Geoscience Reference
In-Depth Information
40, ERA-Interim has increased the number of pressure levels from twenty-
three to twenty-seven; uses improved model physics, new humidity analysis,
more extensive use of satellite radiances, and improved fast radiative transfer
model; and includes additional cloud parameters. Data are available at 0.75
o
resolution.
Arctic System Reanalysis (ASR):
The ASR is intended to provide a high reso-
lution in space (10 km) and time (3 h) data assimilation of the Arctic's atmo-
sphere/sea ice/land system by assimilating observations into Polar WRF. As
of this writing, ASR data are available for the years 2000-2010 and a 30 km
resolution (Bromwich et al.,
2010
).
Analyzed fields (fields directly affected by assimilated data), such as 500 hPa
height, tend to be depicted quite similarly between different reanalyses. However,
because of various factors (including model physics, assimilation approaches, and
resolution), forecast surface fields - such as precipitation, evaporation, and radia-
tion fluxes - can be quite different. This is abundantly clear in
Figure 9.18
, which
shows average March snow water equivalent for the north polar region from six
atmospheric reanalyses. Consequently, it is now common practice in climate studies
to look at output from several reanalyses to establish a range of estimates.
9.8
Ecosystem Models
Terrestrial ecosystems in the Arctic and boreal forest regions are expected to be
highly sensitive to climate change and may play a strong role in biospheric feed-
backs to global climate (Kittel, Steffen, and Chapin,
2000
). This sensitivity arises
from complex interactions between ecosystem structure and function, soil and per-
mafrost processes, and regional climate. Biophysical and biogeochemical dynamics
of these landscapes in turn impact the global climate system through control over
surface-atmosphere exchanges of energy and greenhouse gases. Arctic and boreal
ecosystems are characterized by large carbon stocks. Tundra ecosystems contain a
significant fraction of the world's soil carbon that might react to near-term climate
change. There is strong evidence that recent warming in the Arctic has affected
both the structure and function of Arctic ecosystems. Functional responses include
changes in the biogeochemical cycling of carbon, nutrients, and water in ecosys-
tems, whereas structural responses represent changes in the species composition
across a landscape that may further modify the function of ecosystems (Clein et al.,
2000
). The observed greening of the Arctic tundra is an example of the latter.
The issues are complex. For example, the analysis of A. Betts (
2000
) indicates
that a change from tundra to boreal forest could increase carbon uptake by trees and
help mitigate the warming effects of atmospheric greenhouse gas loading. However,
such changes could be offset by the lower albedo of boreal forest as compared
to tundra. A substantial amount of carbon and nitrogen from the tundra could be
released in inorganic forms. As these soils warm, decomposition rates increase and
the growing season lengthens. A large release of carbon dioxide from these soils
