What long term trends
and decadal changes do you see occurring in the Arctic atmosphere,
and what significance do they have?
Decadal changes and
long-term trends in the Arctic atmosphere
John Walsh
Professor of Atmospheric Sciences
University of Illinois at Urbana-Champaign
Global climate models suggest that the Arctic atmosphere
will respond strongly to increasing concentrations of greenhouse
gases. Hence the Arctic atmosphere could provide an early
indication that greenhouse gases are affecting the earth's
climate. However, the atmosphere of the Arctic, as in other
regions of the globe, can undergo large decadal-scale and
longer variations for reasons unrelated to changes in the
radiative forcing by greenhouse gases. What do the recent
data suggest in this context?
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Much of the Arctic appears to have warmed at the surface
over the past 40 years. The warming has been strongest over
northern Eurasia and northwestern North America (including
Alaska), as shown in the maps at http://faldo.atmos.uiuc.edu/RESEARCH/temptrends.html.
The warming has been strongest in winter and spring, when
it amounts to as much as 23°C per decade since 1960
in the areas of greatest warming. A similar seasonal pattern
of warming has been detected in ice station and buoy data
from the Arctic Ocean (Serreze et al., 2000).
However, much of the warming during this period is likely
due less to the greenhouse effect than to changes in the
atmospheric circulation. A large-scale pattern of atmospheric
pressure and winds, known as the Arctic
Oscillation, brings relatively warm air to northern
Eurasia and, to a lesser extent, northwestern North America
when it is in a particular phase. This phase has been dominant
for the past 20 years, while the opposite phase predominated
in the 1960s. Hence the changing wind pattern since 1960
has favored warming in the same Arctic areas where the strongest
warming has been detected.
While the Arctic warming of the recent decades may have
multiple causes, longer temperature reconstructions from
paleoclimatic data (tree rings, ice cores, temperature records)
indicate that the Arctic was warmer during the 20th century
than at any time since 1600 (Overpeck et
al., 1997). This reconstruction is weighted toward the
summer season, and it does also show a variety of decadal-scale
swings, including a cooling of the Arctic from the 1940s
to the 1960s. The reconstruction suggests that natural variations
occur on the decade-to-century timescale, and it implies
that such variations may have combined with greenhouse-gas
changes during the past century "to drive unprecedented
changes in the Arctic environment" (Overpeck
et al., 1997, p. 1255).
The recent decades have also seen a shift of precipitation
in the Arctic. The atmospheric circulation pattern that
contributed to much of the post-1960 Arctic warming has
also increased precipitation over subarctic Europe and the
eastern North Atlantic (Dickson et al.,
2000), and it has increased storminess in the Arctic.
Recent assessments of 20th-century precipitation (IPCC,
1996) show that precipitation has increased by a greater
percentage in the Arctic (65°N85°N) than in any other
latitudinal zone on the globe. Thus the Arctic has become
a generally wetter as well as a warmer place over the past
few decades, suggesting an acceleration of the hydrologic
cycle in the Arctic. These changes have implications for
soil moisture, vegetation, and river discharge. There are
indeed indications that the seasonal "spike" in river discharge
is occurring earlier in the spring in many Arctic rivers
(Lammers et al., 2000).
Finally, there are recent indications of changes in the
upper atmosphere of the Arctic. As part of an overall hemispheric
trend, ozone concentrations have shown a broad decrease
of 510% in the Arctic stratosphere. Perhaps more noteworthy
are the brief occurrences of migratory ozone holes lasting
several days. While the reductions of ozone during these
Arctic events are large percentages of the total, the reductions
cover much smaller areas and are less persistent than the
Antarctic "ozone hole". The Arctic ozone reduction events
appear to be dynamically forced (AMAP, 1997).
At even higher altitudes, polar mesospheric clouds or "noctilucent"
clouds in the 8090 km layer have received considerable
attention in recent decades. Sample photographs of such clouds,
which are visible only when the sun is several degrees below
the horizon, can be seen at http://cumulus.helsinki.fi/~tpnousia/nlcgal/nlcgal.html.
While the formation mechanisms of these clouds are still being
debated, one hypothesis is that the dissociation of methane
contributes to the formation of the frozen water particles
of which these clouds are composed. Methane concentrations
are increasing in the atmosphere. Since extremely low temperatures
are also required for the formation of polar mesospheric clouds,
an increase in the frequency of polar mesospheric clouds may
be an indication of changes in atmospheric composition or
of changes in the temperature of the upper atmosphere above
the Arctic.
References
AMAP, 1997: Arctic Pollution Issues:
A State of the Arctic Environment Report. Arctic Monitoring
and Assessment Programme, P.O. Box 8100 Dep., N-0032 Oslo,
Norway, 188 pp.
Dickson, R. R., T. J. Osborn, J. W.
Hurrell, J. Meincke, J. Blindheim, B. Adlandsvik, T. Vinje,
G. Alekseev and W. Maslowski, 2000: The Arctic Ocean response
to the North Atlantic Oscillation. J. Climate, 13,
2671-2696.
IPCC, 1996: Climate Change 1995:
The Science of Climate Change (J. T. Houghton, L. G.
Miera Filho, B. A. Callander, N. Harris, A. Kattenberg and
K. Maskell, Eds.), Intergovern- mental Panel on Climate
Change, Cambridge Univ. Press, Cambridge, U.K., 572 pp.
Lammers, R. D., A. I. Shiklomanov, C.
J. Vorosmarty, B. M. Fekete and B. J. Peterson, 2000: Assessment
of contemporary Arctic river runoff based on observational
discharge records. J. Geophysical Research (Atmospheres),
105, in press.
Overpeck, J., K. Hughen, D. Hardy, R.
Bradley, R. Case, M. Douglas, B. Finney, K. Gajewski, G.
Jacoby, A. Jennings, S. Lamoureux, A. Lasca, G. MacDonald,
J. Moore, M. Retelle, S. Smith, A. Wolfe and G. Zielinski,
1997: Arctic environmental change of the last four centuries.
Science, 278, 1251-1256.
Serreze, M. C., J. E. Walsh, F. S. Chapin
III, T. Osterkamp, M. Dyurgerov, V. Romanovsky, W. C. Oechel,
J. Morison, T. Zhang and R. G. Barry, 2000: Observational
evidence of recent change in the northern high-latitude
environment. Climatic Change, 46, 159-207.
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