The 21st Century climate challenge

1The 21 st Century climate challengerapid, non-linear responses of the climate systemto human-induced forcing (box 1.1).Climate scientists have drawn a distinctionbetween ‘imaginable surprises’, which are currentlyseen as possible but unlikely (deglaciation of polarice sheets or MOC reversals are examples) and ‘truesurprises’, or risks that have not been identifiedbecause of the complexity of climate systems. 48Feedback effects between climate change and thecarbon cycle, with changes in temperature givingrise to unpredictable outcomes, are the source ofthese potential surprises.There is growing evidence that natural carbonabsorption will weaken as temperatures rise.Modelling by the Hadley Centre suggests thatclimate change feedback effects could reduce theabsorptive capacity consistent with stabilizationat 450 ppm by 500 Gt CO 2, or 17 years ofglobal emissions at current levels. 49 The practicalconsequence of carbon cycle feedback effects isthat emissions may need to peak at lower levels orbe cut more rapidly, especially at higher levels ofgreenhouse gas concentrations.The focus on potentially catastrophic outcomesshould not divert attention from themore immediate risks. There is a large sectionof humanity that would not have to await theadvanced disintegration of ice sheets to experiencecatastrophe under these conditions. Precisenumbers can be debated, but for the poorest 40percent of the world’s population—around 2.6billion people—we are on the brink of climatechange events that will jeopardize prospects forhuman development. We will develop this pointfurther in chapter 2.Risk and uncertainty as a casefor actionHow should the world respond to the uncertaintiesassociated with climate change?Some commentators argue for a ‘wait-and-see’approach, with the mitigation effort to be scaledup in light of developments. The fact that theIPCC’s assessment and wider climate sciencepoint to uncertain risks with low probabilities ofglobal catastrophe in the medium term is cited asgrounds for delayed action.Such responses fail a number of publicpolicy tests for the development of climatechange mitigation strategies. Consider first theresponse to the range of possibilities identifiedby climate science. These ranges are not aBox 1.1Feedback effects could accelerate climate changeThere are many positive feedback effects that could transform climatechange scenarios for the 21 st Century. High levels of uncertainty aboutpositive feedback effects are reflected in IPCC scenario projections.Multiple feedbacks have been observed in ice sheet disintegration.One example is the ‘albedo flip’—a process that occurs whensnow and ice begin to melt. Snow-covered ice refl ects back tospace most of the sunlight that strikes it. When surface ice melts,darker wet ice absorbs more solar energy. The meltwater producedburrows through the ice sheet, lubricating its base, and speedingthe discharge of icebergs into the ocean. As an ice sheet dischargesmore icebergs into the ocean, it loses mass and its surface sinksto a lower altitude, where the temperature is warmer, causing it tomelt even faster. Meanwhile, warming oceans add yet another positivefeedback to this process, melting the offshore accumulationof ice—ice shelves—that often form a barrier between ice sheetsand the ocean.The accelerated melting of permafrost in Siberia with global warmingis another concern. This could release vast amounts of methane—a highly potent greenhouse gas—into the atmosphere, which wouldincrease warming and the rate at which permafrost melts.The interaction between climate change and the carbon sinkcapacity of rainforests provides another example of positivefeedback uncertainties. Rainforests can be thought of as vast‘carbon banks’. Trees in the Amazon region of Brazil alone store49 billion tonnes of carbon. Another 6 billion tonnes is stored inIndonesia’s forests. As global temperatures rise, changing climatepatterns could generate processes that will lead to the release oflarge amounts of carbon from these reservoirs.Rainforests are already contracting at an alarming ratein the face of commercial pressures, illegal logging and otheractivities. Under a business-as-usual scenario, climate modelsforecast temperatures in most of the Amazon region risingby 4–6°C by 2100. This could convert up to 30 percent ofthe Amazon rain forest into a type of dry savannah, accordingto research carried out under the auspices of Brazil’s NationalSpace Research Institute. Such an outcome would in turn driveup net global emissions of CO 2. Because rainforests recycleat least half of rainfall back into the atmosphere, accelerateddeforestation would also increase drought and fuel the spread ofsavannah areas.Source: FAO 2007b; Hansen 2007a, 2007b; Houghton 2005; Nobre 2007; Volpi 2007.38 HUMAN DEVELOPMENT REPORT 2007/2008