenberg et al. (2001)Science


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Thus, we cant conclude that any of these changes are occurring. ButHenderson-Sellers also concludes

It is emphasized that the popular belief that the region ofcyclogenesis will expand with the 26C [sea surface tem-perature] isotherm is a fallacy.

Henderson-Sellers et al. (1999)

Bulletin of the American Meteorological Society

A hurricane or tropical storm is what we call a latent heat engine; that is,evaporating water condenses and gives off energy that churns up the winds,the circulation, and gives us a cyclonic organization that characterizes hurri-canes. For this to occur you need warm water, and if the warm water areagets bigger, you may have more reasons for cyclogenesis and deeperstorms, which should yield an intensification of storm events. Their conclu-sion: Its a fallacy. That willnot occur. Furthermore

The very modest available evidence points to an expecta-tion of little or no change in global frequency.

Henderson-Sellers et al. (1999)Bulletin of the American Meteorological Society

There have been a number of other studies, which I wont go intodetail, but three suggest an increase of tropical storm activity in a warmerworld, while two different studies argue for a decrease in tropical storm ac-tivity in a warmer world.2 The conclusion is that we dont yet know whatmight happen and we havent seen any changes taking place.

In the Southern Hemisphere, fewer analyses have beencompleted, but they suggest a decrease in extra-tropical cy-

clone activity since the 1970s.IPCC science document (2001)

From the studies that have been completed, extra-tropical cyclone activityhas become less extreme.

2Krishnamurti et al. (1998), Walsh and Ryan (1999), and Meehl et al. (2000) sug-

gest an increase in tropical storm activity in a warmer world while Bengtsson et al.(1996) and Yoshimura et al. (1999) argue for a decrease in tropical storm activity.


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Recent analyses of changes in severe local weather (e.g.,tornadoes, thunderstorm days, and hail) do not providecompelling evidence to suggest long-term changes.

One thing we find as we look at tornado records is that over the lastthirty or forty years, there has been a dramatic increase in the weaker tor-nadoes, the F0s, F1s and F2s. Has that been a climate change signal? It

turns out the answer is no, for two reasons. One is that there is muchmore interest in chasing storms, particularly by meteorology students atvarious universities, including the University of Oklahoma, where I workedfor 9 years. On the first big chase day in spring, nobody came to class;they were all out trying to get video for CNN. There is also much moreinterest in chasing storms and many more spotters helping out the NationalWeather Service by being their eyes in the field.

Probably the biggest impact, however, has been the development ofDoppler weather radar. With Doppler radar, we can see the circulation inthe middle of a wheat field in Kansas when formerly it may have produceda small tornado that disappeared five minutes later without anybody seeing

or recording it. Now we may record it on Doppler and so it becomes partof the tornado statistics. So, in general, trends in severe weather events arenotoriously difficult to detect because they are rare events and it is difficultto detect changes in the rare event portion of the spectrum.

I am now going to read a quote by Sinclair and Waterson (Journalof Climate) in two parts. Doubled CO2 leads to a marked decrease in theoccurrence of intense storms [in the extratropics]. Thats the second timeweve heard that stated. But they give us an exception: One exception isin the South Pacific, where there is a suggestion of an increased incidenceof cyclones at the intense end of the spectrum. Lets investigate the South

Pacific.


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Figure 11

Figure 11 shows a map of topography, what you and I recognize asthe earths surface. The mountains are in red, the lowlands in green, thewater in white.

A standard T42 truncation scheme (Biasutti et al., 2003). The contour interval is400m. Negative altitudes of 200m occur west of the Andes while the highestpeak in the Andes is only 3000m high.

Figure 12


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But what a climate model recognizes as the earths topography isquite a bit different. Figure 12 is a spectral model using what is called T42truncation scheme. Here the Andes mountains are just sort of a generalbump; we dont see the individual mountains, and in fact the Andes onlyreach an altitude of 3000 meters. The mountains of East Africa are repre-sented by a large plateau and the Himalayas appear as simply a rise over alarge area. So we really cannot simulate mountains and irregular topogra-

phy well.

But worse yet, what does the model see over the ocean? Severalyears ago, I talked with Tony Broccoli from the Geophysical Fluid Dynam-ics Lab in Princeton and he said, We were looking for precipitation pat-terns over the oceans, and we saw some weird things happen. We couldnot figure out what those weird things were. Since everyone blocks out thetopography over the oceans, I decided to plot it and see what it looks like.And thats what we see in figure 13.

Surface Elevation (m) Represented in an R30 Climate Model(2.25 of Latitude by 3.75 of Longitude)

Figure 13

In these models, topography must be smooth, so the model pro-duces waveforms out over the Pacific and the Atlantic. Note that all theoceans have topographical variations on the order of several hundreds ofmeters. The Andes Mountains dont stop at the edge of the Pacific Ocean;


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they extend all the way across and you can still see the remnants of them aswe approach Australia. So a wave pattern occurs in topography where inthe real world you have a vast expanse of thousands and thousands of milesof water. No topographical variations. But in the model, the air is forcedup and down, with the rising motions enhancing the condensation process.So increased storminess in this area could simply be a result of topographi-cal forcing and this is incorrectly represented in climate models over the

South Pacific.

Now we come back to the second part of Sinclair and Watersonscomment:

Reductions in average cyclone central pressure that havebeen used in other studies to promote the possibility of en-hanced storminess under greenhouse warming, are morelikely the result of global-scale sea level pressure falls ratherthan any real increase in cyclone circulation strength.

Sinclair and Watterson (1999)Journal of Climate

If we are looking for a cyclone or for the development of storms, weneed to look for an area of low pressure. But is the pressure in certain re-gions, in the average, decreasing over time? Lower pressures imply morestorminess and higher pressures imply less storminess, because high pres-sure produces clear skies. Their conclusion is that increased storminess ismore likely the result of falling sea-level pressure on a global scale. Thereare two physical ways that could happen. One is that we could be losingsubstantial amounts of mass of the atmosphere. We are losing hydrogen tospace, but thats insignificant. The atmosphere still has essentially the samemass as it did hundreds of years ago. So we are not seeing that as a

change. The other possibility is that the atmosphere is expanding, chang-ing the potential energy of the molecules, and hence changing their gravita-tional force on the earths surface. We do not see that happening signifi-cantly either.

How else could that happen? Well, in the climate models, it hap-pens because the models do not conserve mass! Some models in fact donot conform to one of the basic premises of the atmosphere, the law ofmass conservation. What the modelers do is run the model for a bit, stopand re-inflate the atmosphere, sort of like running on tires with a slow leak.The real world doesnt work that way, but some climate models do and


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Sinclair and Wattersons argument is that the drops in general pressure thatmodelers have seen over time are actually the tires deflating. They arenot seeing a real process in the atmosphere or a simulated real process in amodel; what they are seeing is a model fault, in that the model simply can-not reproduce the law of mass conservation.

There is no firm evidence that climate has become more

variable over the last few decades. IPCC Politics (1990)

There is no evidence that extreme weather events, or cli-mate variability, has increased in a global sense, through the20th century, ... data and analyses are poor and not com-prehensive.

IPCC Science (1996)

Variability in much of the Northern Hemisphere's midlati-tudes has decreased as the climate has become warmer.Some computer models also project decreases in variabil-

ity.Karl et al. (1997)

In fact, we have seen less variability, so in many cases, the argu-ment might be that in a warmer world, we will see fewer, not more, ex-treme events. This is actually good news, because it is extremes that causethe most economic damage and cause the most deaths.

Now for the report of the water sector for the United States Na-tional Assessment. Remember that the National Assessment was put to-gether by Mike McCracken and Tom Karl and a few others through the Na-

tional Assessment scientific team and it was commissioned by a group in-cluding Al Gore. Bruce Hayden gave a presentation for the water sector inAtlanta and I was fortunate to be there when he made his presentation.Bruce said,

There has beenno trend in North America-wide storminessor in storm frequency variability for the period 1885-1996 It is not possible to attribute regional changes instorm climate to elevated atmospheric carbon dioxide.


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[Model] projections of North American storminess shows nosensitivity to elevated carbon dioxide. Statements aboutstorminess based on [model] output statistics are unwar-ranted.

Little can or should be said about change in variability ofstorminess in future, carbon dioxide enriched years.

Hayden (1999)Journal of the American Water Resources AssociationReport of the Water Sector for the US National Assessment

One would assume that would make it into the National Assess-ment. But the National Assessment proclaimed, despite Haydens asser-tions,

It is likely that the observed trends toward an intensifica-tion of precipitation events will continue. Thunderstormand other intensive rain events are likely to produce largerrainfall totals. Projections [for hurricanes] are that peak

wind speed and rainfall intensity are likely to rise signifi-cantly.

US National Assessment (2001)

The statements of U.S. National Assessment run directly counter to everyargument we have seen earlier. The National Assessment went on to say,

While it is not clear how the numbers and tracks of hurri-canes will change, projections are that peak wind speed andrainfall intensity are likely to rise significantly.


Here again is the argument by Henderson-Sellers et al., which wasavailable to the National Assessment:

It is emphasized that the popular belief that the region ofcyclogenesis will expand with the 26C [sea surface tem-perature] isotherm is a fallacy. The very modest availableevidence points to an expectation of little or no change inglobal frequency.

Henderson-Sellers et al. (1999)Bulletin of the American Meteorological Society


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So lets summarize: What can we say and what cant we say?

Changes in Total Precipitation?We saw from theory that it may increase and that in certaincases it had increased. The answer I will give is that it is possi-ble we have seen increases in precipitation and we might see in-

crease in precipitation in the future, but it is certainly belowmeasurement bias and natural variability. Precipitation is ahighly noisy field; year to year there is quite a bit of variation, aswe saw, and models just dont simulate that at all.

Changes in Frequency and Intensity?No, we have not seen changes in precipitation frequency or in-tensity and this is in contradiction to the IPCC Science docu-ment (2001). Now in fairness, Kunkel et al.s research, some ofthe arguments made regarding Lins and Slacks findings, andthe context of the questions that we really should be addressingwas not available to the IPCC science assessment in 2001.

Changes in Flood and Drought Frequencies?No, we have not seen changes in flood and drought frequenciesand that is in complete agreement with the IPCC Sciencedocument (2001).

Changes in Tropical Storm Frequencies?No, we have not seen changes in tropical storm frequencies andthat is in agreement with the IPCC Science document (2001).

Changes in Extratropical Storm Frequencies?

No, we have not seen changes in extratropical storm frequen-cies and that is in agreement with the IPCC Science document(2001).

So I will leave you with two pieces of information:


IPCC Politics (2001)


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The observed trends toward an intensification of precipita-tion events will continue. Thunderstorm and other inten-sive rain events are likely to produce larger rainfall totals.Projections [for hurricanes] are that peak wind speed andrainfall intensity are likely to rise significantly.


I ask you the question: Where is the proof for any of these state-ments?

I thank you very much for inviting me here today.

Questions and answers3

Question:What is the problem with the National Assessment estimate?Was it under political pressure?

Legates: Well, I do not know enough about how exactly how it was puttogether, but I have some suspicions. But not being there, I cannot reallycomment directly that this is what people did or not. All I can say is it justseems to be a bit lacking in scientific balance and that is disconcerting be-cause the IPCC and the National Assessment are erroneously held up as ascientific consensus but they dont often reflect the true state of the scienceand they generally give a very biased view the National Assessment inparticular. I cant say why it happened, but it has happened.

Question: You discussed some extreme weather events in the past, forinstance, the dust bowl. How does that compare to what the models pre-dict? Do the models predict the dust bowl? Is this a matter of natural vari-ability or is there some other cause for it?

Legates: The dust bowl was two different things. One centered on naturalvariability that caused an extended dry period. But at the same time therewas ignorance of the ways in which Midwest land should be managed.They treated it like the humid east, they plowed the land and turning overthe topsoil allows it to blow away under very dry conditions. So it was acombination of drier than normal conditions, we saw that in Kunkels graphwhere the 1930s showed a decrease in precipitation, and natural variability.But it was also accentuated by human activities. So I am not sure, in fair-

3 These questions are from Dr. Legates presentations on April 12 and 14, 2004.


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ness to the models, whether they should be expected to simulate the dustbowl.

But remember the models dont get year-to-year variability well.They have years that all look very similar to one another. The problem isthat models tend to simulate precipitation much differently than the wayprecipitation occurs. Most models simulate what is called popcorn pre-

cipitation; it is largely convective and occurs with very little organized sys-tem structure. Our weather map is characterized by the passage of frontsand organized systems, but climate models do not have the spatial resolu-tion to simulate fronts. Yet fronts are important weather-making phenom-ena. So if a model only simulates convective precipitation events, it is miss-ing, by the nature of its coarse resolution, the ability to simulate other pre-cipitation-forming mechanisms that are equally important. We could getchanges, lets say, in precipitation equally if we simply change the large-scale circulation. But models arent able to simulate that; its all popcornconvective rainfall. Thus, there is a tendency for every year to look likeevery other year, so long as the radiation input remains the same, as itgenerally does. But we are not going to see all of the myriad things that

can happen in the real world. So to come back to your question, are theyable to simulate the dust bowl? No, but thats probably too because theyarent able to simulate the processes that existed either on the anthropo-genic side or on the climate side. They are limited in both respects.

Question: Could you go over this effect, the oceans being modeled withdifferent sea levels?

Legates: The sea-level of the oceans or do you mean the topography assimulated by climate models?

Question: The topography of the oceans. Where do those various bumpsthat we see there come from?

Legates: This is from a spectrally based model. That means we are tryingto represent all fields by a series of sines and cosines, spherical harmonics,if you will. In this case, we are trying to describe a very rough field by aseries of smooth curves. If you have ever seen the modeling of a hat usingsmooth curves, we can only simulate the hat up to the point at which thevertical gradient occurs. The more terms you use, the more you narrowthat area but the more undulation you get in the vicinity of that stepchange. Thats what is known as the Gibbs phenomenon. So anywhere


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we have very sharp gradients, it is going to be very difficult for a climatemodeler or for spherical harmonics to resolve that dramatic change. Butwe have that in topography, the Andes in particular; the mountains riseconsiderably and then come back down to sea level and then theres a flatocean for thousands and thousands of miles. So what happens is, there-fore, that wave field extends across the ocean. You can see it dampeningout as it goes. But nevertheless within the modeled oceans, we still have

significant elevations to contend with.

Question: So you are indicating that those bumps that we see every fewhundred miles are artifacts of the process?

Legates: Yes, these are the Andes, as they appear as they the harmonicsdampen out. Here is essentially the Antarctic Peninsula, as the harmonicsdampen out to the north. With distance, things decay. But the sharper thegradient, the longer that field is going to manifest itself into the sphericalharmonics.

Question: In general, the way people try to cure harmonics like that inthe model, is to take more and more samples. Are you at that point?

Legates: Figure 12 shows R30, which simply means for rhomboidal trun-cation at 30 wave numbers. Figure 11 is a little newer analysis with T42truncation but we can still see the problem here: even with higher resolu-tion, we cant resolve the Andes. There is actually a negative altitude of200 meters, which will also propagate out all through the ocean as well. Ifwe took more wave numbers, the oceans would dampen out, but therewould be considerable problems along the steep gradients more so thanwe see here. That is the Gibbs effect. But to increase wave numbers sig-nificantly would require a lot of computational power, to be able to resolve

all those wave numbers, and we just dont have the ability to do that now.

Question: So all models have this problem?

Legates: All spectrally based models do. There are two classes of models.Models that are spectrally based represent everything as waveforms hori-zontally while what are called grid-point models represent everything as lit-tle boxes. Horizontally you look at things going into and out of boxes.Even in both models there are boxes stacked up on top of one another.But in terms of the grid-point model again, you have fluxes into and out ofthe box, and you resolve the fluxes crossing the six sides of the box. Spec-


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tral models have the problem I mentioned; but box models have anotherproblem in that they also have problems with abrupt changes from one cellto another.

Question: The word drought is being used and bandied about. What isthe technical definition of drought? Is that what is going on with the Ogallaaquifer?

Legates: Well, there are all sorts of issues. As you are probably aware,with the Ogalla aquifer, over-pumping leads to lowering in the well waterlevels. The water gets into the aquifer through percolation; excess precipi-tation moves down through the soil and winds up in the water table. Sonormally the excess water would replenish the aquifer and everything wouldbe okay. If we start pumping the aquifer at a rate greater than its recharge,we change the moisture available in the aquifer, which lowers its level. Weput much more demand on the water, which changes the amount of wateravailable in the streams. So artificially we are creating a drought conditionbecause of increased usage. Drought can be brought about by lack of pre-cipitation, but it can also be brought about by increased use of the limitedwater resources you have available to you. Just because we wind up indrought conditions doesnt necessarily mean that precipitation is dropping;it can also mean that the demand for water has increased.

Question: Another western issue with regional impact. Based on argu-ments by the US National Assessment, most of the water storage in thewestern states that comes from snowfall, like in the Sierra Nevada, is de-creasing. So people are saying that in those states you had better get usedto reduced snowfall, because it will never go up again.

Legates: It is true that warmer temperatures obviously in marginal areas

would decrease the amount of snow. At the mountaintops, the tempera-ture is generally cold enough that any precipitation you get is going tocome down in the form of snow. I have heard, for example, people talkabout the melting of the entire Antarctic icecap. I dont know of anymodel, for example, that is predicting above-freezing temperatures at theSouth Pole. So clearly there is still going be an accumulation of ice downthere. In fact, with rising temperatures and with more water available, youcan get increased precipitation, hence increased storage. Thus, its likelythat the Antarctic ice sheets would grow with increased air temperatures.


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But that brings us back to the Sierra Nevada. The idea is, withslightly warmer temperatures in the midwinter, it is possible to get moresnowfall. The issue, though, is most of their water in the year comes fromthe snowfall. Again if there is an increased demand with more population,that can lead to a lack of water, which isnt necessarily climatologicallydriven. So there is some truth in the argument that you have to be awarein the future that there may not be water reserves. But it may not be

global-warming-induced; it may simply be increased demand. Again, thereis that two-level issue associated with all water resources. There arechanges in supply and changes in demand. So even though the supplymay remain the same, if demand goes up, your reserves go down, and thatis not necessarily an indicator that the supply has changed.

Question: You talked about how the results from different researchersachieve different conclusions. Are they working on different assumptionsof CO2 levels warming and that leads to different results?

Legates: Not necessarily that they are working on different assumptionsof CO2 warming, but a different understanding of the climate system. Theclimate system is inherently complex. If it were simply a radiation budgetissue, they can say increased carbon dioxide will necessarily increase tem-peratures and that will necessarily lead to this, that and the other. But theclimate is a series of feedback mechanisms, some positive, some negative.For example, if atmospheric circulation were to change, it may decreasethe precipitation, which in turn, may decrease the evaporation, which maymake the temperature rise even greater. By contrast, if it changes so youget more precipitation, it may decrease the temperature because more en-ergy can go into evaporating moisture. So it is a really complicated seriesof what-if scenarios. And the various researchers are each determiningwhat they think this is more important, but others are saying, no I think

that is more important than this. So it gets down into an area of uncer-tainty. Of course, any model can predict what you what it to, if you makethis model be more attuned to this and less attuned to that, you might beable to make it say what you want. But it doesnt necessarily mean any ofit is right. It is just a very complicated issue and the different researchersuse different assumptions on how the climate system may respond.

Question: You were talking about the National Academy of Sciences re-port; it was basically put together in a matter of two weeks and is basically aregurgitation of the IPCC report. It should not be considered an independ-ent report of any sort.


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Legates: I would argue it is a regurgitation of the IPCC Summary for Poli-cymakers rather than a scientific document. In fairness, thats what it wasintended to be, a political document, not a scientific document. I thinkTom Karl made that statement in his congressional testimony several yearsago.

Kueter: Thank you very much for coming.

* * *


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The Marshall Institute Science for Better Public Policy


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Board of Directors

Robert Jastrow, Chairman

Mount Wilson Institute (ret.)

Frederick Seitz, Chairman Emeritus

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William OKeefe, President

Solutions Consulting

Bruce N. Ames

University of California, Berkeley

Sallie Baliunas

Marshall InstituteSenior Scientist

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Los AlamosNational Laboratories

Thomas L. Clancy, Jr.

Author

Will Happer

Princeton University

Willis Hawkins

Lockheed Martin (ret.)

Bernadine Healy

U.S. News & World Report

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President EmeritusGrove City College

Robert L. Sproull

University of Rochester (ret.)

Chauncey StarrElectric Power Research Institute

Documents

1.Study of Hydro Logical Cycle