The volume of Arctic sea ice increased by a third after the summer of 2013 as the unusually cool air temperatures prevented the ice from melting, according to UCL and University of Leeds scientists. This suggests that the ice pack in the Northern hemisphere is more sensitive to changes in summer melting than it is to winter cooling, a finding which will help researchers to predict future changes in its volume. The study, published in Nature Geoscience today and funded by the Natural Environment Research Council (NERC), used 88 million measurements of sea ice thickness recorded by the European Space Agency's CryoSat-2 mission between 2010 and 2014. It showed that there was a 14% reduction in the volume of summertime Arctic sea ice between 2010 and 2012, but the volume of ice jumped by 41% in 2013 (relative to the previous year), when the summer was 5% cooler than the previous year.

Lead author and PhD student, Rachel Tilling from the Centre for Polar Observation and Modelling (CPOM), UCL Earth & Planetary Sciences, said: "The summer of 2013 was much cooler than recent years with temperatures typical of those seen in the late 1990s. This allowed thick sea ice to persist northwest of Greenland because there were fewer days when it could melt. Although models have suggested that the volume of Arctic sea ice is in long term decline, we know now that it can recover by a significant amount if the melting season is cut short."

The volume of Arctic ice has been steadily falling since the late 1970s but was difficult to assess accurately before CryoSat-2, which measures ice-thickness across the whole region. When compared to ~772,000 readings from an airborne laser, 430 measurements from electromagnetic sensors and 80 million upward-looking sonar observations, the team found that CryoSat's measurements of sea ice thickness agreed to within 2mm.

Miss Tilling added: "Until CryoSat-2 was launched, it was tricky to measure the volume of Arctic sea ice as the pack drifts and measurements could not be taken across the whole region. Together with maps of sea ice extent, our measurements of sea ice thickness now complete the picture because they reveal what's going on below the water, where most of the action happens."