Friday, August 26, 2016

How Predictable Is the First Ice-Free Arctic Summer?

Average September Arctic sea ice extent, from 1979 to 2015. (Credit:  NSIDC) Click to Enlarge.
Around this time each year, many people turn their attention to the Arctic in anticipation of the annual minimum for sea ice cover.

After reaching its annual peak extent at the end of winter, Arctic sea ice melts as temperatures rise through spring and into summer.  Sea ice then hits its smallest extent sometime in September.

Since the satellite record began in 1979, the Arctic sea ice cover in September has declined by around 13% per decade.  The current record low was recorded on 16 September 2012, when sea ice dwindled to 3.41m square kilometers.

Such a stark drop off in sea ice has prompted the question of when the Arctic will first see an ice-free summer.  In our new study, published last week in Geophysical Research Letters, we consider whether it’s possible to pin this down to a specific year.

When does ice-free mean ice-free?

First, we need to clarify what exactly an “ice-free” Arctic summer is.

By “ice-free”, scientists usually mean a sea ice extent of less than one million square kilometers, rather than zero sea ice cover.

Arctic sea ice extent for 14 August 2016 (5.61m square kilometres). The orange line shows the 1981 to 2010 median extent for that day. The black cross indicates the geographic North Pole. (Credit: NSIDC) Click to Enlarge.
There’s a good reason for this.  Arctic sea ice isn’t just found in the central Arctic Ocean, but also along the northern coastlines of the US, Greenland, Russia and Canada, and in the narrow channels of the Canadian Arctic Archipelago.  And it is thicker in these regions than in the central Arctic Ocean.

Scientists expect, therefore, that sea ice will be present there a little longer than it will out in the central Arctic Ocean.  This means as sea ice continues to decline, we will reach a point where the central Arctic Ocean will be largely ice-free, but remnants of ice will still remain along the northern coastlines of Canada, Alaska, and Greenland.  Scientists therefore chose the one million square kilometer threshold to represent a practically ice-free Arctic Ocean.

Uncertainty in predictions

The emissions pathways have a big impact on when we might see consecutive ice-free summers.  For example, under the high emissions pathway, the Arctic is consistently ice-free during September from the late 2060s in our simulations.  In contrast, under the moderate pathway, consistently ice-free summers only occur in a few model simulations by 2080.

In conclusion, our findings suggest that we cannot predict the timing of an ice-free Arctic summer with an uncertainty of less than about 25 years.

But while natural fluctuations of weather and climate will affect exactly when an Arctic summer will first be ice free, we can be fairly certain that it will happen well before the end of this century without significant cuts to greenhouse gas emissions.

Read more at How Predictable Is the First Ice-Free Arctic Summer?

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