Wednesday, 15 March 2017

Arctic Ice Melt: Up to half might be totally natural

An article in Popular Science under the heading:

Scientists have known for a while that the Arctic is melting.  
But while scientists are certain that the Arctic is melting at an alarming rate, they aren't really sure why. 
Although anthropogenic climate change accounts for some of the melting, Arctic ice is disappearing much faster than climate change models predict it should. A new study in Nature Climate Change sheds new light on the mystery.

Simulated impact of atmospheric circulation on summertime Arctic sea-ice trends.

Influence of high-latitude atmospheric circulation changes on summertime Arctic sea ice (link)

Nature Climate Change
(2017) doi:10.1038/nclimate3241 Qinghua Ding, et al 13/3/2017

The Arctic has seen rapid sea-ice decline in the past three decades, whilst warming at about twice the global average rate. Yet the relationship between Arctic warming and sea-ice loss is not well understood. Here, we present evidence that trends in summertime atmospheric circulation may have contributed as much as 60% to the September sea-ice extent decline since 1979. A tendency towards a stronger anticyclonic circulation over Greenland and the Arctic Ocean with a barotropic structure in the troposphere increased the downwelling longwave radiation above the ice by warming and moistening the lower troposphere. Model experiments, with reanalysis data constraining atmospheric circulation, replicate the observed thermodynamic response and indicate that the near-surface changes are dominated by circulation changes rather than feedbacks from the changing sea-ice cover. Internal variability dominates the Arctic summer circulation trend and may be responsible for about 30–50% of the overall decline in September sea ice since 1979.

Popular Science quoted the lead author Qinghua Ding, 
a professor in the Geography Department at the University of California Santa Barbara.:

“There is a mismatch between the model's output and the observation. Observation shows very fast, very abrupt sea ice melting, whereas the climate model cannot capture the fast melting.” 
Science Daily: (link)
"What we've found is that a good fraction of the decrease in September sea ice melt in the past several decades is most likely natural variability. That's not really a surprise," said co-author David Battisti, a UW professor of atmospheric sciences.

"The method is really innovative, and it nails down how much of the observed sea ice trend we've seen in recent decades in the Arctic is due to natural variability and how much is due to greenhouse gases."