Scientists at the British Antarctic Survey say that the melting of the Pine Island Glacier ice shelf in Antarctica has suddenly slowed right down in the last few years, confirming earlier research which suggested that the shelf's melt does not result from human-driven global warming.
The Pine Island Glacier in West Antarctica and its associated sea ice shelf is closely watched: this is because unlike most of the sea ice around the austral continent, its melt rate has seemed to be accelerating quickly since scientists first began seriously studying it in the 1990s.
Many researchers had suggested that this was due to human-driven global warming, which appeared to be taking place rapidly at that time (though it has since gone on hold for 15 years or so, a circumstance which science is still assimilating).
However back in 2009 the British Antarctic Survey sent its Autosub robot probe under the shelf (famously powered by some 5,000 ordinary alkaline D-cell batteries on each trip beneath the ice, getting through no less than four tonnes of them during the research). The Autosub survey revealed that a previously unknown marine ridge lay below the shelf, over which the icepack had for millennia been forced to grind its way en route to the ocean. However in relatively recent times the ice had finally so ground down the ridge that the sea could flow in between shelf and ridge, freeing the ice to move much faster and warming it too.
As we reported at the time, this caused BAS boffins to suggest that the observed accelerating ice flow and melt seen since the '90s was actually a result of the ridge's erosion and sea ingress, rather than global warming.
Now, the latest BAS research has revealed that rather than accelerating, "oceanic melting of the ice shelf into which the glacier flows decreased by 50 per cent between 2010 and 2012".
The BAS goes on to explain:
Observations made in January 2012, and reported now in [hefty boffinry mag] Science, show that ocean melting of the glacier was the lowest ever recorded. The top of the thermocline (the layer separating cold surface water and warm deep waters) was found to be about 250 metres deeper compared with any other year for which measurements exist.
This lowered thermocline reduces the amount of heat flowing over the ridge. High resolution simulations of the ocean circulation in the ice shelf cavity demonstrate that the ridge blocks the deepest ocean waters from reaching the thickest ice ...
In January 2012 the dramatic cooling of the ocean around the glacier is believed to be due to an increase in easterly winds caused by a strong La Ninã event in the tropical Pacific Ocean.
Dr Pierre Dutrieux of the BAS adds, bluntly:
"We found ocean melting of the glacier was the lowest ever recorded, and less than half of that observed in 2010. This enormous, and unexpected, variability contradicts the widespread view that a simple and steady ocean warming in the region is eroding the West Antarctic Ice Sheet."
The Science paper can be read by subscribers to the journal here. The BAS announcement of the results can be read here. Readers unfamiliar with the rules of the climate game should note that the term "climate variability" as used in those documents means for this purpose "climate effects not caused by humans".