Environmental Engineering Reference
In-Depth Information
of this period. Polyakov et al. estimate that as much as 50% of the trend
over this period in the North Atlantic is internal variability. The modeling
work by Zhang and Knight also indicate that the influence of the Atlantic,
despite its small size, can spread preferentially over Eurasia and contribute
to global temperature signals. For example, the model analyzed by Knight
generates a 0.1 K global mean warming for an increase of 1 Sverdrup (about
5% of the 20 Sverdrup mean value) in the Atlantic overturning.
Volcanic responses and the response to the 11-year solar cycle can also
be used to constrain TCR, as can the autocorrelations of internal fluctua-
tions (rather than watching the volcanic response decay in time to estimate
the strength of restoring forces, one can watch internal fluctuations decay).
Paeloclimatic evidence constrains equilibrium sensitivity, and with model-
ing guidance and heat uptake measurements constraining the ratio of TCR
to the equilibrium response, one can also use these to constrain TCR. But
there are also issues related to the decoupling of transient and equilibrium
responses and to issues of “Earth system sensitivity”, that come into play
when considering paleoclimatic constraints (see Chapter 6). We judge the
constraints that directly involve fits to the temperature record over the past
century and the last few decades to be the most useful in constraining TCR
at this time.
The magnitude of many of the impacts discussed in this report scale
with the value of TCR. We estimate TCR by starting with the distribution in
the CMIP3 ensemble, but lowering the low end of the distribution slightly
to take into account the possibility suggested by some recent studies of
internal variability that a portion of the most recent Northern Hemisphere
warming is internal. This results in a best estimate of
1.65°C,
likely lying
in the range
1.3-2.2°C
(i.e., with 2/3 probability) and very likely lying in
the range of
1.1-2.5°C
(i.e., with 90% probability). The estimate in the
WG1/AR4 report is a very likely range of 1-3°C. Our reduction of the upper
limit to this range is consistent with our critique of very high equilibrium
sensitivities in Section 3.2.
3.4 CUMULATIVE CARBON
Introduction: Why Use Cumulative Carbon Emissions?
The temperature response to anthropogenic carbon emissions is deter-
mined by: (1) the response of the carbon cycle to emissions; (2) the climate
response to elevated CO
2
concentrations; and (3) the feedback between
climate change and the carbon cycle. There has been significant attention
