Holes in Climate Science

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The e-mails leaked from the University

of East Anglias Climatic Research Unit(CRU) in November presented an early Christmas present to climate-change

denialists. Amid the more than 1,000 messageswere several controversial comments that taken out of context seemingly indicate thatclimate scientists have been hiding a mound of dirty laundry from the public.

A fuller reading of the e-mails from CRU inNorwich, UK, does show a sobering amount of rude behaviour and verbal faux pas, but noth-ing that challenges the scientific consensus of climate change. Still, the incident provides agood opportunity to point out that as in any

active field of inquiry there are some majorgaps in the understanding of climate science.In its most recent report in 2007, the Intergov-ernmental Panel on Climate Change (IPCC)highlighted 54 key uncertainties that compli-cate climate science.

Such a declaration of unresolved problemscould hardly be called hidden. And some of these such as uncertainties in measurementsof past temperatures have received consid-erable discussion in the media. But other gapsin the science are less well known beyond thefields circle of specialists. Such holes do notundermine the fundamental conclusion that

humans are warming the climate, which isbased on the extreme rate of the twentieth-

century temperature changes and the inability

of climate models to simulate such warmingwithout including the role of greenhouse-gaspollution. The uncertainties do, however, ham-per efforts to plan for the future. And unlike themyths regularly trotted out by climate-changedenialists (see Enduring climate myths, page286), some of the outstanding problems may mean that future changes could be worse thancurrently projected.

Researchers say it is difficult to talk openly about holes in understanding.Of course there are gaps inour knowledge about Earthsclimate system and its com-

ponents, and yes, nothing hasbeen made clear enough to thepublic, says Gavin Schmidt, aclimate modeller at NASAsGoddard Institute for SpaceStudies in New York and oneof the moderators and con-tributors to the influentialRealClimate blog. But thisclimate of suspicion were working in is insane.Its really drowning our ability to soberly com-municate gaps in our science when somepeople cry fraud and misconduct for theslightest reasons.

Naturehas singled out four areas regionalclimate forecasts, precipitation forecasts,

aerosols and palaeoclimate data that some

say deserve greater open discussion, both withinscientific circles and in the public sphere.

Regional climate predictionThe sad truth of climate science is that themost crucial information is the least reliable.To plan for the future, people need to know how their local conditions will change, nothow the average global temperature will climb.Yet researchers are still struggling to develop

tools to accurately forecastclimate changes for the twenty-first century at the local andregional level.

The basic tools used tosimulate Earths climate aregeneral circulation models(GCMs), which representphysical processes in the glo-bal atmosphere, oceans, icesheets and on the lands sur-face. Such models generally have a resolution of about 13

in latitude and longitude too coarse to offermuch guidance to people. So climate scientistssimulate regional changes by zooming in onglobal models using the same equations, butsolving them for a much larger number of grid

points in particular locations.However, increasing the resolution in this

THE REAL HOLES INCLIMATE SCIENCE

Like any other field, research on climate change has some fundamentalgaps, although not the ones typically claimed by sceptics. Quirin

Schiermeier takes a hard look at some of the biggest problem areas.

This climate ofsuspicion were

working in is insane.Its drowning ourability to soberlycommunicate gaps inour science.

Gavin Schmidt

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way can lead to problems. Zooming in fromGCMs bears the risk of blowing up any inher-ent weakness of the mother model. If the

model does a poor job of simulating certainatmospheric patterns, those errors will be com-pounded at the regional level. Most expertsare therefore cautious when asked to makeregional predictions.

Our current climate models are just not upto informed decision-making at the resolu-tion of most countries, says Leonard Smith, astatistician and climate analyst at the LondonSchool of Economics and Political Science.

You need to be very circumspect aboutthe added value of downscaling to regionalimpacts, agrees Hans von Storch, a climatemodeller at the GKSS Institute for Coastal

Research in Geesthacht, Germany, who hasrecently contributed to a regional climate assess-ment of the Hamburg metropolitan region. If the simulations project future changes in linewith the trends already observed, von Storchhas more confidence in them. But if researchersrun the same model, or an ensemble of models,multiple times and the results diverge from eachother or from the observed trends, he cautions,planners should handle them with kid gloves.Whenever possible, theyd rather wait withspending big money on adaptation projectsuntil there is more certainty about the thingsto come.

Downscaled climate models face particularuncertainty problems dealing in regions with

complex topography, such as where mountainsform a wall between two climatically differentplains. Another potential source of error comesfrom projections concerning future green-house-gas emissions, which vary depending onassumptions about economic developments.

All the problems, however, do not makeregional simulations worthless, as long astheir limitations are understood. They arealready being used by planners at the local andnational levels (see graphs, left). Simulationsremain an important tool for understand-ing processes, such as changes in river flow,that global models just cannot resolve, saysJonathan Overpeck, a climate researcher at theUniversity of Arizona in Tucson. Overpeck ispart of a research team that is using statisticaltechniques to narrow down divergent modelprojections of how much average water flow in the Colorado River will decrease by 2050.Researchers hope that by improving how they simulate climate variables such as cloud cov-erage and sea surface temperatures, they willfurther reduce the uncertainties in regionalforecasts, making them even more useful forpolicy-makers.

PrecipitationRising global temperatures over the next few decades are likely to increase evaporation andaccelerate the global hydrological cycle a

change that will dry subtropical areas andincrease precipitation at higher latitudes.These trends are already being observed andalmost all climate models used to simulateglobal warming show a continuation of thisgeneral pattern 1.

Unfortunately, when it comes to precipita-tion, that is about all the models agree on. Thedifferent simulations used by the IPCC in its2007 assessment offer wildly diverging picturesof snow and rainfall in the future (see graphic,right). The situation is particularly bad forwinter precipitation, generally the most impor-tant in replenishing water supplies. The IPCC

simulations failed to provide any robust projec-tion of how winter precipitation will change atthe end of the current century for large partsof all continents 2.

Even worse, climate models seemingly underestimate how much precipitation haschanged already further reducing confi-dence in their ability to project future changes.A 2007 study 3, published too late to be includedinto the last IPCC report, found that precipita-tion changes in the twentieth century bore theclear imprint of human influence, includingdrying in the Northern Hemisphere tropicsand subtropics. But the actual changes were

larger than estimated from models a find-ing that concerns researchers.

If the models do systematically under-estimate precipitation changes that wouldbe bad news, because the existing forecastswould already cause substantial problems,says Gabriele Hegerl, a climate-system scien-tist at the University of Edinburgh, UK, and aco-author on the paper. This is, alas, a verysignificant uncertainty, she says.

Climate scientists think that a main weak-ness of their models is their limited ability tosimulate vertical air movement, such as con- vection in the tropics that lifts humid air intothe atmosphere. The same problem can tripup the models for areas near steep mountainranges. The simulations may also lose accuracybecause scientists do not completely under-stand how natural and anthropogenic aerosolparticles in the atmosphere influence clouds.Data on past precipitation patterns around theglobe could help modellers to solve some ofthese issues, but such measurements are scantin many areas. We really dont know natural variability that well, particularly in the trop-ics, says Hegerl.

The uncertainties about future precipita-tion make it difficult for decision-makersto plan, particularly in arid regions such asthe Sahel in Africa and southwestern NorthAmerica. Mega-droughts lasting several decades have struck these areas in the past andare expected to happen again. But the models

in use today do a poor job of simulating suchlong-lasting droughts. Thats pretty worry-ing, says Overpeck.

Increasing the resolution of models will notbe enough to resolve the convective processesthat lead to precipitation. To forecast pre-cipitation more accurately, researchers aretrying, among other things, to improve thesimulation of key climate variables such as theformation and dynamics of clouds. Further-more, high-resolution satellite observationsare increasingly being used to validate andimprove model realism.

Projections of precipitation change for 209099.Blue indicates increases in precipitation andbrown denotes drying. White represents areas ofuncertainty, where less than two-thirds of modelsagreed on whether precipitation would increaseor decrease. Stippled areas indicate where 90%of the models agreed on the sign of the change.

10.5

50302015

1050+5+10

+50%+30+20+15

0+0.5+1+1.5+2+2.5+3+3.5+4+5+7+10 C

Climate simulations for Europe for the end ofthe current century suggests warming (top) ofmore than 3 C relative to the end of the twentiethcentury. Precipitation projections (bottom)indicate drying of southern Europe and wetterconditions in northern Europe.

I P C C C L I M A T E C H A N G E 2 0 0 7 : T H E P H Y S I C A L S C I E N C E B A S I S

. C H

. 1 1 ( 2 0 0 7 )

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AerosolsAtmospheric aerosols airborne liquid or

solid particles are a source of great uncer-tainty in climate science. Despite decades of intense research, scientists must still resort tousing huge error bars when assessing how par-ticles such as sulphates, black carbon, sea saltand dust affect temperature and rainfall.

Overall, it is thought that aerosols cool cli-mate by blocking sunlight, but the estimates of this effect vary by an order of magnitude, withthe top end exceeding the warming power of all the carbon dioxide added to the atmosphereby humans.

One of the biggest problems is lack of data.We dont know whats in the air, says Schmidt.

This means a major uncertainty over key processes driving past and future climate.To measure aerosols in the sky, satellite and

ground-based sensors detect the scattering andabsorption of solar radiation. But researcherslack enough of this kind of data to completea picture of aerosols across the globe. And acomplex set of coordinated experiments isrequired to determine how aerosols alter cli-mate processes.

Some aerosols, such as black carbon, absorbsunlight and produce a warming effect thatmight also inhibit rainfall. Other particles suchas sulphates exert a cooling influence by reflect-

ing sunlight. The net effect of aerosol pollutionon global temperature is not well established.

And various studies have produced conflictingconclusions over whether global aerosol pollu-

tion is increasing or decreasing.The relationship between aerosols andclouds adds another layer of complication.Before a cloud can produce rain or snow, raindrops or ice particles must form and aerosolsoften serve as the nuclei for condensation. Butalthough some aerosols enhance cloudiness,others seem to reduce it. Aerosols could alsohave a tremendous impact on temperaturesby altering the formation and lifetime of low-level clouds, which reflect sunlight and cool theplanets surface.

Scientists have yet to untangle the interplay between pollution, clouds, precipitation and

temperature. However, NASAs Glory satellite,an aerosol and solar-irradiance monitoringmission scheduled for launch in October, will

provide some greatly anticipated data. Still,atmospheric researchers say that ground-based

sensors capable of determining the abundanceand composition of aerosols in the atmosphereare needed just as much.

The tree-ring controversyMany of the e-mails leaked from the CRUcomputers came from a particular group of cli-mate researchers who work on reconstructingtemperature variations over time. The e-mailsrevealed them discussing some of the uncertain-ties in centuries worth of climate informationgleaned from tree rings and other sources.

Records of thermometer measurements overthe past 150 years show a sharp temperature rise

during recent decades that cannot be explainedby any natural pattern. It is most likely to havebeen caused by anthropogenic greenhouse-gasemissions. But reliable thermometer recordsfrom before 1850 are scarce and researchersmust find other ways to reveal earlier tempera-ture trends.

Palaeoclimatology relies on records culledfrom sources such as tree rings, coral reefs, lakesediments, stalagmites, glacial movements andhistorical accounts. As trees grow, for exam-ple, they develop annual rings whose thicknessreflects temperature and rainfall. Proxies suchas these provide most knowledge of past cli-

mate fluctuations, such as the Medieval WarmPeriod from about 800 to 1300 and the Little

Climate scientists grapple with realuncertainties, but those who doubtthe reality of human-driven globalwarming usually ignore thoseissues and, instead, perpetuate aseries of claims that do not holdup to scrutiny. What follows is aselection of myths about climate:

Climate models cant provideuseful information about the realworld.Models can reproduce much ofthe climate variation over thepast millennium, but projections

for the future are subject to well-described uncertainties, both inthe understanding of climate andin estimates of future economicdevelopment. They cannottherefore provide decision-makerswith exact information of the rateof future changes, but they canoffer useful general informationand they unconditionally predict awarmer world.

Global warming stoppedten years ago.Climate is not weather. The climateis the multi-decade average of theconstantly changing state of theatmosphere. Natural variationscan cause temperatures to riseand fall from year to year ordecade to decade. Although globaltemperatures did not rise as quicklyin the past decade as in previousones, the most recent decade wasthe warmest on record.

Temperatures were higher in

pre-industrial times.The consensus of proxy-basedreconstructions of pre-industrialclimate is that the second halfof the twentieth century wasprobably warmer than any otherhalf-century in more than amillennium. Warmer periods didoccur in the more distant past,albeit under different orbital andgeological conditions. In any

case, warm spells in the past donot disprove human influence onclimate today. The cause of anyparticular climate change needs tobe investigated separately.

Temperature records taken in thelower atmosphere indicate thatthe globe is not warming.A decade ago, there seemed to bea discrepancy between surface andtropospheric temperatures. Butthis issue was resolved when long-standing calibration problems withsatellite sensors were discovered.

Satellite measurements show thatthe lower atmosphere is warmingat a rate consistent with thepredictions of climate models.

A few degrees of warming are nota big deal.In the most recent ice age, the worldwas only a few degrees cooler onaverage than it is today. The currentrate of warming is in all likelihood

unique in the history of humankind.There may be no such thing as anoptimum temperature for theplanet, but modern human societiesare adapted to the weather patternsand sea levels of the past millennia.The rapidity of global warmingsubstantially adds to the problem.

Measured increases intemperature reflect the growthof cities around weather stationsrather than global warming.Climate researchers have takengreat care to correct for the impact

of urbanization in temperaturerecords by matching data frommore-urban stations with data fromrural ones. Moreover, some of thelargest temperature anomalies onEarth occur in the least populatedareas, including around the Arcticand the Antarctic Peninsula.Measurements also show warmingof the surface ocean and deepermarine layers. Q.S.

Enduring climate myths

The thin white lines show how aerosols from shipexhausts brighten clouds over the Atlantic Ocean.

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Ice Age, centred on the year 1700.When proxy records for the Northern

Hemisphere are stitched together, they show a pattern resembling a hockey stick, with tem-peratures rising substantially during the latetwentieth century above the long-term meanconditions. This type of work was pioneeredin 1998 by Michael Mann, a climate researcherthen at the University of Virginia in Charlottes- ville, and his co-authors 4. In a subsequentpublication 5, they concluded that the decadeof the 1990s was probably the warmest decade,and 1998 the warmest year, in at least a millen-nium. That work figured prominently in the2001 assessment by the IPCC.

But the use and interpretation of such proxy records has generated considerable contro- versy. One notable critic, Stephen McIntyre, aretired Canadian mining consultant and edi-tor of the Climate Audit blog, has spent muchof the past decade challenging the work of Mann and other scientists whose e-mails wereleaked. McIntyre has doggedly attacked theproxy records 6, particularly the statistics usedto analyse tree-ring data.

Many scientists are tired of the criticisms,and the IPCC concluded that it is likely thatthe second half of the twentieth century wasthe warmest 50-year period in the NorthernHemisphere during the past 1,300 years. Butlegitimate questions remain about paleoclimate

proxies, according to the IPCC7

.Climate scientists are worried in particularabout tree-ring data from a few northern sites.By examining temperature measurements fromnearby, researchers know thattree growth at these locationstracked atmospheric tempera-tures for much of the twentiethcentury and then diverged fromthe actual temperatures duringrecent decades. It may be thatwhen temperatures exceed acertain threshold, tree growthresponds differently.

The divergence issue alsomade an appearance in the CRUaffair. In the most frequently quoted of theCRU e-mails, the former director of the centre,Phil Jones, mentioned a trick namely usingactual observations of late-twentieth-century temperatures instead of tree ring data tohide the decline in the response of trees to thewarming temperatures.

On the surface, Joness phrasing seems damn-ing. Indeed, a graph of Northern Hemispheretemperature produced for the World Meteoro-logical Organization in 2000 with Joness helpfails to make clear that instrumental records

from the nineteenth and twentieth centurieswere spliced onto proxy data for the past mil-

lennium because of the divergence issue. Thefigure did, however, contain clear references topapers that discussed the divergence issue.

They show what was, at the time, the bestestimate of how temperatures evolved overtime, says Hegerl. However, with hindsight,they could have been a bit clearer how this wasdone, given the high profile that figures likethis can have.

Aside from the issue of clarity, the decision to

exclude the tree-ring records that diverge fromthe instrumental data makes sense, says Tho-mas Stocker, co-chair of the IPCCs workinggroup on the physical basis of climate change.

The tree ring divergence prob-lem is restricted to a few high-latitude regions in the NorthernHemisphere and is not ubiqui-tous even there, he says.

Still, the divergence issueremains a source of debatewithin the scientific community.Im worried about what causesthe divergence, says Hegerl.

As long as we dont understandwhy they diverge, we cant besure that they accurately represent the past. Soimproving the usefulness of proxies will requirea better understanding of how different speciesof trees grow and respond to climate change.

Another outstanding problem in proxy research is the large range of uncertainty fortemperatures from before about 1500. Studiespublished in 2004 (ref. 8) and 2005 (ref. 9), basedon a combination of proxies of different resolu-tion, suggest that fluctuations in global tempera-ture during the past millennium may have beenlarger than initially thought. However, these

studies still show late twentieth century warm-ing to be unprecedented, says von Storch. And

the most recent decade was warmer still.Even with ongoing questions about the proxy

data, the IPCCs key statement that most ofthe warming since the mid-twentieth century is very likely to be due to human-causedincreases in greenhouse-gas concentration remains solid because it rests on multiple linesof evidence from different teams examiningmany aspects of the climate system, says SusanSolomon, the former co-chair of the IPCC team

that produced the 2007 physical science reportand a climate researcher with the US NationalOceanic and Atmospheric Administration inBoulder, Colorado.

The IPCCs team of scientists, she sayswould not have said that warming is unequiv-ocal based on a single line of evidence evenif came from Moses himself. Quirin Schiermeier is a senior reporter withNature based in Munich.1. Meehl, G.A.et al. in Climate Change 2007: The Physical

Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel onClimate Change (eds Solomon, S. et al.) Ch. 10, 760789(Cambridge Univ. Press, 2007).

2. IPCCClimate Change 2007: The Physical Science Basis.Contribution of Working Group I to the Fourth AssessmentReport of the Intergovernmental Panel on Climate Change (Cambridge Univ. Press, 2007).

3. Zhang, X.et al. Nature 448, 461465 (2007).4. Mann, M. E., Bradley, R. S. & Hughes, M. K.Nature 392,

779787 (1998).5. Mann, M. E., Bradley, R. S. & Hughes, M. K.Geophys. Res.

Lett. 26, 759762 (1999).6. McIntyre, S. & McKitrick, R.Geophys. Res. Lett. 32, L03710

(2005).7. Jansen, E.et al. in Climate Change 2007: The Physical

Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on ClimateChange (eds Solomon, S. et al.) Ch. 6, 466475 (CambridgeUniv. Press, 2007).

8. Von Storch, H. et al. Science 306, 679682 (2004).9. Moberg, A., Sonechkin, D. M., Holmgren, K., Datsenko,

N. M. & Karln, W. Nature 433, 613617 (2005).

See Editorial, page 269.

oC

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Year1600 1800 2000

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Northern Hemisphere temperature estimates made using data from tree rings and other sources graphed relative to the average of 196190. The curve includes data from 10 studies, with darkercolours representing greater consensus. There is considerable disagreement about conditions around

1000AD; most studies indicate temperatures were substantially cooler then, but a few suggestwarming comparable to that of the twentieth century. There is much stronger agreement for the warmconditions since 1900. The heavy black line is temperature recorded by thermometers.

As long as we dontunderstand why therecords diverge, wecant be sure thatthey accuratelyrepresent the past.

Gabriele Hegerl

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