Environmental Engineering Reference
In-Depth Information
in the future, there would be a lock-in to further warming of comparable
magnitude to that already occurring at the time of stabilization.
The instantaneous response of Earth's atmosphere and oceans to in-
creases in greenhouse gases and net radiative forcing represents a transient
climate change, which can be linked to “transient climate response.”
8
The
transient climate response is smaller than the longer-term “climate sensitiv-
ity” that includes adjustments by the oceans to the added heat (Table S.1).
For example, if carbon dioxide equivalent concentrations (including aerosols
and other gases) were to increase from today's best estimate levels of about
390 parts per million by volume (ppmv) to 550 ppmv at rates of growth
similar to those occurring today, averaged warming would be expected to
increase in a manner that scales with the change in radiative forcing rela-
tive to the transient climate response; for 550 ppmv the best estimate total
warming since pre-industrial times is about 1.6°C (within a likely uncertainty
range of 1.3-2.2°C). In the hypothetical case where concentrations are then
immediately stabilized at 550 ppm, further warming would subsequently
occur over the next several centuries, reaching a best estimate “climate
sensitivity” of about 3°C (likely in the range of 2.1-4.3°C). The horizontal
arrow in Figure S.4 depicts such a transition from transient to equilibrium
warming. {2.2, 2.4, 3.2, 3.3}
Climate sensitivity remains subject to considerable uncertainty.
The esti-
mated “likely” range presented in this report corresponds to the range of
model results in the CMIP3 global climate model archive, and it is roughly
consistent with paleoclimate evidence. However, the possibility of climate
sensitivities substantially higher than this range cannot at present be ruled
out. This report should be read with this proviso in mind, as these high
sensitivities, if realized, would amplify many of the impacts discussed and
associated risk. {3.2, 3.3, 6.1}
3. Climate Changes, Future Impacts, and
Choices among Stabilization Targets
Increases in global mean temperature caused by higher anthropogenic
greenhouse gas concentrations would be expected to lead to a diverse range
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The transient climate response is defined as the warming at the time of doubling of CO
2
concentration (compared to a pre-industrial value of 278 ppm this is about 550 ppm). Scaled
by radiative forcing, the same relationship characterizes warming that has occurred during
the 20th century as well as further warming that is projected to continue with growing CO
2
concentrations in the 21st century for a broad range of plausible scenarios.
