Geoscience Reference
In-Depth Information
distribution of near-surface Northern Hemisphere permafrost and projected changes
through the twenty-first century, even with the same RCP scenario. In their study,
near-surface permafrost is taken to be present if the soil at 3.5 m has a temperature
of 0
o
C or less for the present and prior year. The scatter between models reflect both
differences in simulated surface climates (e.g., air temperature and surface radia-
tion fluxes) as well as the ability of the lands surface models to properly represent
permafrost physics. There is a large range in complexity in the land surface models,
with soil column depths ranging from 3-47 m, with between three and twenty-three
soil layers. Horizontal resolutions in the models are also quite variable. Within these
limitations, the models indicate that by the year 2100, for RCP 4.5, near-surface
permafrost will retreat from the present day discontinuous permafrost zone, while
for RCP 8.5 it may only be found in the Canadian Arctic Archipelago, the Russian
Arctic coast, and east Siberian uplands.
As has been addressed in several places in this textbook, recent decades have
seen a stronger rise in surface and lower tropospheric air temperatures over the
Arctic compared to the globe as a whole, most notable in autumn and winter, a
process termed Arctic amplification (
Figure 1.5
). As introduced in
Chapter 2
, this
outsized warming appears to involve a suite of processes, including reductions in
sea ice extent, leading to stronger summer heat gain in the ocean mixed layer, with
this heat then released back to the atmosphere in autumn and winter, changes in
atmospheric circulation leading to increased atmospheric heat flux convergence,
changes in cloud cover and water vapor, and increased concentrations of black car-
bon aerosols and soot on snow. Arctic amplification is in turn a near universal fea-
ture of global climate model projections for the twenty-first century that include ris-
ing concentrations of atmospheric carbon dioxide (Holland and Bitz,
2003
; Serreze
and Barry,
2011
). On the basis of averaging all of the CMIP3 simulations with the
A1B emissions scenario, by the last two decades of the twenty-first century, surface
air temperatures over the Arctic during winter are expected to be 5
o
C or more above
those for the 1980-1999 period, with the largest warming over the Arctic Ocean, in
considerable part because of having less sea ice (
Figure 9.15
). Arctic amplification
is by contrast not a prominent feature of summer. However, a closer look at the data
again reveals large differences between different models; some show a particularly
strong cold-season Arctic amplification, whereas others show it to be weaker.
Figure 9.15
also shows projected changes in precipitation and in sea level
pressure across the globe relative to 1980-1999. The pattern of projected precip-
itation changes is complex and manifests the increased moisture-holding capac-
ity of the warmer atmosphere along with changes in atmospheric circulation. In
areas in which there are efficient precipitation-generating mechanisms, precipita-
tion is expected to increase. This includes regions near the equator where there is
strong convection, and in middle and Arctic latitudes where ascent and cooling of
poleward-moving airmasses linked to extratropical cyclone activity allows water
vapor to condense. Increased Arctic precipitation may also result from sea ice loss
because the open water provides a regional moisture source. The expected increase
in evapotranspiration in the Arctic is more than compensated by the increase in
