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speak on behalf of the scientificcommunity that carries out assessmentsof all aspects of climate change under the collective direction of all thegovernments of the world, which are members of the IPCC. The assessments of the Panel involve a mammothhuman effort. To appreciate the scale and complexityof this effort may I mention that the FourthAssessment Report (AR4) of the IPCC completed in2007 involved approximately 3,750 experts includinglead authors, contributing authors and expertreviewers, all of whom volunteered their time withoutcompensation by the IPCC. The AR4 referred toapproximately 18,000 items of published literatureand dealt with about 90,000 comments provided atvarious stages of drafting by reviewers fromgovernments and the scientific community.Let me highlight two important findings of the AR4:"Warming of the climate system is unequivocal as isnow evident from observations of increases in globalaverage air and ocean temperature, widespreadmelting of snow and ice and rising global average sealevel;" and: "Most of the observed increase in globalaverage temperatures since the mid twentieth centuryis very likely due to the observed increase inanthropogenic GHG concentrations. It is likely thatthere has been significant anthropogenic warming over the past 50 years averaged over each continent(except Antarctica)".The AR4 found that the resilience of many ecosystemsis likely to be exceeded this century by anunprecedented combination of climate change andother global change drivers. Over the course of thiscentury, net carbon uptake by terrestrial ecosystems islikely to peak before mid century and then weaken oreven reverse thus amplifying climate change.Approximately 20 to 30 per cent of plant and animalspecies assessed so far are likely to be at increased risk of extinction if increases in global averagetemperatures exceed 1.5 to 2.5°C. Anthropogenicwarming could lead to some impacts that are abrupt or irreversible depending upon the rate and magnitudeof the climate change. Partial loss of ice sheets on polar land could imply meters of sea level rise,major changes in coast lines and inundation of low-lying areas, with greatest effects in river deltas andlow-lying islands.It is noted that the Copenhagen Accord aimed "to holdthe increase in global temperatures below 2°C" andrecognised "that deep cuts in global emissions arerequired" and countries "should co-operate inachieving the peaking of global and national emissionsas soon as possible". In fact, the least cost trajectoryfor stringent mitigation assessed in the AR4 clearlyestimated that global emissions should peak no laterthan 2015 and decline thereafter.The AR4 assessed a wide range of impacts based on past observations and projected those that are likelyto occur in the future for different levels andmagnitudes of climate change. Some of these areextremely important to bear in mind, because indeed the ultimate objective of the UNFCCC as stated in Article 2 is to prevent dangerousanthropogenic interference with the climate system.Science cannot determine what constitutes"dangerous", but it can provide substantial scientificevidence and insights on the basis of which negotiatorscan determine how to integrate this information in thecontext of Article 2.THE SCIENCE OF CLIMATE CHANGE056GLOBAL VOICESIDR RAJENDRA K. PACHAURI, CHAIRMAN, INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC)Photo: © European Union 2011

GLOBAL VOICES057? To achieve that goal, mitigation efforts andinvestments over the next two to three decades willhave a large impact on opportunities to achieve lowerstabilisation levels. Delayed emissions reductionsignificantly constrains the opportunities to achievelower stabilisation levels and increases the risk of moresevere climate change impacts. Neither adaptation notmitigation alone can avoid all climate change impacts;however they can complement each other and togethercan significantly reduce the risks of climate change. Responding to climate change, therefore, involves aniterative risk management process that includes bothadaptation and mitigation and takes into accountclimate change damages, co-benefits, sustainability,equity and attitudes. Changes in lifestyle and behaviorpatterns can contribute to climate change mitigationacross all sectors. Policies that provide a real orimplicit price of carbon could create incentives forproducers and consumers to significantly invest in lowGHG products, technologies and processes. Mitigation options are associated with a range of co-benefits, which include lower levels of air pollution andassociated health benefits, higher levels of energysecurity, higher levels of employment and higher levelsof agricultural production. The AR4 has assessed thatfor a stabilisation level of between 445 to 535 ppm ofCO2equivalent the reduction of average annual GDPgrowth rates up to 2030 would be less than 0.12 percent. The range of global GDP reduction in 2030,therefore, would be less than three per cent as part ofa least cost trajectory towards different long termstabilisation levels. The association of co-benefits,such as those related to the objectives of development,sustainability and equity should also be seen in thecontext of estimated costs.The AR4 has assessed a number of mitigationportfolios for achieving stabilisation of GHGconcentrations, and energy conservation andefficiency are some of the most attractive optionsavailable. It was also found that the buildings sectorBelow: A supervisor using a 3D viewer with a panoramic view ofthe Earth during theUnited Nations ClimateChange Conference inCancun 2010