Instrumental records analyzed by Jacka and Budd (1998) and summarized in Figure 16-4 have shown overall warming at permanently occupied stations on the Antarctic continent (1959-1996) and Southern Ocean island stations (1949-1996). Using two different statistical techniques, they found that the 16 Antarctic stations have warmed at a mean rate of 0.9-1.2°C per century, and the 22 Southern Ocean stations have warmed at 0.7-1.0°C per century. Antarctic Peninsula stations show a consistent regional rate of warming that is more than twice the average for other Antarctic stations. King and Harangozo (1998) suggest that this warming is associated with an increase in the northerly component of the atmospheric circulation over the Peninsula and perhaps changes in sea-ice extent. For Antarctic stations, warming trends are largest in winter and smallest in autumn. For the Southern Ocean stations, warming trends are largest in autumn and smallest in spring and summer. However, three GCMs were unable to reproduce these trends (Connolley and O'Farrell, 1998).
Another analysis of a 21-station data set from Antarctica by Comiso (1999) found a warming trend equivalent to 1.25°C per century for a 45-year record beginning in the 1950s but a slight cooling trend from 1979 to 1998. The slight cooling trend for this later 20-year period also was confirmed via analysis of surface temperatures over the whole continent, as inferred from satellite data. These changes can be placed in a long-term context by comparison with results from the high-accumulation ice coring sites on Law Dome, East Antarctica, which show clear climate signals with sufficient resolution to identify seasonal variations (van Ommen and Morgan, 1996, 1997; Curran et al., 1998). There was a cooling period in the late 1700s and 1800s and a warming over the 19th century, with greater variability and change in winter than in summer.
Changes in precipitation in the Antarctic are more poorly understood. Model
estimates from Smith et al. (1998a) indicate that the accumulation rate for
the East Antarctic ice sheet surface has increased by a rate of 1.9 mm yr-1
(water equivalent) over the period 1950-1991. Their estimate of sensitivity
is 12.5 mm yr-1 per degree of warming. Examination of water-mass
properties of oceans shows that significant changes have occurred over the past
30 years. Bindoff and McDougall (2000) and Wong et al. (1999) point out that
sub-Antarctic mode water (SAMW) and Antarctic intermediate water (AAIW) have
become less saline and cooler, and both water masses are now deeper. These changes
indicate surface warming in the source region of SAMW and increased precipitation
in the source region of AAIW.
|Table 16-1: IPCC SRES climate scenarios for 2080. Values are changes from present climate summarized from Carter et al. (2000), and are scaled output of nine AOGCMs.|
|- Arctic Ocean||+2-25b||+0.5-4.5||+2-45c||+3.0-16.0|
|a CSIRO-mk2 model
b CSIRO-mk2 model predicts +42%.
c ECHAM4 model predicts +70%.
Jacka and Budd (1998) found no significant trends in Antarctic sea-ice data
over the satellite era (1973-1996). Although the mean trend is zero, the sector
from 0° to 40°E has a clear trend toward increased sea ice. This is
matched by a larger sector of decreasing extent near the Bellingshausen and
Amundsen Seas, from about 65°W to 160°W. Elsewhere, sea-ice extent trends
are relatively small (see Figure 16-4)a finding
that also is supported by Cavalieri et al. (1997). Analysis of whaling
records by de la Mare (1997) suggests that the Antarctic summer sea-ice edge
has moved southward by 2.8 degrees of latitude between the mid-1950s and the
early-1970s. This suggests a decline in the area covered by sea ice of 25%.
It should be noted, however, that the data used in this analysis span two distinct
periods, during which differing whale species were harvested. Using atmosphere-ocean
sea-ice models, the computations of Wu and Budd (1998) indicate that sea ice
was more extensive over the past century, on the annual average by 0.7-1.2 degrees
of latitude. It also was thicker by 7-13 cm than at present. Wu et al.
(1999) conclude that the sea-ice extent reduced by 0.4-1.8 degrees of latitude
over the 20th century.
The IPCC commissioned a Special Report on Emissions Scenarios (SRES). Four
"marker scenarios" representing different world storylines are used
to estimate emissions and climate change to 2100 (IPCC, 2000). Table
16-1 summarizes these climate projections for the polar regions. In almost
all cases, predicted climates are well beyond the range of variability of current
climate. However, these estimates cover a very large range of precipitation
and temperatures, so future climate remains uncertain except that it will be
wetter and warmer. Some of the projected increases in precipitation and temperatures
are larger than for any other part of the globe.
Models predict that land areas in the Arctic will receive substantially increased snowfall in winter and that the climate will be markedly warmer. Summer could be much warmer and wetter than present. The climate over the Arctic Ocean does not change as dramatically, but it will become warmer and wetter by 2080. For the Antarctic continent, the models tend to predict more snow in winter and summer. Although temperatures are forecast to increase by 0.5°C, there will be little impact on melt because they will remain well below freezing, except in limited coastal localities. The Southern Ocean warms least, especially in summer. Precipitation increases by as much as 20%, so there will be more freshwater input to the ocean surface. This chapter also refers to other climate models. Some are equilibrium models for the atmosphere only; others are transient, coupled atmosphere-ocean models. Some deal with aerosols and other do not. In polar regions there can be large differences in predictions, depending on the model chosen, although most predict large changes in climate over the next 100 years. Assessments of impacts will vary, depending on the climate model chosen. This should be kept in mind in assessing the impacts described in this chapter.
Discrepancies among climate models and problems of downscaling (Shackley et al., 1998) mean that alternative methods of prediction that are based on analysis of empirical climate data (e.g., palaeoclimatic analogs and extrapolation of recent instrumental records) still have value. Anisimov and Poljakov (1999) analyzed modern temperature trends over the northern hemisphere; they suggest that warming in the Arctic will be most pronounced in the continental parts of North America and Eurasia. The potential impacts of continued deepening of the winter polar vortex would include weakening of the wind-driven Beaufort Gyre. This would further reduce the extent and thickness of the Arctic pack ice (McPhee et al., 1998) and change ocean temperatures and sea-ice boundaries (Dickson et al., 1999).
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