Geoscience Reference
In-Depth Information
times more effective than carbon dioxide in trapping heat near the surface of the
earth, any change in the number and distribution of lakes will affect methane emis-
sions, and thus potentially global climate.
One model used in a number of Arctic studies is the Terrestrial Ecosystem
Model (TEM), an ecosystem model that uses biogeochemistry to describe carbon
and nitrogen dynamics of plants and soils. It uses spatially referenced information
on climate, elevation, soils, vegetation, and water availability as well as soil- and
vegetation-specific parameters to estimate monthly fluxes of carbon and nitrogen
and pool sizes at high resolution (0.5 degrees latitude/longitude). TEM has greatly
evolved through the years to include soil thermodynamic and hydrologic processes.
A fairly recent Arctic application is that of Yi et al. (
2009
), who used TEM to sim-
ulate the dynamics of evapotranspiration, soil temperature, active layer depth, soil
moisture, and water table depth over the Alaska tundra in response to both climate
variability and fire disturbance.
D. McGuire and his colleagues (
2010
) used TEM as part of a model-based study of
the carbon balance for the Arctic as a whole over the 1997-2006 period. They consid-
ered a linked framework of carbon dioxide and methane exchange across the Arctic
land, ocean, and atmosphere. The study was driven by recognition that the Arctic
carbon balance may be influenced by the strong warming being observed there. They
concluded that, over the 1997-2007 period, the terrestrial areas of the Arctic were a
source of carbon to both the atmosphere and to the ocean (via dissolved carbon in
runoff), while - at the same time - the ocean sequestered carbon from both the land
and atmosphere, with the ocean sink exceeding the land source by about 30 Tg per
year. When exchanges were broken down into carbon dioxide and methane, a more
complex picture emerged. Although both the land and ocean were sinks of carbon
dioxide, the land sink diminished over the study period because of increased fire
disturbance while the ocean sink strengthened because of a stronger phytoplankton
uptake linked to diminished sea ice extent. In turn, terrestrial areas were a growing
source of methane to the atmosphere. They concluded that the radiative effects of the
methane emissions exceeded the radiative effects of the carbon dioxide sink, so that
the Arctic was a source of greenhouse gas forcing to the climate system. The key ques-
tions from their study are whether carbon dioxide emissions attributed to increased
fire frequency will overwhelm the increased ocean uptake as sea ice extent continues
to decline, and whether enhanced GPP because of rising atmospheric carbon dioxide
will compensate for enhanced climate forcing linked to the methane emissions.
An issue of growing interest is the potential for a permafrost carbon feedback
(PCF) on climate. Permafrost regions of the Northern Hemisphere contain about
1,700 Gt of carbon as organic matter, roughly double the amount of carbon in the
atmosphere and nearly all of it frozen in the permafrost (Tarnocai et al.,
2009
). As
temperature continue to rise in the Arctic, some of this organic matter will thaw and
begin to decay, releasing carbon dioxide and methane into the atmosphere, accel-
erating climate warming and promoting further permafrost thaw. K. Schaefer et al.
(
2011
) examined the PCF using the Simple Biosphere/Carnegie-Ames-Stanford
Approach (SiBCASA) model in simulations of the terrestrial carbon cycle from
1973 to 2200 for regions of continuous and discontinuous permafrost north of 45
o
N.
