Environmental Engineering Reference
In-Depth Information
backs, and thus were able to generate a probabilistic estimate of the emis-
sions associated with various temperature targets. To restrict the probability
of exceeding 2°C to 33%, Zickfeld et al. concluded that emissions from 2001
to 2500 must be kept to a median estimate of 590 GtC, with a range of 200
to 950 GtC owing to different estimates of climate sensitivity uncertainty, as
well as uncertainty in climate-carbon feedbacks. For an exceedence prob-
ability of 50%, the median estimate from this study was approximately 840
GtC (range: 500 to 1,210). Including also historical CO
2
emissions up to
2000 (~460 GtC; Houghton, 2008; Boden et al., 2009), this best estimate
for 2°C from Zickfeld et al. corresponds to emissions of approximately 1,050
GtC (1,300 GtC) for an exceedence probability of 33% (50%). Meinshausen
et al. (2009) also presented an estimate of the cumulative carbon emissions
required to meet a 2°C temperature target. This study used a simpler model
and a narrower time window (2000-2050) but considered also the effect
of non-CO
2
greenhouse gases and aerosols; Meinshausen et al. estimated
that cumulative emission from 2000-2050 must be restricted to 390 GtC to
avoid 2°C warming with 50% likelihood.
“Stabilization” Framework Based on Cumulative Carbon
The cumulative carbon framework is well suited to relating instanta-
neous global temperature change to a given level of cumulative carbon
emitted. Based on the above studies, we select 1.75°C global temperature
change per 1,000 GtC emitted to be a representative best estimate for the
climate response to cumulative carbon emissions. There is large uncertainty,
however, in this estimate of the temperature response to carbon emissions
owing both to uncertain carbon cycle response to elevated CO
2
and cli-
mate changes (Section 2.4) as well as to uncertainty in the physical climate
system response to CO
2
forcing (Sections 3.2 and 3.3). We use here a
very
likely
uncertainty range on this central estimate of 1 to 2.5°C per 1,000 GtC
emitted, based on the lower and upper 5-95% confidence limits given in
Matthews et al. (2009) and Allen et al. (2009).
This estimate of the temperature response to cumulative emissions
corresponds to approximately 1,150 GtC (4200 billion tons of CO
2
) in al-
lowable emissions consistent with 2°C temperature change (Figure 3.6). It is
critical to recognize, however, that the uncertainty range on this number is
very large, with a possible lower limit of 500 GtC (1,800 Gt CO
2
) inferred
from Allen et al. (2009) and a possible upper limit of 1,900 GtC (7000 Gt
CO
2
) from Matthews et al. (2009); this range of CO
2
emissions (500-1,900
GtC) can be considered to be a
very likely
range for emissions consistent
with 2°C global warming. We can narrow this range somewhat and apply
