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Antarctic Temperature Trends
Reference
Thompson, D.W.J. and Solomon, S.  2002.  Interpretation of recent Southern Hemisphere climate change.  Science 296: 895-899.

What was done
The authors explore a number of different data sets - "30 years (1969-1998) of monthly mean radiosonde data from seven stations located over Antarctica, 32 years (1969-2000) of monthly surface temperature data observations, 30 years (1969-1998) of ground-based total column ozone measurements from Halley station, and 22 years (1979-2000) of tropospheric geopotential height data" - in an attempt to (1) define the nature of climate change in Antarctica since 1969 and (2) determine the reasons for the observed changes.

What was learned
With respect to thermal characteristics, the authors say that "at the surface, the Antarctic Peninsula has warmed by several [degrees C] over the past several decades, while the interior of the Antarctic continent has exhibited weak cooling."  In addition, they note that "ice shelves have retreated over the peninsula and sea-ice extent has decreased over the Bellingshausen Sea, while sea-ice concentration has increased and the length of the sea-ice season has increased over much of eastern Antarctica and the Ross Sea."  With respect to atmospheric circulation, they say there is "a systematic bias toward the high-index polarity of the SAM," or Southern Hemispheric Annular Mode, such that the ring of westerly winds encircling Antarctica has recently been spending more time in its strong-wind phase.

What it means
The heightened strength and persistence of the SAM would seem to explain most of the cooling experienced over the bulk of Antarctica over the past several decades, as well as much of the warming of the Antarctica Peninsula, as the latter location experiences fewer cold-air outbreaks under such conditions while simultaneously receiving increased advective warmth from the Southern Ocean.  But what has caused the strengthening of the SAM?  The authors speculate it is related to "recent trends in the lower stratospheric polar vortex, which are due largely to photochemical ozone losses."  Other scientists, however - as noted by Kerr (2002) in an accompanying "News of the Week" story - are not yet ready to make that connection, although they do not totally discount it.

As for us, we are keeping an open mind as well.  In the interim, however, we note that the work of Thompson and Solomon has at least advanced our knowledge to the point that we can better understand how the Antarctic Peninsula could have warmed so dramatically over the last few decades while the great mass of the continent to which it is attached has cooled.  Finally, we note that before man can be blamed - or thanked - for either of these temperature trends, considerably more data, either forward or backward in time, will be required to determine whether or not the observed changes of the past 30 years are but manifestations of the most recent phase of a natural multidecadal cycle.

Reference
Kerr, R.A.  2002.  A single climate mover for Antarctica.  Science 296: 825-826.


Reviewed 15 May 2002