The 21st Century climate challenge

and other activities release greenhouse gases,which are continuously recycled between theatmosphere, oceans and land biosphere. Currentconcentrations of greenhouse gases are the netresults of past emissions, offset by chemicaland physical removal processes. The Earth’ssoils, vegetation and oceans act as large ‘carbonsinks’. Emissions of CO 2are the primary sourceof increased concentrations. Other long-livedgreenhouse gases like methane and nitrousdioxide generated from agricultural activitiesand industry, mix with CO 2in the atmosphere.The total warming or ‘radiative forcing’ effectis measured in terms of CO 2equivalence,or CO 2e. 28 The sustained rate of increase inradiative forcing from greenhouse gases over thepast four decades is at least six times faster thanat any time before the industrial revolution.The global carbon cycle can be expressed interms of a simple system of positive and negativeflows. Between 2000 and 2005 an average of26 Gt CO 2was released into the atmosphereeach year. Of this flow, around 8 Gt CO 2wasabsorbed into the oceans and another 3 Gt CO 2was removed by oceans, land and vegetation. Thenet effect: an annual increase of 15 Gt CO 2in theEarth’s atmospheric stocks of greenhouse gases.Global mean concentration of CO 2in2005 was around 379 ppm. Other long-livedgreenhouse gases add about 75 ppm to thisstock measured in terms of radiative forcingeffects. However, the net effect of all humaninducedgreenhouse gas emissions is reduced bythe cooling effect of aerosols. 29 There are largedegrees of uncertainty associated with thesecooling effects. According to the IPCC, they areroughly equivalent to the warming generated bynon-CO 2greenhouse gases. 30Atmospheric concentrations of CO 2are ona sharply rising trend. 31 They are increasing ataround 1.9 ppm each year. For CO 2alone theannual concentration growth rate over the past10 years has been around 30 percent faster thanthe average for the past 40 years. 32 In fact, inthe 8,000 years prior to industrialization, atmosphericCO 2increased by only 20 ppm.Current rates of absorption by carbon sinksare sometimes confused with the ‘natural’ rate.In reality, carbon sinks are being overwhelmed.Take the world’s largest sink—its oceans. Thesenaturally absorb just 0.1 Gt more CO 2per yearthan they release. Now they are soaking upan extra 2 Gt a year—more than 20 times thenatural rate. 33 The result is serious ecologicaldamage. Oceans are becoming warmer andincreasingly acidic. Rising acidity attackscarbonate, one of the essential building blocksfor coral and small organisms at the start of themarine food chain. Based on current trends,future carbon dioxide releases could producechemical conditions in the oceans that have notbeen witnessed in the past 300 million years,except during brief catastrophic events. 34The future rate of accumulation in greenhousegas stocks will be determined by the relationshipbetween emissions and carbon sinks. There is badnews on both fronts. By 2030 greenhouse gasemissions are set to increase by between 50 and100 percent above 2000 levels. 35 Meanwhile, thecapacity of the Earth’s ecological systems to absorbthese emissions could shrink. This is becausefeedbacks between the climate and the carboncycle may be weakening the absorptive capacityof the world’s oceans and forests. For example,warmer oceans absorb less CO 2and rainforestscould shrink with higher temperatures andreduced rainfall.Even without taking into accountuncertainties over future carbon absorption weare heading for a rapid increase in greenhousegas stock accumulation. In effect, we areopening the taps to increase the flow of waterinto an already overflowing bath. The overflowis reflected in the rate at which CO 2is enteringand being locked into the Earth’s atmosphere.Climate change scenarios—theknown, the known unknowns,and the uncertainThe world is already committed to futureclimate change. Atmospheric stocks ofgreenhouse gases are rising with increases inemissions. Total emissions of all greenhousegases amounted to around 48 Gt CO 2e in2004—an increase of one-fifth since 1990.Rising concentrations of greenhouse gasesmean that global temperatures will continueAtmospheric concentrationsof CO 2are on asharply rising trend1The 21 st Century climate challengeHUMAN DEVELOPMENT REPORT 2007/2008 33