A history of Arctic land temperature anomalies from 1880 Natural climate variability is organized into spatial pat-
through 2006 is shown in Figure 3.2. In the late 1800s the terns of high and low pressure regions, represented by
Arctic was relatively cold, although there is some uncer- the Arctic Oscillation (also called the Northern Annular
tainty around these early temperature estimates. The Arctic Mode) and North Pacific patterns in the Northern Hemi-
warmed by about 0.7ºC over the 20th century. There was a sphere, and the Southern Annular Mode in the South-
warm period in the 1920s to 1940s and cold periods in the ern Hemisphere. The patterns of surface temperature
early 1900s and in the 1960s. Over the last decade the tem- anomalies when the Arctic Oscillation and Northern
peratures were about 1.0ºC above the 20th century average. Pacific patterns are in their positive extreme are shown
in Figure 3.4. When either of the patterns is in its posi-
Figure 3.3 shows that the largest recent gains in annual tive extreme, the pattern contributes to an overall Arctic
temperatures for the planet are over the North American warm period. In recent years (2000–2005), however, the
Arctic, north central Siberia, and on the Antarctic Pe- pattern of warm temperature anomalies is circumpo-
ninsula. These recent increases in temperature are con- lar in distribution and different from either of the two
firmed by changes in other features: loss of sea ice, shift major 20th century climate patterns. We are truly in a
of tundra to shrub vegetation, and migration of marine new and uncertain climate state for the northern polar
and terrestrial ecosystems to higher latitudes
5
. region
6,7
.
Temperature increases
2001-2005
1.6 - 2.1
1.2 - 1.6
0.8 - 1.2
0.4 - 0.8
0.2 - 0.4
-0.2 - 0.2
-0.4 - -0.2
-0.8 - -0.4
Insufficient data
Mean surface
temperature anomaly (ºC)
Figure 3.3: Increases in annual temperatures for a recent five-year period relative to 1951–1980. Warming is widespread, generally
greater over land than over oceans, and greatest at high latitudes in the Northern Hemisphere.
Source: based on Hansen and others 2006
8
CHAPTER 3 WHY ARE ICE AND SNOW CHANGING? 33