Environmental Engineering Reference
In-Depth Information
centuries (see Section 2.2). In a framework of cumulative carbon emissions,
CO
2
concentrations do not necessarily “stabilize” but rather change over
time in response to a given CO
2
emissions scenarios; in this case, it is the
total cumulative carbon emitted over time, rather than the atmospheric CO
2
concentration itself, that indicates the level of expected climate warming.
The clear advantage of the cumulative carbon framework is that a given
level of cumulative emissions corresponds to a unique temperature change,
which remains approximately constant for several centuries after the point
of zero emissions (Matthews et al., 2008; Solomon et al., 2009). As can be
seen in Figure 3.7, for this particular model, cumulative emissions of 1,000
GtC from 1750 to 2100 result in a year-2100 global temperature change of
1.8°C over pre-industrial temperatures, which corresponds to a year-2100
CO
2
concentration of 460 ppm; both the year-2100 temperature change
and the year-2100 CO
2
concentration are independent of the shape of the
CO
2
emissions scenario and depend only on the total cumulative carbon
emitted. By contrast, the rate of temperature change, as well as the peak
CO
2
concentration in these simulations, varied as a results of differences in
the rates of increase and decline of emissions in each scenario.
Figure 3.8 illustrates how the concept of CO
2
stabilization can be recon-
ciled with the cumulative emissions framework. This figure shows idealized
CO
2
concentration scenarios that reach between 350 and 1,000 ppm at
the year 2100, along with the cumulative carbon emissions and tempera-
ture changes associated with each scenario.
4
From this analysis, restricting
global temperature change to 2°C requires best-guess cumulative emissions
of 1,150 billion tons of carbon between the years 1800 and 2100; this cor-
responds to a “stabilization” CO
2
concentration of between 450 and 550
ppm at the year 2100, with the caveats that CO
2
concentrations changes
and warming after the year 2100 would depend on the level of additional
post-2100 emissions. In general, at a given time (e.g., the year 2100) both the
atmospheric CO
2
concentration and the associated temperature change can
be inferred from cumulative carbon emissions to date. If carbon emissions
were subsequently eliminated, atmospheric concentrations would slowly
decrease over time, whereas temperature would remain elevated for several
4
The temperature responses to cumulative emissions shown in Figure 3.8 include both the
carbon cycle and climate sensitivity to emissions, but do not correspond directly to either
the transient climate response or the equilibrium climate sensitivity associated with a given
CO
2
concentration. In general, for higher emissions scenarios where forcing is still increas-
ing rapidly at 2100, this temperature change will more closely reflect the transient climate
response. For lower emissions scenarios with stable or declining forcing during the latter half
of the 21st century, the temperature change at 2100 will more closely reflect the equilibrium
climate sensitivity.
