Cool Arctic summer brought brief recovery in sea ice loss in 2013, study suggests

A colder-than-expected Arctic summer led to a brief let up in the decline of sea ice, a new study says. State-of-the-art satellite data shows that after a 14% decrease in sea ice volume between 2010 and 2012, summer sea ice recovered to pre-2010 levels in 2013.

The study shows how natural fluctuations in Arctic weather will cause many ups and downs, scientists tell Carbon Brief. But the long-term patterns is a decline in sea ice through the century.

Ice thickness

Scientists have used satellites to monitor polar sea ice since the 1970s. It’s how we know that the summer of 2012 saw a new record low in sea ice extent of 3.41m square kilometers – 44% below the 1981-2010 average. Compare the dotted orange line with the thick grey one in the graph below.

Arctic sea ice extent as of September 30, 2012, along with daily ice extent data for the previous five years. The 1979 to 2000 average is in dark gray. The grey area around this average line shows the two standard deviation range of the data. Source: NSIDC.

But the area covered by sea ice is only part of the story. To understand how the volume of sea ice is changing, you also need to know its thickness.

The new study, published today in Nature Geoscience, is the first to use satellite data to derive sea ice thickness and volume results for the entire northern hemisphere, says lead author Rachel Tilling from University College London.

And the results suggest that Arctic sea ice volume rebounded to similar levels seen before the record low in 2012.

Chilly summer

Tilling and her colleagues used 88m individual measurements, from October 2010 to November 2014, taken by the European Space Agency CryoSat-2 satellite. The data show that between the summers of 2010 and 2012, sea ice volume declined by 14% – that’s 1,279 cubic kilometers of ice melting into the sea.

But after this dramatic drop, the Arctic experienced a much cooler summer in 2013 – one more typical of a summer in the 1990s, Tilling says.

The chillier conditions cut the summer melt season short and allowed thick ice to hold steady on the northern coast of Greenland. The satellite data shows that by the end of the summer in 2013, the sea ice volume had grown 41% bigger than its diminished state in 2012. Sea ice then decreased again by 6% (of the 2013 volume) by the summer of 2014.

You can see this in the graphs below. The upper graph shows the CryoSat data as red stars against the long-term trend in sea ice volume from computer models (red line). You can see the decline towards the record sea ice low in 2012 and the subsequent increase in 2013.

The lower graph shows the trend in the number of days above zero degrees (purple line), when sea ice is likely to melt. You can see a clear drop in days of melting in the summer of 2013, which corresponds to the growth in sea ice seen that year.

Graphs show a) Time series of Arctic sea ice volume for autumn 1980-2014 (solid line) and spring 1981-2014 (dashed line) from computer model, with CryoSat-2 volume estimates (red stars) for 2010-2014, and b) Time series of days below zero degrees across the Arctic Ocean for 1980-2014 (solid purple line) with long-term average (solid black line) and standard deviation (dashed black lines), and CryoSat-2 autumn ice volume for 2010-2014 (red stars). Source: Tilling et al. (2015)

Bouncing ball

The research highlights how how natural fluctuations in the weather can affect the volume of Arctic sea ice in a particular year, says Dr Ed Hawkins, Associate Professor at the University of Reading, who also wasn’t involved in the study. He tells Carbon Brief:

“As temperatures in the Arctic increase, the amount of sea ice will continue to reduce further over the coming decades. But, we do not expect these changes to happen smoothly – we fully expect there to be shorter periods where the sea ice temporarily bucks the long-term trend.”

Hawkins likens the decline of Arctic sea ice to a ball bouncing down a bumpy hill. He explains in a recent article:

“Gravity will ensure that the ball will head downwards. But, if the ball hits a bump at a certain angle it might move horizontally or even upwards for a time, before resuming its inevitable downward trajectory.”

The image below shows the rise and fall of September Arctic sea ice extent during its decline over the last 25 years.

Observations (black) of Arctic sea-ice extent in September. The dashed black line represents the long-term linear trend and various deliberately chosen 8-year linear trends are shown in colours. Source: Climate Lab Book

Uncertainties

Despite the advances that CryoSat-2 brings, other polar scientists tell us why we still need to be careful when interpreting satellite measurements of ice thickness.

The study assumes the depth of snow using averages from other observations collected between 1954 and 1991. This could cause problems with the thickness estimates, says Prof Mark Serreze, the director of the National Snow and Ice Data Center (NSIDC ), who wasn’t involved in the study. He tells Carbon Brief:

“I find it hard to justify this assumption, for the depth and density of the snow atop the sea ice can be highly variable both spatially and from year-to-year.”

Working out snow depth is one of the largest sources of error when calculating ice thickness, says Dr Julienne Stroeve, a senior research scientist at NSIDC. With just four years of CryoSat-2 data to work with, it might be a bit early to inform how sea ice copes with variable Arctic weather. She tells Carbon Brief:

“I think assuming climatology for snow cover based on data collected several decades ago – especially for such a short time-series – and drawing firm conclusions about how ice thickness has changed is premature.”

Nonetheless, understanding how Arctic sea ice is affected by year-to-year variability helps scientists make reliable predictions of when we’re likely to see a summer with no sea ice, says co-author Prof Andy Shepherd:

“Although the jump in volume means that the region is unlikely to be ice free this summer, we still expect temperatures to rise in the future, and so the events of 2013 will have simply wound the clock back a few years on the long-term pattern of decline.”

Even with such a short record, the paper demonstrates the highly variable conditions in the Arctic, says Serreze, and that the eventual slide to an ice-free summer Arctic Ocean will not be smooth, but will occur in fits and starts.

Main image: Open passage in pack ice.