The 21st Century climate challenge

personal and social circumstances”. 23 Climatechange is a profound denier of freedom ofaction and a source of disempowerment. Onesection of humanity—broadly the poorest1.2 Climate science and future scenariosUnderstanding the scientific evidence on climatechange is a starting point for understandingthe human development challenges of the21 st Century. There is a vast amount of scientificliterature on the subject. Here we focus on theconsensus set out by the IPCC, while drawingattention to the large areas of uncertaintyover future outcomes. In looking at the futureunder climate change there are many ‘knownunknowns’—events that can be predicted butwithout any certainty as to their timing ormagnitude. It should come as no surprise thatscientists cannot be certain about precisely howthe Earth’s ecological systems will respond tohuman-induced greenhouse gas emissions: weare living with an experiment that has neverbeen conducted before.One of the ‘knowns’ is that we are on atrajectory that, if uncorrected, will lead to a veryhigh probability of dangerous climate changeoutcomes. Those outcomes would provide acontinuum from near-term human developmentsetbacks to long term ecological disaster.Human-induced climate changeThroughout its history, the earth has experiencedoscillations between warm and coolperiods. These shifts in climate have beentraced to a wide variety of ‘climate forcings’,including orbital variations, solar fluctuations,volcanic activity, water vapour, and the atmosphericconcentration of greenhouse gases, suchas CO 2. The changes that we see happeningtoday are occurring at a more rapid rate, withstronger magnitudes and patterns that cannotbe explained by natural cycles.Average global surface temperature isthe most fundamental measure of climate2.6 billion—will have to respond to climatechange forces over which they have no control,manufactured through political choices incountries, where they have no voice.change. Temperatures in the past half-centuryhave probably been the highest of any 50-yearperiod for the past 1,300 years. The world isnow at or near the warmest level on recordin the current interglacial period, whichbegan around 12,000 years ago. There isstrong evidence that the process is accelerating.Eleven of the twelve warmest years since1850 occurred between 1995 and 2006. Overthe past 100 years the Earth has warmed by0.7°C. There are large interannual variations.However, on a decade-by-decade basis, thelinear warming trend for the past 50 yearsis nearly twice that for the past 100 years(figure 1.1). 24There is an overwhelming body of scientificevidence linking rising temperatures to increasedatmospheric concentrations of CO 2and othergreenhouse gases. The effect of these gases inthe atmosphere is to retain part of the outgoingsolar radiation, thereby raising the temperatureof the Earth. This natural ‘greenhouse effect’is what keeps our planet habitable: without it,the planet would be 30°C colder. Throughoutthe Earth’s four previous glacial and warmingcycles, there has been a high correlationbetween atmospheric concentrations of CO 2and temperature. 25What is different about the currentwarming cycle is the rapid rate at which CO 2concentrations are increasing. Since preindustrialtimes, atmospheric CO 2stocks have increasedby one-third—a rate of increase unprecedentedduring at least the last 20,000 years. Evidencefrom ice cores shows that current atmosphericconcentrations exceed the natural range of thelast 650,000 years. The increase in stocks of CO 2has been accompanied by rising concentrationsof other greenhouse gases.The world is now at or nearthe warmest level on recordin the current interglacialperiod, which beganaround 12,000 years ago1The 21 st Century climate challengeHUMAN DEVELOPMENT REPORT 2007/2008 31