12 LTT462 / 15 February – 28 February 2007

feature www.lttonline.co.uk

Everyone’s clamouring to cut CO2

but what does the science say?

This month’s report by theIntergovernmental Panel onClimate Change points the fingerfirmly at man-made carbon diox-ide emissions for rising globalaverage temperatures and other

climate changes in the last 50 years. “Most ofthe observed increase in globally averagedtemperatures since the mid-20th century isvery likely due to the observed increase inanthropogenic greenhouse gas concentra-tions,” said the Summary for policy-makers(SPM) of the first volume of the fourthassessment report (AR4). In IPCC parlance“very likely” means there is a greater than90% (but less than 95%) likelihood of thestatement being correct. This represents astronger statement than the SPM of theIPCC’s third assessment report (TAR) pub-lished in 2001. It said most of the observedwarming over the last 50 years was “likely” (a66%-89% likelihood) to have been due to theincrease in greenhouse gas concentrations.

The AR4 report predicts that the contin-ued burning of fossil fuels at or above currentrates will cause further warming “and inducemany changes in the global climate systemduring the 21st century that would very likelybe larger than those observed during the 20thcentury.” There will be more hot extremes, adecline in Arctic and Antarctic sea ice, moreintense tropical cyclones, and extra-tropicalstorm tracks will move poleward.

Problems indeed, but none of this seemsquite on the scale of global catastrophe thatfeatures so prominently in the climate changecommentary provided by much of themedia, politicians, pressure groups and otherpundits. It’s hard to reconcile TheIndependent’s front page headline just before

the report’s release that “2,000 climate changescientists deliver the most terrifying reportever” with the measured tone of the reportitself, though whether this dichotomy saysmore about media hyperbole or the conser-vative language of scientists might be a matterof some debate.

In some important respects the latest SPMis less alarming than the one published as partof the TAR. The upper end of predicted sealevel rise has been cut by a third and majorchanges to the Atlantic Ocean’s thermohalinecirculation (which transports heat to the highlatitudes of the Northern Hemisphere) in the21st century are now regarded as “veryunlikely” and longer-term changes “cannot beassessed with confidence” (the TAR said thatbeyond 2100 it could shut down completely).

Nevertheless, New Scientist magazinereported last week that some of scientists’“more scary scenarios” were “left on the cut-ting room floor” as the IPCC strove to reachunanimous agreement on the report. “Thebenefit – that there is now little room left forsceptics – comes at what many see as a dan-gerous cost: many legitimate findings havebeen frozen out,” it said.

The SPM observes that :• Eleven of the last 12 years (1995-2006) rankamong the 12 warmest years in the record ofglobal surface temperature since 1850.• The global surface temperature is estimatedto have risen by 0.74ºC between 1906 and2005 [within a range 0.56 to 0.92ºC].• Sea level is estimated to have risen by 17cmduring the 20th century. The IPCC says thereis high confidence that the rate increasedbetween the 19th and 20th centuries but it isunclear whether there has been any accelerat-ing trend in the 20th century.

Arctic temperatures increased at almosttwice the global average rate in the past 100years, says the SPM. “Satellite data since 1978show that annual average Arctic sea ice extenthas shrunk by 2.7 [2.1 to 3.3]% per decade.”There have been increases in precipitationobserved in eastern parts of North and SouthAmerica, northern Europe and northern andcentral Asia. But drier conditions have beenobserved in the Sahel, the Mediteranean,southern Africa and parts of southern Asia.

Evidence on tropical cyclones (typhoonsand hurricanes) is ambiguous. “There isobservational evidence for an increase ofintense tropical cyclone activity in the NorthAtlantic since about 1970... [but] there is noclear trend in the annual numbers of tropicalcyclones,” says the SPM.

Attributing blame

Changes in the atmospheric concentra-tions of greenhouse gases, aerosols, in solar

The IPCC: bringing scientists and governments together

The IPCC was set up in 1988 by the World MeteorologicalOrganisation and the United Nations’ environmental pro-gramme with a remit to assess on a “comprehensive,

objective, open and transparent basis the scientific, technicaland socio-economic information relevant to understanding thescientific basis of risk of human-induced climate change, itspotential impacts and options for adaptation and mitigation”.

The reports are written by teams of authors that are nomi-nated by governments and international organisationsaccording to their expertise. The IPCC’s work takes place inthree workstreams. Working Group 1 examines the scientificaspects of the climate system and climate change (theSummary for policy-makers of which was published thismonth). Working Group 2 assesses the vulnerability of socio-economic and natural systems to climate change, negativeand positive consequences of climate change, and optionsfor adapting to it. Working Group 3 assesses options for limit-

ing greenhouse gas emissions and mitigating climate change.The latter two working groups will report later this year.

All the reports are reviewed by more specialists as well asby Governments. The Summaries for policy-makers are pre-pared concurrently with the main reports and are also subjectto expert and government review. Line-by-line approval of theSPM takes place in a plenary session but lead authors mustapprove any changes. The main report is then modified tomake it correspond with the SPMs. That explains why themain report of working group 1 has not yet been released.

The IPCC reports are widely seen to be the most authorita-tive assessments of the science of climate change. However,some critics say the process of preparing the SPMs allowsscientific evidence to be ‘cherry picked’. Last week, forinstance, the Fraser Institute, a Canadian free market think-tank, published its own Independent Summary forPolicy-makers which, its authors say, summarises the second

order draft of the IPCC working group 1 main report (thesame report used by the IPCC to write its SPM). The FraserInstitute’s report comes to somewhat different conclusions.It says the hypothesis that greenhouse gas emissions haveproduced a significant warming of the planet is credible butthat available evidence allows this to be “credibly disputed”.Furthermore, it says there is “no compelling evidence thatdangerous or unprecedented changes are underway”.

Meanwhile, the impression that the assessment reportsare shaped by hundreds of scientists is misleading, accordingto climate scientist Roger Pielke Snr of the University ofColorado, Boulder. “The assessment reports are managed byonly a small subset of climate scientists who often use a plat-form as Lead Author to promote their research and theirparticular perspective,” he says.

Independent summary for policymakers is available atwww.lttonline.co.uk

“Thedebateover thescience ofclimatechange iswell andtruly over.” EnvironmentsecretaryDavid Miliband(pictured)

More and more transport policies are being justified on the basis of cutting carbondioxide emissions, yet the science underpinning the actions is rarely discussed intransport circles.Andrew Forster looks at the latest UN report on climate change

LTT462 / 15 February – 28 February 2007 13feature

radiation and changes to land-use can all alterthe energy balance of the climate system,notes the SPM. Climate computer modelshave been used to try and explain how suchinfluences shape the global temperaturerecord. These models are not discussed indetail in the SPM but their importance inhelping to shape understanding wasexplained by Alan Thorpe, chief executive ofthe Government’s Natural EnvironmentResearch Council, during last month’s‘Climate Change Challenge’ run on theorganisation’s website. “The size of therecently observed global warming, over a fewdecades, is significantly greater than the natu-ral variations in long simulations with climatemodels if CO2 is kept at pre-industrial levels,”Thorpe explained. “Only if the human inputof greenhouse gases is included does the sim-ulated climate agree with what has beenrecently observed. Clearly factors currentlyunknown to science can’t be included but wehave no reason to suppose they exist.”

The SPM says changes in solar irradiancesince 1750 have caused a radiative forcing (theinfluence a factor has in altering the balanceof incoming and outgoing energy in the Earthatmosphere system) of only +0.12 Watts persquare metre. Of far greater importance, saysthe SPM, are the increasing global atmos-pheric concentrations of carbon dioxidebrought about largely by fossil fuel use andland-use change, and emissions of methaneand nitrous oxide arising from agriculture.

CO2 is the most important anthropogenicgreenhouse gas, says the SPM, with an esti-mated radiative forcing value of +1.66 wattsper square metre. The SPM says that globalatmospheric concentrations of CO2 rose froma pre-industrial level (in 1750) of 280 partsper million (ppm) to 379ppm in 2005 andgrew by 1.9ppm per year between 1995 and2005. Burning of fossil fuels is the primarysource of increased atmospheric CO2 emis-sions, it says, with land-use change providinganother significant but smaller contribution(the ratio was about 4:1 in the 1990s).

Looking ahead

The SPM estimates that a global averagesurface warming following a doubling of CO2concentrations is “likely [that is, more than66%] to be in the range 2 to 4.5ºC with a bestestimate of about 3ºC and is very unlikely[less than 10% chance] to be less than 1.5ºC.”

The final section of the 21-page SPM dis-cusses predictions of future changes inclimate, drawing on climate modelling.Critics believe that too much credence isgiven to the model results (see panel right)when some important aspects of the climatesystem are, by the IPCC’s own admission, stillpoorly understood (the SPM notes, forinstance, that there is still medium-lowunderstanding of important radiative forcingssuch as aerosols).

The media made much play of the 6.4ºCtemperature change cited in the SPM but thisis the upper limit of change predicted in a sce-nario in which CO2equivalent concentrationswould reach 1,550ppm by 2100 – a nearquadrupling from today’s levels of 430ppm.Furthermore, the SPM says the likely temper-ature range in this scenario is actually

2.4ºC-6.4ºC with a best estimate of 4ºC.Concentrations of 600ppm CO2e are pre-dicted to raise temperatures by 1.8ºC under abest estimate on a range of 1.1-2.9ºC.

The predicted sea level rise presented inthe SPM now ranges from just 18cm to 59cm(comparing 2090-2099 with 1980-1999)depending on the emission scenario mod-elled. This is a narrower band than the9-88cm presented in the TAR. The AR4emphasises that the new figures do notassume significant increases in melting of theGreenland and Antarctic ice sheets, which itsays is a possibility as global temperatures rise.

The SPM predicts that:• sea ice will shrink in both the Arctic andAntarctic. “In some projections, Arctic late-summer sea ice disappears almost entirely bythe latter part of the 21st century.”• hot extremes, heat waves and heavy precip-itation events will become more frequent.• tropical cyclones are likely to become moreintense, with larger peak wind speeds andmore heavy precipitation associated withongoing increases of tropical sea surfacetemperatures. Extra-tropical storm tracks areprojected to move poleward.• increases in the amount of precipitation arevery likely in high latitudes, while decreases

are likely in subtropical land regions.

Political consensus

Environment secretary David Milibandsaid the SPM represented “another nail in thecoffin of the climate change deniers and rep-resents the most authoritative picture to date,showing that the debate over the science ofclimate change is well and truly over.”Miliband’s statement is hard to defend scien-tifically because even if one acceptsanthropogenic warming there remain greatuncertainties about the size of impacts. Buthis statement is a more accurate portayal ofthe policy debate. Benny Peiser, a climatechange policy analyst at Liverpool JohnMoores University, says that with policy-makers around the world agreeing (for thetimebeing at least) that the science is settled,the debate is now on policy responses.“Froma policy perspective this is now about themost cost-effective approach to dealing withclimate change,” says Peiser. “Do we go forrevolutionary change as advocated by climatealarmists, or gradual adjustment as suggestedby climate moderates?”

Summary for policymakers is available atwww.lttonline.co.uk

“Theoversellingof modelsas robustprojectionsadds to theexistingpoliticis-ation ofclimatescience.” Roger PielkeSnr, Universityof Colorado

Cosmic rays and concerns about modelling

Last month the National Environment Research Councilinvited those sceptical of the science underpinning man’seffect on the climate to challenge NERC’s panel of experts

in a web-based Climate Change Challenge. Alan Thorpe, NERC’schief executive, told contributors: “We are confident about thegreenhouse effect. We are confident that warming is going on.We are confident that human activity is adding carbon dioxide tothe atmosphere. Carbon dioxide is a greenhouse gas. The onusis now on those who deny this to say why that additional green-house effect is not responsible for warming the planet.”

Although the perception is often of a polarised debate withinthe scientific community between a majority who endorse theIPCC viewpoint and a minority who attribute change to natural fac-tors, this oversimplifies a discussion with many different facetsand a diversity of views.

Some scientists do believe that natural factors can explain cli-mate change. In the book The chilling stars, to be published nextmonth, Danish physicist Henrik Svensmark and Nigel Calder, theformer editor of New Scientist, will point the finger at cosmicrays. Calder, writing in The Sunday Times last weekend, said:“More cosmic rays [equals] more clouds. The sun’s magneticfield bats away many of the cosmic rays and its intensificationduring the 20th century meant fewer cosmic rays, fewer clouds,and a warmer world... We are not exaggerating, we believe, whenwe subtitle the book ‘A new theory of climate change’.”

Climate scientist Roger Pielke Snr of the University ofColorado, Boulder believes the IPCC and policy-makers generallyare too focused on CO2 emissions. “Humans are significantlyaltering the global climate but in a variety of diverse waysbeyond the radiative effect of carbon dioxide,” says Pielke, aformer co-chief editor of the Journal of Atmospheric Science, onhis Climate Science weblog. He praises the analysis in the USNational Research Council’s 2005 report Radiative Forcing ofClimate Change, which concluded that changes in land-use andindustry emissions (aerosols) have much larger regional climateimpacts than revealed in the way the IPCC calculates radiativeforcing.

Pielke says the spatial concentration of aerosol emissions andland-use changes means that they present a greater threat ofbringing about ‘threshold’ changes to the climate system thanrising global CO2 concentrations. “As a simple example of this,we find a greater impact of sunlight on a piece of paper when wefocus it with a lens,” he told LTT. He says the emphasis on cut-

ting CO2 to control temperature is “scientifically flawed”, estimat-ing that man’s CO2 emissions have accounted for about 30% ofwarming up to the present – substantially lower than the IPCCestimates.

Pielke (who is a co-author of the book Human impacts onweather and climate, the second edition of which is to be pub-lished by Cambridge University Press this month) also disputesmany of the observational records that underpin the IPCC analy-sis, such as global surface land surface temperature, glacierretreat and ocean temperature. “The reported ‘warming’ from theHadley Centre/University of East Anglia’s Climatic Research Unitdata [the source of the IPCC’s 0.7ºC warming estimate] has awarm bias of significant value (certainly tenths of a degree) in itsconstruction,” he says. On glacier evidence he says recent peerreviewed research shows that “the general message that glaciersare receding almost everywhere is clearly not accurate when thedata is evaluated in detail”.

Climate physicist Professor Richard Lindzen of theMassachusetts Institute of Technology in the US disagrees vehe-mently with alarming global warming predictions. Lindzen, whowas a lead author for the IPCC’s third assessment report, saysthere is broad agreement that the world warmed in the 20th cen-tury, that CO2 is a greenhouse gas and that man has beenresponsible for recent increases in CO2. But he says the temper-ature change of a few tenths of a degree recorded in the late20th century is so small that it could be explained by nothingmore than “natural, internal, unforced variability”.

Lindzen says climate models vastly overestimate temperaturechanges resulting from increases in CO2. All other things beingequal, Lindzen says a doubling of CO2 should result in a globalmean warming of just 1ºC. “Alarming predictions all require thatwater vapour and clouds act so as to greatly amplify the impactof CO2,” he (and fellow critics) say in a recent critique of theStern Review published in World Economics. “But it is freelyacknowledged, including by the IPCC, that water vapour and espe-cially clouds are poorly modelled, while the underlying physics fordetermining their behaviour is missing or even unknown.”

Pielke believes many scientists, policy-makers, journalists andother commentators place too much confidence in climate modelresults. “The overselling of regional and global models as robustprojections rather than as sensitivity simulations, adds to theexisting politicisation of climate science and provides justifiablecriticism of the [IPCC] assessment reports,” he says.