International Polar Year 2007–2008 - WMO

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IPY 20 07–20 08
tion documented repeated cycles of ice sheet collapse
and growth and some new IPY studies provide direct
evidence for orbitally induced oscillations in the West
Antarctic Ice Sheet (Naish et al., 2009). This large marine
ice sheet appears to have collapsed and reformed
during the interval between 3 and 5 million years ago
when the planetary temperatures were 3°C warmer
than today and the atmospheric CO 2 concentrations
reached values as high as 400 ppm. Parallel IPY modeling
efforts indicate that during periods with elevated
temperatures and atmospheric CO 2 , the West Antarctic
ice sheet can collapse repeatedly producing ~5m of
global sea level rise (Pollard and DeConto, 2009).
The general trend at the landscape level across the
Arctic is that the most rapid decadal changes have
occurred where there are fine-grained soils, strong
natural and anthropogenic disturbance regimes,
and relatively ample water and nutrients (Fig. 5.1-4).
Nevertheless, not all changes are caused by climate
shifts. For example, in Barrow, Alaska, some of the
vegetation changes may have been caused by
residents changing the hydrological system. Similarly
some of the wetlands changes may have been caused
by increased goose populations and their effect on
eutrophication. Again, shrub and tree abundance
shifts in some areas may be related to changes in
herbivory. Identification of clear causes of ecosystem
changes will require post-IPY investigations. Changes
in ecosystems are relatively easy to document, but
clear simple attribution to specific causes is often
difficult.
Change has to be addressed by projecting
IPY observations onto the background of past
observations and by considering a wide range of
natural variability from interannual to multidecadal
time scales. Sea ice extent is a popular indicator
of change, although attribution of its change can
be globally as well as regionally controversial. The
drastic changes in the Arctic Ocean are evidenced by
the record minimum summer sea ice extent in 2007,
which was followed by a slight recovery later during
the IPY period. Over the longer-term a clear trend of
decreasing ice extent and thinning has continued. In
the Arctic Ocean the mobility of sea ice increased to
the extent that the transpolar ice drift accelerated by a
factor of two. In contrast, the sea ice cover extent in the
Southern Ocean has tended to increase slightly each
year and has shown a slight hemispheric increase of
about 1% by decade over 30 years (Turner et al., 2009b)
Superimposed on this overall trend there are marked
regional differences. There has been a diminishing sea
ice cover west of the Antarctic Peninsula (Amundsen
and Bellingshausen seas) and an increase in the
eastern Weddell Sea and the Ross Sea. There have also
been changes to the annual persistence of Antarctic
sea ice in some regions (Chapter 2.3).
The surface air temperature over the Antarctic
continent seems to have increased by around
0.5ºC between 1957 and 2006, although there are
substantial local differences and the trend is not
significantly different from zero at the 95% confidence
level (Steig et al., 2009). This result changes the
previous accepted vision of the general cooling over
the same period (Thompson and Solomon, 2002). The
studies carried out during IPY have highlighted the
potential of satellite observations together with in situ
measurements to contribute to monitoring of weather
and climate over the polar areas (Chapter 3.1).
During IPY, studies in the snow and firn from Devon
Island in the Canadian Arctic allowed tracing human
impacts in the Arctic over several millennia. Data back
to 4,000 BP show that lead contamination in the High
Arctic pre-dated the use of leaded gasoline additives
and the Industrial Revolution. Several lead peaks linked
to human activity ~3,100 years ago correspond to the
Roman period and late 19th-20th centuries. Although
the decrease in the use of leaded gasoline diminished
the Pb in precipitation in the studied area, Pb isotope
data show that at least 90% of the Pb in the High Arctic
is still from anthropogenic sources (Chapter 2.1).
The Southern Ocean is warming and freshening
throughout most of the ocean depth, although
significant regional differences exist. Major currents
are shifting to the south, causing regional changes
in sea-level and supplying additional heat to melt ice
around the rim of Antarctica (Chapter 2.3). The future
of the Southern Ocean carbon sink is under debate.
In the north, shifts in exchanges between the Arctic
and Atlantic via subarctic seas are impacting the
Arctic Ocean. The changing poleward ocean heat flux
is central to determining the present and future of
the perennial Arctic sea-ice. Changes in atmospheric
conditions caused by warming have affected ocean
stratification and circulation. Increased heat gain by