Geoscience Reference
In-Depth Information
Figure 9.12.
CMIP5 hindcasts (1900-2006) and projections (2006-2007) of annual
surface air temperature anomalies for the Arctic (the region poleward of 65
o
N), along
with the Arctic record from the ERA-Interim reanalysis. See text for details (courtesy of
A.P. Barrett, NSIDC, Boulder, CO). (See plate section for color version.)
GCMs have been widely used to evaluate hindcasts of Arctic sea ice extent and
projections out to the year 2100 and beyond. In a widely cited paper, J. Stroeve et al.
(
2007
) examined simulations of September Arctic sea ice extent for September from
the CMIP3 model archive, including ensemble members, for the 1900-2100 period.
For the twenty-first century, use was made of simulations with the A1B emissions
scenario, the most widely examined scenario used in the CMIP3 simulations. The
primary conclusions were that: (1) essentially all of the models show declining
September ice extent over the period of observations, providing evidence for a role
of external forcing on the observed trend; and (2) the simulated trends, viewed as
a group, are smaller than the observed trend. In a subsequent effort, Stroeve et al.
(
2012
) examined output from the later generation of models contained in the CMIP5
archive, focusing on the RCP 4.5 scenario for projections. Although there is a wide
range in modeled sea ice extents between the various CMIP5 models, the CMIP5
multimodel ensemble mean was found to be more consistent with the historical
observations, both in terms of the observed average extent and the rate of decline
over the past several decades (
Figure 9.13
). Trends from most ensemble members
and models are nevertheless still smaller than the observed trend. Pointing to strong
impacts of internal climate variability, 16 percent of the ensemble member trends as
evaluated over the period of satellite observations were statistically indistinguishable
from zero. When the analysis was constrained by rejecting those models that do an
especially poor job at reproducing the observed climatological mean September ice
extent, the spread in projected ice extent throughout the twenty-first century from the
CMIP5 and CMIP3 models is similar. Furthermore, a similar number of these ensem-
ble members reach nearly ice-free September conditions (less than 1.0 × 10
6
km
2
)
