Benjamin Rabe and colleagues analysed more than 5,000 salt-concentration profiles collected using sensors mounted on ships, submarines and large ice floes. Much of the data was collected during the International Polar Year 2007–08. It was thanks to the dense network of observations collected over this period that a comparative assessment of the freshwater content of the Arctic Ocean was made possible for the first time.

The results showed that the freshwater content rose by around 20 per cent, which corresponds to a rise of about 8,400 cubic kilometres of water – the same magnitude as the volume of freshwater that is exported from the ocean each year. The researchers also noticed a thickening of the low-salt layers in the ocean. They suspect that two factors are at work: reduced export of freshwater from the ocean – in the form of sea ice or in liquid form, and increased input from near-coastal regions of Siberia.

The findings have important implications for global ocean circulation and the Arctic climate as the freshwater content of the ocean’s surface layer controls whether heat from the ocean is emitted into the atmosphere or to ice.

May 11