Geoscience Reference
In-Depth Information
Annual J-D
L-OTI(°C) Change 1960-2012
0.76
-4.1
-4
-2
-1
-.5
-.2
.2
.5
1
2
4
4.1
Figure 1.5.
Linear trends in annual averages surface air temperature over the 1960-2012
period. The inset shows linear trends averaged by latitude. The larger trends over the
Arctic region (Arctic amplification) are obvious (based on the National Aeronautics and
Space Administration Goddard Institute for Space Sciences [NASA GISS] temperature
analysis(
http://data.giss.nasa.gov/gistemp
)
. (See plate section for color version.)
It is well-known that the globe as a whole is warming; the upward trend in sur-
face air temperature over the past fifty years has been especially strong in the Arctic
(
Figure 1.5
), with the strongest trends in autumn and winter. As assessed over the
satellite passive microwave record (1979 to present), sea ice extent shows down-
ward linear trends in all months, the largest (12 percent per decade) at the end of the
melt season in September (
Figure 1.6
). The temperature of the intermediate-depth
(150-900 m) Atlantic water of the Arctic Ocean has increased in recent decades
(Polyakov et al.,
2010
). Data from marine sediments indicate that the Atlantic
inflow is now the warmest of the past 2,000 years (Spielhagen et al.,
2011
). The
mass balance of the Greenland ice sheet has shown accelerating mass loss, contrib-
uting to sea level rise (e.g., Rignot and Kanagaratnam,
2006
; Rignot et al.,
2008
;
Jiang, Dixon, and Wdowinski,
2010
; Shepherd et al.,
2012
). Satellite data show
that the extent of seasonal surface melt over the Greenland Ice Sheet is increas-
ing (Tedesco et al.,
2010
). As a consequence of Arctic warming, growth of alder,
willow, and dwarf birch shrubs has increased in northern Alaska over the past fifty
years. Satellite-derived vegetation indices also suggest this is occurring in west-
ern Canada, Scandinavia, and parts of Russia (Tape, Sturm, and Racine,
2006
). As
demonstrated by increasing values of the satellite-derived Normalized Difference
Vegetation Index (NDVI), which represents the fraction of photosynthetically active
radiation absorbed by the plant canopy, the Arctic tundra is greening (Bhatt et al.,
2010
). Permafrost temperatures that have been monitored since the middle of the
twentieth century exhibit a general pattern of warming in Alaska, northwest Canada,
