MEASURING sea ice is difficult. Not only does it appear in the most remote, inhospitable parts of the world, it is constantly either melting or forming. Since 1979, satellites have made the job easier, but they can give a misleading picture. Using satellite images to tot up the total area of sea ice risks mistaking surface melt for open water during the summer melting season. Scientists at the National Snow and Ice Data Center (NSIDC) in Colorado instead measure sea-ice extent by dividing the images into grids and counting any squares with more than 15% ice concentration as “ice covered”. Sea-ice extent is always larger than sea-ice area, but this method eliminates melt-season inaccuracies.

Scientists are interested in sea ice as a marker—and amplifier—of climate change. Its bright surface reflects 80% of the sunlight that hits it back into space. When it melts, the uncovered dark ocean surface absorbs 90% of the sunlight, which heats it up, causing more ice to melt. In recent years, the melting season in the Arctic has been ending later in the year, leading to less time for new ice to form. As a consequence, the total sea-ice extent in September 2016 was over 3m km² smaller than in September 1980, although not as small as in September 2012, the worst year on record. A recent graph tweeted by a PhD student showing a dramatic drop in global sea-ice area over the past three months has caused a stir in the climate science community, although the NSIDC reportedly takes issue with the way its Arctic and Antarctic data are combined. Less controversial but equally alarming is the NSIDC's latest data release, which indicates that sea-ice extent in the Arctic is currently around 2m km² below average, the lowest November figure on record.