Geoscience Reference
In-Depth Information
Figure 13.17
Time series of global average surface air temperature, expressed as anomalies relative to
the base period 1980-1999, as simulated by global climate models participating in the IPCC Fourth
Assessment Report. Results for the twentieth century are based on observed radiative forcings,
projections for the twenty-first century employ different emissions scenarios. The solid lines represent the
multi-model means, while the shading indicates the spread between different models based on the +/- 1
standard deviation.
Source: IPCC (2007). Reproduced by permission of the IPCC (Summary for Policymakers, Report of WG1 1, IPCC, p. 14, fig.
SPM.5).
next two decades. This warming is essentially the
heat 'in the pipeline' that would ensue as the system
comes into radiative equilibrium with the radiative
forcing for the year 2000.
As is clear from
Figure 13.18
, the magnitude of
projected surface warming has distinct spatial
patterns which stay largely consistent through the
twenty-first century. The expectation is for
amplified warming, relative to the global mean,
over the north polar region (a pattern already seen
in observed trends: see
Figure 13.9
). Building on
earlier discussion, this in large part reflects loss of
the Arctic's sea ice cover. For most of the year, sea
ice acts to insulate a relatively warm Arctic Ocean
expected to have increased by 1.8
o
C (B1 scenario)
to 4.1
o
C (A2 scenario). It is important to note that
as time progresses, the uncertainty in greenhouse
gas emissions (the range in projections from the
different scenarios) starts to become of increasing
importance relative to the range between
simulations from different models for a given
scenario. Phrased differently, uncertainty as to
how much warmer it will be in 2100 is more a
function of uncertainties in human behavior than
uncertainties in how well the climate system can
be modeled. If one assumed that greenhouse gas
concentration could have been kept at levels for the
year 2000, a small warming would occur over the
