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Arctic Sea Ice: Long-term decline held back

Dr David Whitehouse, GWPF Science Editor

Any day now the Arctic sea-ice will cease melting and start to refreeze as the brief boreal summer draws to a close. It’s been a cool and stormy summer in those parts yet this year’s sea-ice minimum will be one of the highest of the past decade at around 4.73 million sq km which is greater than in 2020, 2019, 2018, 2017, 2016, 2015, 2012, 2011, 2008 and 2007.

So what is all this we read and hear about the Arctic sea-ice decreasing at an alarming rate and ice-free summers due in the near-future?

Looking at the data from the satellite era (post-1979) it’s obvious it has been declining but the standstill of the past decade or so cannot be ignored. What some see as a continued decline others see as a bottoming out. But this is frequently denied or overlooked, unwisely extrapolated through or cherry-picked out of existence.

According to Schroeder writing in Climate Change the Arctic sea ice extent has never been as low as at present during the past 1450 years and that natural variability alone cannot explain the decline with an increase of atmospheric CO2 resulting in an increase in long-wave radiation producing an increase in ice melt. However current climate models suggest that natural climate variability and the impact of anthropogenic climate change are of similar magnitude with no consensus as to which is the strongest indicating that those promoting the Arctic sea ice as a prominent indicator of climate change are being misleading.

This situation is often not reflected even in peer-reviewed scientific literature. Take for instance the first line of a recent paper by Yang et al in Science Bulletin “While Arctic sea ice has been decreasing in recent decades that is largely due to anthropogenic forcing…”

Despite this, the main thrust of Yang et al is to point out the differences between the behaviour of Arctic and Antarctic sea ice extent. They say that contrary to what was happening in the north and in the face of global warming the extent of Antarctic sea ice showed an increasing trend during 1979–2015, followed by an abrupt decrease. Using ice cores they find that for the entire Antarctic there is a decreasing trend in ice-extent of −0.03° per decade, which is not a lot, for the 20th century, with a rapid decline in the mid-1950s. It was not until the early 1980s that the observed increasing trend occurred.

Yang however do not tell the whole story in that it omits to say that whilst there was a decline in Antarctic Ice it was short lived and as Simmonds and Li write in the Annals of the New York Academy of Sciences 2021 the Antarctic sea-ice extent increased up to 2014, then commenced a remarkable retreat (the annual mean ice extent decreased by 2.03 million sq km in the 3 years to 2017), and subsequently increased to near its long-term average value in 2020.

Polar events are globally important, Marcianesi at al 2021 writing in Polar Science point out that the overall warming effect due to Arctic land and ocean albedo change corresponds to adding about 44% to the direct effect of human CO2 emissions during the same period with a global effect.

The climate models used to analyse the polar regions and provide future trajectories are not as good as some think they are. Cai et al writing in the Journal of Climate examine the near-surface mean temperatures in the Arctic as predicted by 22 models participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6). The models it must be said leave a lot to be desired when it comes to one of their primary functions which is to provide a way to see what might happen in the future. Most of them underestimate the observed mean temperature in the Arctic during 1979–2014 and it’s clear that there is a very large model spread and uncertainties exist in the CMIP6 models.

Undaunted by this Diebold and Rudebusch produce forecasts of Arctic sea ice extent for the rest of this century. Their best fitting statistical model indicates that overall sea ice coverage is declining at an increasing rate which is an interesting conclusion when one views the data of the past decade! They estimate an almost a 60 percent chance of an effectively ice-free Arctic Ocean sometime during the 2030s—much earlier than the average projection from other global climate models. Ravindran et al 2021 writing in Polar Science expect ice-free conditions during September by the year 2050.

Putting the Arctic sea-ice variability into a longer perspective few realise that during the early 20th century the Arctic experienced a period of remarkable warming which is not well understood. Aizawa et al reporting in Geophysical Research Letters use what they call a “state-of-the-art” climate model that appears to successfully reproduce the warming event using internal climate variability and the influences of the Sun and volcanoes, in doing so it casts light on the balance of the effects on the region today.

A Longer Pattern?

Looking on the longer term Helama et al in Climate Dynamics 2021 note that Holocene climate variability is punctuated by episodic climatic events such as the Little Ice Age (LIA) predating the industrial-era warming. Their dates and causes are controversial. Even more crucially they add, it is uncertain whether earlier events represent climatic regimes similar to the LIA. So they analyse a new 7500-year long palaeoclimate record tailored to detect LIA-like climatic regimes from northern European tree-ring data.

As well as the actual 17th Century LIA, they found many 100–800 year periods with cold temperatures combined with clear sky conditions from 540 CE, 1670 BCE, 3240 BCE and 5450 BCE onwards. In total these LIA-like regimes covered 20% of the study period. They notes that the ongoing decline in Arctic sea-ice extent is mirrored in their data which shows reversal of LIA-like conditions since the late nineteenth century.

So in a real sense the Arctic sea-ice extent is an emblem of climate change and the various forces contributing to it. Today, as the well-defined ice edge that stretches along most of the Russian side of the Arctic Ocean starts to freeze, the only certain thing is the oncoming cold and dark.