The polar oceans are among the most rapidly changing oceans in the world. The yearly cycle of the build-up and melting of Arctic sea ice is one of the earth’s vital signs and a key climate variable monitored by scientists.
While they may be far from the world’s major population centres, changes in the polar regions have global implications: they affect the world’s climate through carbon storage and/or release, heat balance, and other environmental and ecological impacts.
One of the most visible indicators of this change is the reduction in the extent of Arctic sea ice by the end of September each year.
Since 1979, scientists have observed a decrease in the extent of Arctic sea ice in all months of the year. The September minimum extent is 36.5 per cent smaller in the period 2010–2019 than it was in the 1980s.
In a recent Intergovernmental Panel on Climate Change report (IPCC, 2019), scientists observed a reduction trend of 12.8 per cent (± 2.3 per cent) per decade. With 4.1 million square kilometres, the 2019 minimum arctic sea ice extent is the second lowest (after the record low of 2012) since the beginning of satellite monitoring in 1979. This rate of decline is likely (between a 66 per cent and 90 per cent probability) to be unprecedented in at least the past 1,000 years.
Under a scenario of global heating of 1.5°C above pre-industrial levels, by the end of the 21st century, the Arctic ocean would only be ice-free in September once in every 100 years. Under a global warming scenario of 2°C, this would occur as much as one year in three, according the IPCC.
Loss of sea ice increases global heating. White snow reflects sunlight, whereas water absorbs it. Declining Arctic sea ice, therefore, amplifies the warming up of the Arctic. Temperatures increased by around 0.5°C per decade between 1982 and 2017, primarily due to increased absorbed solar radiation accompanying sea ice loss since 1979. This is twice as fast as the global average.
Not only is the extent of Arctic sea ice reducing, but it is also much thinner, and scientists observe a transition to younger ice. Five-year-old sea ice has declined by about 90 per cent since 1979.
This has several impacts. It reduces the reflection of light, thus contributing to thermal expansion of the oceans. The change in Arctic sea ice extent may influence mid-latitude weather, and it may affect the regional composition of species, their spatial distribution, and the abundance of many marine species, with impacts on the structure of ecosystems.
For instance, reduced ice cover is adversely affecting the habitat of polar bears which now need to travel greater distances and swim more than previously, particularly threatening young cubs.
While the science tells us that global heating is happening, the exact impacts on fragile habitats and ecosystems are much harder to predict.
“There is an urgent need to reduce our greenhouse gases emissions,” says Peduzzi.