Around 320BC, a Greek merchant called Pytheas set off for a long journey north. He brought back reports of a land called Thule, six days north of Scotland, “where... there are no nights during the [summer] solstice...and also no days during the winter solstice”. It is unclear if Pytheas made it there himself, or merely heard tales. But for this and his account of a “congealed sea”, he is said to have been one of the first Arctic explorers. Were he to return today, he would find a very different landscape.
Temperatures in the Arctic are warming twice as fast as the global average. One driver is the melting of floating sea ice. When it vanishes, it exposes deep blue waters, which absorb more solar energy than white ice does. In turn, this speeds up melting: a classic positive-feedback loop. The ice recedes to an annual minimum extent every September. The record low was set in 2012; 2007 and 2016 are joint second. This year is expected to be level with them.
The best-known consequence of Arctic heating is rising sea levels. Melting sea ice does not raise the water level, for the same reason that melting ice cubes do not make a cup overflow. However, water trapped on land in Greenland’s ice cap does increase the sea level when it melts into the ocean.
Greenland has 2.85m cubic kilometres of ice, enough to lift sea levels seven metres. For now, it is melting slowly. Sea levels are rising by an average of 3.3mm per year; owing to an unusually hot summer in 2019, Greenland will contribute about 1mm.
Another feedback loop involves frozen soil. Normal garden soil consists of 5% carbon; soil in Arctic permafrost regions, rich in organic matter, contains 20-50%. It is thought to hold a total of 1.1-1.5trn tonnes of carbon, more than the atmosphere and ten times as much as the Amazon.
As the Arctic warms, bacteria in the soil consume organic matter faster, releasing more carbon dioxide and methane. These gases can then speed up the greenhouse effect—heating the permafrost further and causing more emissions. This July the Siberian tundra warmed and dried enough to catch fire for weeks, a very unusual event.
The third threat posed by Arctic warming is less scientifically certain but more immediate. Higher Arctic temperatures are thought to affect weather patterns in the northern mid-latitudes, where weather systems form as a result of the temperature gap between the hot tropics and cool pole. The jet stream pushes them west to east.
Some evidence suggests that as the temperature difference shrinks, the jet stream weakens and its wavy pattern deepens. This allows “tongues” of frigid air to reach south, and warm pockets to approach the Arctic Circle. It may also cause both storms and clear skies to stay in place for longer, leading to extended floods and dry spells.
Climate-change sceptics point to cold snaps in North America as evidence that concern about global warming is overheated. In fact such days, caused by chilly air escaping polar latitudes, may be a consequence of Arctic warming.
This article is from our Graphic detail section.