The 21st Century climate challenge

judgements over what is an unacceptable costin social, economic and ecological terms at anygiven level of warming. For millions of peopleand for many ecosystems the world has alreadypassed the danger threshold. Determining whatis an acceptable upper-limit target for futureglobal temperature increases raises fundamentalquestions about power and responsibility. Theextent to which those facing the greatest risks areable to articulate their concerns, and the weightattached to their voice, matters a great deal.Yet with all of these caveats any successfulclimate change mitigation effort has to start byestablishing a target. Our starting point is thegrowing consensus among climate scientistson the threshold marker for dangerous climatechange. That consensus identifies 2°C (3.6°F) asa reasonable upper-bound. 16Beyond this point, the future risks ofcatastrophic climate change rise sharply.Accelerated melting of the Greenland and WestAntarctic ice sheets could set in train irreversibleprocesses, leading eventually to sea levels risingby several metres—an outcome that would causeforced human resettlement on a vast scale. Largeareas of rainforest could be transformed intosavannah. The world’s already shrinking glacierswould be set on course for rapid decline. Above the2°C threshold, the pressure on ecological systemssuch as coral reefs and biodiversity would intensify.Complex carbon on biodiversity feedback effectslinked to the warming of the oceans, the loss ofrainforests and melting ice sheets would acceleratethe pace of climate change.Crossing the 2°C threshold would be a stepacross the boundary that marks significant riskof catastrophic outcomes for future generations.More immediately, it would trigger setbacks inhuman development. Developing countries are ata double disadvantage in this area: they are locatedin tropical areas that stand to experience some ofthe most severe early impacts from climate change;and agriculture—the sector most immediatelyaffected—plays a far greater social and economicrole. Above all, they are characterized by highlevels of poverty, malnutrition and disadvantage inhealth. The combination of acute deprivation onthe one side, with weak social insurance provisionand limited infrastructural capacity to containclimate risks on the other, points to a high potentialfor human development reversals.From climate change to stalledhuman progress—the transmissionmechanismsClimate change is global but the effects will belocal. Physical impacts will be determined bygeography and microlevel interactions betweenglobal warming and existing weather patterns.The immense scope of these impacts makesgeneralization difficult: drought-prone areas insub-Saharan Africa will face different problemsfrom flood-prone areas in South Asia. Humandevelopment impacts will also vary as changesin climate patterns interact with pre-existingsocial and economic vulnerabilities. However,five specific risk-multipliers for human developmentreversals can be identified:• Reduced agricultural productivity. Aroundthree-quarters of the world’s populationliving on less than US$1 a day dependdirectly on agriculture. Climate changescenarios point to large losses in productivityfor food staples linked to drought andrainfall variation in parts of sub-SaharanAfrica and South and East Asia. Projectedrevenue losses for dryland areas insub-Saharan Africa amount to 26 percentby 2060, with total revenue losses of US$26billion (in constant 2003 terms)—in excess ofbilateral aid transfers to the region. Throughits impact on agriculture and food security,climate change could leave an additional600 million facing acute malnutrition bythe 2080s over and above the level in ano-climate change scenario. 17• Heightened water insecurity. Exceeding the2°C threshold will fundamentally change thedistribution of the world’s water resources.Accelerated glacial melt in the Himalayaswill compound already severe ecologicalproblems across northern China, India andPakistan, initially increasing floods beforereducing the flow of water to major riversystems vital for irrigation. In Latin America,accelerated melting of tropical glaciers willthreaten water supplies for urban populations,agriculture and hydroelectricity,Through its impact onagriculture and foodsecurity, climate changecould leave an additional600 million facing acutemalnutrition by the 2080s1The 21 st Century climate challengeHUMAN DEVELOPMENT REPORT 2007/2008 27