Last month was the warmest month recorded globally in the last 136 years.
The Northwest Passage, a mythical link through the Canadian Arctic between the Pacific and Atlantic until 1854, is now a viable commercial route.
Until 2009, the Arctic pack ice prevented regular marine shipping throughout most of the year but the Arctic sea ice decline has rendered the waterways more navigable.
The sea ice melts and freezes on a seasonal cycle. From winter maximum to summer minimum, a typical yearly drop in Arctic sea-ice extent in recent years would be from around 15 to five million square kilometres.
The polar weather is still more than cold enough each winter to re-cover the Arctic Ocean with sea ice.
Unfortunately, the quality and thickness of that return ice has been declining, and the amount that survives has plummeted.
Currently, the Arctic sea-ice extent shows a record low for late October, as calculated by the National Snow and Ice Data Center after measurements began by satellite measurements in 1979.
Arctic v Antarctic
This behaviour is not really such a shock, given that Arctic sea ice has been declining for decades in the midst of notable high-latitude warming.
It has been a comforting thought, to some, that the ice cover in the Antarctic has been increasing during the same period.
The stark difference between yearly patterns of sea ice in the Arctic and the Antarctic is mainly a function of where the land sits.
Northern sea ice melts and freezes within the Arctic Ocean, which surrounds and includes the North Pole.
Southern ice melts and freezes in a ring around the continent of Antarctica. As a result, southern sea ice covers a larger area each winter, yet more than 80 per cent of it disappears each summer.
Climatologists have long expected the Antarctic’s sea ice to decline along with that of the Arctic.
Instead, it’s actually expanded to record-high extents at times over the last few years. Even the best computer models have been flummoxed by this trend.
The trend this year
This year though, there has been a huge ice loss. The year’s Antarctic ice extent peaked very early, on August 31, and is now at its second-lowest value on record for late October, beaten only in 1986.
The complex reasons behind the Antarctic’s sea ice’s previously inexplicable behaviour are now becoming clear, just as it is showing signs of a decline in winter sea ice.
In a review paper published in Nature Climate Change in September, the following changes in climatic conditions were cited:
- A strengthening of the mid-latitude westerly winds that encircle Antarctica. These have generated an upwelling of cold subsurface waters in the Southern Ocean, which allows sea ice to expand more readily.
- Increased melt-water flowing from Antarctica into the Southern Ocean. This reduces the salinity of water near the coast, allowing the surface to freeze at a higher temperature.
- Strengthening of a prevailing low in the Amundsen Sea. The flow around the Amundsen Sea Low pulls cold air off the continent and into the Ross Sea, resulting in an increase in sea ice.
The same study also noted unmistakable signs of climate change in the Antarctic, including warming of the subsurface ocean, thinning of ice shelves, and the acceleration of outlet glaciers that ring the ice sheet.
Three of the authors have said “a much more ominous change deeper down in the ocean [is that] worrying rates of subsurface ocean warming have been detected up against the base of ice sheets. There are real fears that subsurface melting could destabilise ice sheets, accelerating future global sea level rise”.
The permanent Antarctic ice sheet stores around 45 metres of potential sea-level rise. The fact that even Antarctic sea ice is in decline and the land ice is melting should remove any lingering feelings of comfort.
With thanks to Bob Henson, WunderBlog.