Notícias do Árctico (II)

Uma das coisas que tenho vindo a acompanhar com mais atenção é a evolução do comportamento do gelo no árctico, particularmente, à luz da dimuinuição inopinada sucedida em 2007. Não é seguro que tal venha a suceder com a mesma intensidade este ano, embora seja segura a tendência de longo prazo de diminuição da extensão do gelo - há quem mesmo afirme que essa tendência de longo prazo possa ser contrariada (episodicamente) na próxima década. A razão para o meu interesse é óbvia: o Árctico poderá funcionar (sem grandes custos aparentes - salvaguardada a situação dos ursos polares) como um bom "canário da mina" em relação ao aquecimento global. A informação abaixo (texto, fotografias e gráficos) é deste mês e vem no Arctic Sea Ice News & Analysis.

"Daily sea ice extent; the blue line indicates 2008; the black line indicates extent from 1979 to 2000; the dotted line shows extent for 2007"

"Although ice extent is slightly greater than this time last year, the average decline rate through the month of May was 8 thousand square kilometers per day (3 thousand square miles per day) faster than last May. Ice extent as the month closed approached last May’s value."

"This United States National Ice Center analysis shows the percentage of multi-year sea ice in yellow, green, and dark blue. Light blue with red outline indicates ice extent; land and ocean are white."

"The relative lack of thick, resilient multi-year ice in the Arctic discussed in earlier postings finds further support in the latest analysis from the United States National Ice Center (NIC). ... NIC scientist Todd Arbetter suggests that much of the first-year ice is likely to melt by the end of summer, saying that despite the total ice extent appearing normal, the relative amount of multi-year ice going into this summer is very low when compared to climatological averages. NIC has found that the relative fraction of multi-year ice in the central Arctic has plummeted since the mid-1990s, creating an Arctic prone to increased melt in summer. Arbetter said, “This may be a primary reason for record summertime minimums in recent years.”

However, the unusual location of some of this year's first-year ice may help more of it survive than otherwise might be expected. This year, much of the first-year ice is farther north than normal, and those northern areas receive weaker solar radiation. So, northern first-year ice may be less vulnerable to melt than first-year ice in typical locations."

"Open water is clearly seen near Alaska and Banks Island, and in the North Water polynya, in this visible-band satellite image mosaic on May 20, 2008."

"Thinner ice already showing weakness As mentioned, the thin ice that covers much of the Arctic Ocean is showing signs of early breakup, with large polynyas off the coast of Alaska, the Canadian Archipelago, and Baffin Bay.

Coastal polynyas are not unusual, at this time of year, but the polynyas we are currently seeing appear larger and more numerous than usual. This is partly because of the thinner, weaker ice cover.

Thorsten Markus at the NASA Goddard Space Flight Center has noted the size of the North Water polynya at the northern end of Baffin Bay, which typically forms in May. The polynya is much larger than normal, possibly nearing its largest area on record. Inuit report that sea ice is starting to break up near Baffin Bay much earlier than normal this year. They have observed wide cracks in the ice already forming, according to NSIDC scientist Shari Gearheard, who lives and works in the Baffin Island hamlet of Clyde River.

Polynyas are a source of heat for the atmosphere in spring; in summer, however, they are large absorbers of solar energy. Resultant warm ocean surface waters then eat away at the ice edge, accelerating melt."