The report demonstrates that stabilizing atmospheric carbon dioxide concentrations will require deep reductions in the amount of carbon dioxide emitted. Because human carbon dioxide emissions exceed removal rates through natural carbon “sinks,” keeping emission rates the same will not lead to stabilization of carbon dioxide. Emissions reductions larger than about 80 percent, relative to whatever peak global emissions rate may be reached, are required to approximately stabilize carbon dioxide concentrations for a century or so at any chosen target level (see Figure Syn.3).
But stabilizing atmospheric concentrations does not mean that temperatures will stabilize immediately. Because of time lags inherent in the Earth’s climate, warming that occurs in response to a given increase in the concentration of carbon dioxide (“transient climate change”) reflects only about half the eventual total warming (“equilibrium climate change”) that would occur for stabilization at the same concentration (see Figure Syn.4). For example, if concentrations reached 550 ppmv, transient warming would be about 1.6ºC, but holding concentrations at 550 ppmv would mean that warming would continue over the next several centuries, reaching a best estimate of an equilibrium warming of about 3ºC.
Estimates of warming are based on models that incorporate “climate sensitivities”—the amount of warming expected at different atmospheric concentrations of carbon dioxide (Table 1). Because there are many factors that shape climate, uncertainty in the climate sensitivity is large; the possibility of greater warming, implying additional risk, cannot be ruled out, and smaller warmings are also possible. In the example given above, choosing a concentration target of 550 ppmv could produce a likely global warming at equilibrium as low as 2.1ºC, but warming could be as high as 4.3ºC, increasing the severity of impacts. Thus, choices about stabilization targets will depend upon value judgments regarding the degree of acceptable risk.
This report provides a scientific evaluation of the implications of various climate stabilization targets. The report concludes that certain levels of warming associated with carbon dioxide emissions could lock Earth and many future generations of humans into very large impacts; similarly, some targets could avoid such changes. It makes clear the importance of 21st century choices regarding long-term climate stabilization.