Melting Arctic sea ice, a keenly watched measure of global climate change, has “paused”, sharpening debate on whether humans or natural variability are to blame for the earlier decline.
After shrinking 35 per cent over several decades, the low point reached in Arctic ice cover each year appears to have stabilised. This is despite a record low maximum ice extent this winter and new research that shows the annual melt was beginning days earlier each decade.
Scientists who first identified the “hiatus” in global average surface temperatures are claiming a new climate change “pause”.
Summer melts are still retreating to levels that put them at the extreme low end of the relatively short satellite record and attention increasingly is being focused on the loss of ice thickness.
But the “pause” in summer ice melt extent has been widely conceded. A paper published in Nature by Neil Swart from Environment Canada said “from 2007-13 there was a near-zero trend in observed Arctic September sea-ice extent, in large part due to a strong uptick of the icepack in 2013 which has continued into 2014”.
Climate scientists do not believe the long-term downward trend in Arctic sea ice has been broken, however.
The Swart et al (2015) paper said “cherrypicking” such short periods could be “misleading about longer-term changes, when such trends show either rapid or slow ice loss”. It says claims a pause in Arctic ice loss disproved climate change were not true.
Ed Wawkins, co-author of the Swart paper and a researcher at Britain’s University of Reading, said it was “quite conceivable that the current period of near-zero sea-ice trend could extend for a decade or more, solely due to weather-induced natural variability hiding the long-term human caused decline”.
The surface temperature “pause” is now widely accepted, including by the Intergovernmental on Climate Change, which said it could be explained by a range of factors including natural variability, ocean heat take-up, volcanoes and the cooling effect of aerosols.
But, as surface temperatures have plateaued, melting Arctic sea ice has become a key message to support urgent action to tackle climate change.
Evidence of a “pause” is therefore drawing a lot of attention.
“Examining the sea-ice extent data for the past eight years it is obvious that there has not been any statistically significant downward trend, even though there is more noise (interannual variability) in the data,” Dr Whitehouse said.
“There are inter-annual variations, but they do not form a trend. For the 2002-06 period, the annual differences are mostly in the extent of maximum and not minimum ice cover.”
A key question posed both by Mr Swart and Dr Whitehouse is the extent of natural variability.
“Is it (the pause) caused by internal variability masking continuing human-induced sea-ice loss?” Dr Whitehouse says.
“Or has internal variability over decades periods since 1979 been misinterpreted as human-induced decline?”
Meanwhile, the US National Snow and Ice Data Centre said this year’s Arctic sea ice extend had reached a maximum of 14.54 million square kilometres, making it the lowest on the satellite record. The maximum extent was 1.10 million square kilometres below the 1981-2010 average of 15.64 million.