Friday, 13 November 2009

Bring back those fluorocarbon sprays —  the Antarctic ice needs them


It’s a complicated business, this global warming. If there are such people as regular readers of these snippets they will be aware of my fascination with measurements showing ice coverage in the Arctic declining at a frightening rate while down south in the Antarctic in recent years there has been a small but steady growth. The phenomenon is illustrated below in these graphs from the website of the National Snow and Ice Data Center at the University of Colorado.


I have read many attempts at explaining the different course of ice at the two polls and now have come across an intriguing new one. Marco Tedesco of Earth and Atmospheric Sciences, City College of New York, New York, New York, USA and Andrew J. Monaghan of the National Center for Atmospheric Research, Boulder, Colorado, USA published their explanation in the September edition of the journal Geophysical Research Letters and they have written asummary for the Real Climate website, which I will quote from here.
Their study demonstrates that low melt years during the 1979-2009 satellite record are related to the strength of the westerly winds that encircle Antarctica, known as the Southern Hemisphere Annular Mode (SAM). When the SAM is in a positive phase  — meaning that the belt of winds is stronger than average  — it has a cooling effect on Antarctic surface temperatures. The SAM was especially strong in austral spring and summer 2008-2009, and subsequently the 2008-2009 snowmelt was lower than normal.
During the past 30-40 years, the SAM has gradually strengthened during austral summer, due mainly to human-caused stratospheric ozone depletion. In turn, the increasing SAM has weakened longer-term summer warming over Antarctica.
The two scientists argue:
The ozone hole is projected to recover significantly during the next 25-50 years due to the Montreal Protocol, which limits ozone-depleting substances used in industrial and household applications. As the ozone hole “heals”, the increasing summer SAM trends are projected to subside.
As this happens, it is likely that summer temperature increases over Antarctica will become stronger and more widespread because the warming effect from greenhouse gas increases will no longer be kept in check by the dynamic cooling impact of the SAM.
Therefore, the linkage between the SAM and snowmelt leads to our key conclusion: that enhanced snowmelt is likely in Antarctica as the SAM trends subside during the 21st century and summer temperatures become warmer. Our results agree with studies that have noted cooling and/or slower warming during the past three decades due to increasing SAM trends over the same period.
Additionally, our conclusions do not contradict findings showing strong regional warming on the Antarctic Peninsula and in West Antarctica for the past 50 years, and warming over the entire continent for the past century. Our record is limited to the satellite era only, during which ozone depletion has dominated Antarctic summer temperature trends, and as already noted above, the observed warming in the last 50-100 years has occurred mostly in winter and spring. This context is important.
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