Last week the extent of Arctic sea ice reached its summer minimum, the second lowest since satellite records began in 1979. The lowest was recorded in 2012 when the sea ice shrunk to an area of just 3.41 million square kilometres.
As in the rest of the world, as climate change progresses, 2016 has seen some of the highest temperatures ever recorded in the Arctic. During parts of the year, the sea ice extent has tracked well below monthly averages. Cloudy weather since June is thought to have slowed the melt rate (which in May was two to four weeks ahead of schedule), preventing a new record from being set this autumn.
Yet while new records draw attention to the Arctic, it is the long-term trend that should be the cause of most concern. With climate change, the Arctic is warming twice as fast as the mid-latitudes. The past decade has seen nine of the ten lowest summer sea ice minima on record. This suggests that low levels of sea ice are becoming the norm, within a small range of year-to-year variation. However, a longer-term perspective helps put their significance into context.
Since the beginning of satellite records, the summer minimum has declined by 13.7% per decade. Although there will likely always be sea ice during the long Arctic winter when there is no sunlight north of the Arctic Circle, the winter peak is also declining, by 3.2% per decade. Put differently, the summer minimum has fallen from around 6-7 million square kilometres in the 1980s and 1990s, to between 3-5 million square kilometres since 2007. The winter peak has fallen from around 17.5 million square kilometres to approximately 15 million square kilometres over the same period.
This means that Arctic sea ice is not just in decline, but seasonal variation is increasing (as the difference between the minimum and the peak widens), with as yet uncertain consequences for both marine and terrestrial ecosystems. Even so, substantial changes to vegetation cover and fish migration are already being observed.
Another consequence of the greater seasonal changes is that the sea ice is also getting younger. First year ice is thinner and far more fragile than compact multi-year ice (which has survived more than one melt season), and as such, much more vulnerable to being broken up by inclement weather (as happened in 2012).
It also means that it is easier for ships to break through the ice (if they have to pass through it all), opening up the possibility of increased shipping activity across the Arctic (last week also saw the British explorer, David Hempleman-Adams and his team complete their attempt to sail the Northeast and Northwest Passages in a single summer). Even at high latitudes, the sea ice is significantly thinner than previously recorded.
The international scientific community disagrees about what will happen next. Most scientists predict we will not see a consistently ice-free Arctic until circa. 2020 at the earliest. However, they also argue that focussing on these predictions of a relatively arbitrary “ice free” value can distract attention from the relentless downward trend, and the fact that the Arctic environment is already changing in potentially irrevocable ways.