The 21st Century climate challenge

Figure 1.2Global temperature forecast:three IPCC scenariosMean surface warming projections (°C)43210Dangerous climate changeRelative to preindustrial levelsIPCC scenario A1BIPCC scenario A2IPCC scenario B1Note: IPCC scenarios describe plausible future patterns ofpopulation growth, economic growth, technological changeand associated CO 2emissions. The A1 scenarios assume rapideconomic and population growth combined with reliance on fossilfuels (A1FI), non-fossil energy (A1T) or a combination (A1B).The A2 scenario assumes lower economic growth, lessglobalization and continued high population growth. The B1 andB2 scenarios contain some mitigation of emissions, throughincreased resource efficiency and technology improvement (B1)and through more localized solutions (B2).Source: IPCC 2007a.Uncertainty rangeRelative to 1990 levels2000 2025 2050 2075 2100Heading for dangerous climate changeIn two important respects the IPCC’s bestestimaterange for the 21 st Century mightunderstate the problem. First, climate changeis not just a 21 st Century phenomenon.Temperature adjustments to rising concentrationsof CO 2and other greenhouse gases willcontinue to take place in the 22 nd Century.Second, IPCC best-estimates do not rule outthe possibility of higher levels of climate change.At any given level of stabilization, there is a probabilityrange for exceeding a specified temperature.Illustrative probability ranges identified inmodelling work include the following:• Stabilization at 550 ppm, which is belowthe lowest point on the IPCC scenarios,would carry an 80 percent probability of3.52.51.50.5–0.5overshooting the 2°C dangerous climatechange threshold. 40• Stabilization at 650 ppm carries a probabilityof between 60 and 95 percent of exceeding3°C. Some studies predict a 35–68 percentlikelihood of overshooting 4°C. 41• At around 883 ppm, well within the IPCCnon-mitigation scenario range, there wouldbe a 50 percent chance of exceeding a 5°Ctemperature increase. 42Probability ranges are a complex device forcapturing something of great importance forthe future of our planet. An increase in averageglobal temperature in excess of 2–3°C wouldbring with it enormously damaging ecological,social and economic impacts. It would alsocreate a heightened risk of catastrophic impacts,acting as a trigger for powerful feedback effectsfrom temperature change to the carbon cycle.Temperature increases above 4–5°C wouldamplify the effects, markedly increasingthe probability of catastrophic outcomesin the process. In at least three of the IPCCscenarios, the chances of exceeding a 5°Cincrease are greater than 50 percent. Putdifferently, under current scenarios, there isa far stronger likelihood that the world willovershoot a 5°C threshold than keep withinthe 2°C climate change threshold.One way of understanding these risks is toreflect on what they might mean in the lives ofordinary people. We all live with risks. Anybodywho drives a car or walks down a street faces avery small risk of an accident that will createserious injury. If the risk of such an accidentincreased above 10 percent most people wouldthink twice about driving or taking a stroll:a one in ten chance of serious injury is not anegligible risk. If the odds on a serious accidentincreased to 50:50, the case for embarking uponserious risk reduction measures would becomeoverwhelming. Yet we are on a greenhousegas emission course that makes dangerousclimate change a virtual certainty, with a veryhigh risk of crossing a threshold for ecologicalcatastrophe. This is an overwhelming case forrisk reduction, but the world is not acting.In the course of one century or slightlymore, there is a very real prospect that current1The 21 st Century climate challengeHUMAN DEVELOPMENT REPORT 2007/2008 35