The 21st Century climate challenge

since the advent of the industrial era—and therate of increase is quickening. There is overwhelmingscientific evidence linking the rise intemperature to increases in the concentrationof greenhouse gases in the Earth’s atmosphere.There is no hard-and-fast line separating‘dangerous’ from ‘safe’ climate change. Many ofthe world’s poorest people and most fragile ecologicalsystems are already being forced to adaptto dangerous climate change. However, beyonda threshold of 2°C the risk of large-scale humandevelopment setbacks and irreversible ecologicalcatastrophes will increase sharply.Business-as-usual trajectories will take theworld well beyond that threshold. To have a50:50 chance of limiting temperature increaseto 2°C above preindustrial levels will requirestabilization of greenhouse gases at concentrationsof around 450ppm CO 2e. Stabilizationat 550ppm CO 2e would raise the probabilityof breaching the threshold to 80 percent. Intheir personal lives, few people would knowinglyundertake activities with a serious injuryrisk of this order of magnitude. Yet as a globalcommunity, we are taking far greater risks withplanet Earth. Scenarios for the 21 st Centurypoint to potential stabilization points in excessof 750ppm CO 2e, with possible temperaturechanges in excess of 5°C.Temperature scenarios do not capture thepotential human development impacts. Averagechanges in temperature on the scale projectedin business-as-usual scenarions will triggerlarge-scale reversals in human development,undermining livelihoods and causing massdisplacement. By the end of the 21 st Century,the spectre of catastrophic ecological impactscould have moved from the bounds of the possibleto the probable. Recent evidence on theaccelerated collapse of ice sheets in the Antarcticand Greenland, acidification of the oceans, theretreat of rainforest systems and melting of Arcticpermafrost all have the potential—separately orin interaction—to lead to ‘tipping points’.Countries vary widely in their contributionto the emissions that are driving up atmosphericstocks of greenhouse gases. With 15 percent ofworld population, rich countries account foralmost half of emissions of CO 2. High growthin China and India is leading to a gradual convergencein ‘aggregate’ emissions. However, percapita carbon footprint convergence is morelimited. The carbon footprint of the UnitedStates is five times that of China and over 15times that of India. In Ethiopia, the average percapita carbon footprint is 0.1 tonnes of CO 2compared with 20 tonnes in Canada.What does the world have to do to get onan emissions trajectory that avoids dangerousclimate change? We address that question bydrawing upon climate modeling simulations.These simulations define a carbon budget forthe 21 st Century.If everything else were equal, the globalcarbon budget for energy-related emissionswould amount to around 14.5 Gt CO 2annually.Current emissions are running at twice thislevel. The bad news is that emissions are on arising trend. The upshot: the carbon budget forthe entire 21 st Century could expire as early as2032. In effect, we are running up unsustainableecological debts that will lock future generationsinto dangerous climate change.Carbon budget analysis casts a new light onconcerns over the share of developing countriesin global greenhouse gas emissions. While thatshare is set to rise, it should not divert attentionfrom the underlying responsibilities of richnations. If every person in the developing worldhad the same carbon footprint as the averageperson in Germany or the United Kingdom,current global emissions would be four timesthe limit defined by our sustainable emissionspathway, rising to nine times if the developingcountry per capita footprint were raised toCanadian or United States levels.Changing this picture will require deepadjustments. If the world were a single countryit would have to cut emissions of greenhousegases by half to 2050 relative to 1990 levels,with sustained reductions to the end of the21 st Century. However, the world is not a singlecountry. Using plausible assumptions, we estimatethat avoiding dangerous climate changewill require rich nations to cut emissions by atleast 80 percent, with cuts of 30 percent by 2020.Emissions from developing countries would peakaround 2020, with cuts of 20 percent by 2050.By the end of the21 st Century, the spectreof catastrophic ecologicalimpacts could have movedfrom the bounds of thepossible to the probableHUMAN DEVELOPMENT REPORT 2007/2008 7