Environmental Engineering Reference
In-Depth Information
STABILIZATION REQUIRES DEEP EMISSIONS REDUCTIONS
The report demonstrates that stabilizing atmospheric carbon dioxide
concentrations will require deep reductions in the amount of carbon di-
oxide 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 concentra-
tions for a century or so at any chosen target level (see Figure Syn.3).
But stabilizing atmospheric concentrations does not mean that tem-
peratures 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 1.6°C, 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 3°C.
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 possi-
bility 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 2.1°C, but warming could be as high as 4.3°C, in-
creasing the severity of impacts. Thus, choices about stabilization targets will
depend upon value judgments regarding the degree of acceptable risk.
CONCLUSION
This report provides a scientific evaluation of the implications of vari-
ous 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.
