Past Climate Variability and Change in the Arctic and at High Latitudes

34
The U.S. Climate Science Program Chapter 3
The short time interval
for which instrumental
data are available
in the Arctic is not
sufficient to characterize
natural variability, so a
paleoclimatic perspective
is required.
Figure 3.1. Median extent
of sea ice in September
2007, compared with
averaged intervals during
recent decades. Red
curve = 1953–2000; orange
curve = 1979–2000;
green curve = September
2005. Inset: sea ice
extent time series plotted
in square kilometers,
shown from 1953–2007
in the graph below
(Stroeve et al., 2008).
The reduction in Arctic
Ocean summer sea ice
in 2007 was greater
than that predicted by
most recent climate
models. [Copyright 2008
American Geophysical
Union, reproduced by
permission of American
Geophysical Union.]
3.1 INTRODUCTION
Recent instrumental records show that during
the last few decades, surface air temperatures
throughout much of the far north have risen
more rapidly than temperatures in lower latitudes
and usually about twice as fast (Delworth
and Knutson, 2000; Knutson et al., 2006). The
remarkable reduction in Arctic oceAn* summer
sea ice in 2007 (Figure 3.1) has outpaced the
most recent predictions from available climate
models (Stroeve et al., 2008), but it is in concert
with widespread reductions in glacier length,
increased borehole temperatures, increased
coastal erosion, changes in vegetation and
wildlife habitats, the northward migration of
marine life, and degradation of permafrost. On
the basis of the past century’s trend of increasing
greenhouse gases, climate models forecast
continuing warming into the foreseeable future
(Figure 3.2) and a continuing amplification in
the Arctic of global changes (Serreze and Francis,
2006). As outlined by the Arctic Climate
*For bold terms, refer to Glossary; for italic terms, refer
to Plate 1; for geologic ages, refer to Plate 2.
9
8
7
6
5
SEA ICE EXTENT (10 6 KM 2 )10
1953 - 2000 Sept. Median
1979 - 2000 Sept. Median
2005 - September Median
SEPTEMBER SEA ICE EXTENT
4 1953 1960 1970 1980 1990 2000 2007
Impact Assessment (Arctic Climate Impact
Assessment, 2005), the sensitivity of the Arctic
to changed forcing is due to strong positive
feedbacks in the Arctic climate system (see
section 2.2). These feedbacks strongly amplify
changes to the climate of the Arctic and also
affect the global climate system.
Because strong Arctic feedbacks act on climate
changes caused by either nature or by humans,
natural variability and human-caused changes
are large in the Arctic, and separating them
requires understanding and characterization
of the natural variability component. The short
time interval for which instrumental data are
available in the Arctic is not sufficient to characterize
that natural variability, so a paleoclimatic
perspective is required.
This chapter focuses primarily on the history
of temperature and precipitation in the Arctic.
These topics are important in their own right,
and they also set the stage for understanding
the histories of the GreenlAnd ice Sheet and the
Arctic oceAn sea ice, which are described in
Chapters 5 (History of the Greenland Ice Sheet)
September, 2007