On 15 September, the Arctic sea ice has most likely reached its minimum extent for this year. On that day, the surface was 3.74 million square kilometers, which was the second lowest amount since 1979 (the year when they started satellite measurements of sea ice extent). Only 2012 had a lower minimum extent with 3.39 million square kilometers. What do these numbers mean and what effect does this have on the Arctic ecosystem? In the next series of blog posts, I’ll try to answer some of these questions. First the numbers, how much sea ice do we have?
In early years, sea ice records were mainly kept by sailors and were quite more sporadic. In late 1978 this changed with the launch of the Nimbus-7 satellite carrying the Scanning Multichannel Microwave Radiometer (SMMR), starting the continuous measurement of the Arctic sea ice. Since 1978, several other satellites have taken over this task.
In the graph above, you can see the Arctic sea ice extent for the past few months, compared with the record low year of 2012 and with the ‘normal’ range of 1981-2010. You can see it reaches its low point somewhere mid-September and it looks like it starts to rise again now. The question is always what do you call sea ice, in this case they’ve taken the amount of sea where at least 15% was covered with ice.
Looking at all of the data collected since 1979, we see a clear downward trend, both in maximum (March) as in minimum (September) extent.
Most of these graphs only look at ice extent, but don’t take volume or age into account. First year sea ice is quite thin (less as a meter thick) and will easily melt in summer. The older the ice gets, the thicker it gets and, usually, the more life it can support. Over the past decades, we’ve also seen a steep decline in older sea ice, making the remaining ice more vulnerable for quick melt in a warmer year.