Sea ice is a marker and amplifier of climate change. In recent years the melting season in the Arctic has been ending later in the year. As a result, the total sea-ice extent in September 2016 was over 3m sq km smaller than in September 1980. In the Arctic, it is around 2m sq km below average
As Bill McKibben writes in The Boston Globe for 22 November 2016, it’s a pretty simple chart. What it shows is the total area of global sea ice, as measured by satellites dating back to 1979. The red line at the bottom is this year, with some weeks still to go. "And it’s disturbing, of course, because this year’s line is very different. It’s not following the normal pattern—there’s a lot less ice. If you look behind the numbers, you find that the volume of both Arctic and Antarctic sea ice is extremely low for the date—and the fact that it’s happening at both poles is what makes the chart so dramatic. It seems to imply that something very unusual is happening.
The National Snow and Ice Data Centre a crucial world resource in keeping track of the cryosphere. Its research and scientific data management is supported in particular by NASA, the National Science Foundation (NSF), the National Oceanic and Atmospheric Administration (NOAA), and other US government agencies.