Arctic sea-ice extent on an extremely low course

Thursday 17 September 2020

The Arctic sea-ice extent in September is the lowest level in the annual cycle and reflects the melting processes during the previous summer season since March, when the melting begins. Since the start of continuous satellite monitoring in 1979, the summertime sea-ice extent in September has been on the decline, showing a loss of nearly 14% per decade, about 50% over the whole time period. The all-time lowest mean September extent, at 3.49 million km², was reached in 2012 (see Table 1), followed by the years 2019 and 2007. Due to ice feedback effects, global climate warming is especially pronounced in the Arctic, producing twice the warming and widespread sea-ice retreat. This year the sea-ice extent in the Arctic once again reached an extremely low level (see Figure 1).

After the summer months June and July 2020 had yielded minimum values in the 42-year time series of sea-ice extent, the trend continued in September, underscoring the importance of Arctic warming as an early-warning system for global climate change. The mean value in August was 5.03 million km², making it the third-lowest extent (after 2012 and 2019) since 1979 (Fig. 2) and placing it ca. 2.1 million km2 below the average for the month in the years 1981 to 2010. The extent on 31 August was 4.19 million km² and had declined by ca. 1.85 million km² – an area roughly 5 times the size of Germany – in the course of the month alone. Though sea-ice retreat slowed somewhat in late July and early August, from 6 August it accelerated. Substantial sea-ice retreat could be observed in the Canadian Basin, which led to major losses of sea-ice cover, particularly in the second half of the month.

One cause of the significant sea-ice retreat: the meteorological conditions in July and August, which included a distinct temperature anomaly over the Central Arctic. Over the course of the summer, the anomaly produced fluctuations in air temperatures at 925 hPa (ca. 760 m) of more than 6° Celsius above the long-term average for the years 1981 to 2010 (Fig. 3, top row). These conditions in the Central Arctic manifested in a stable, substantial pressure anomaly, especially in July, when the high-pressure cell was directly over Siberia and the Central Arctic. The accompanying air currents pushed the warm continental air masses over Siberia into the Central Arctic (Fig. 3, middle row). Since the ice was already quite thin at the edges by this time of year, this could have been conducive to melting. The melting was also influenced by the warm ocean surface temperatures (see Figure 3, bottom row), which were above the long-term average in the Barents, Kara and Laptev Seas, as well as the Siberian Sea. “It’s safe to assume that, due to the stable high-pressure cell over the Central Arctic in July and August, there were far more cloudless days, and that, as a result, the solar radiation, which is normally buffered by cloud cover, was an additional factor in this year’s melting,” says Dr Monica Ionita-Scholz, a climatologist at the Alfred Wegener Institute in Bremerhaven.  

In addition, a storm broke up the ice north of Alaska, making it far more vulnerable to melting processes. Especially in the second half of August, rapid sea-ice retreat could be observed here. Moreover, the high atmospheric temperatures affected the ocean’s surface, which absorbed the heat, producing more intense melting in the marginal ice zones. The ocean surface temperature anomaly reached values exceeding 4° Celsius and was, in addition to atmospheric warming, intensified by inflows of warm water from the Bering See into Chukchi Sea and from the Atlantic sector into the Barents Sea (Fig. 3, bottom row). A trend referred to as ‘Atlantification’ describes the growing inflow of warmth from the North Atlantic into the Arctic. To date, this process has taken place at depths of up to 150 m, but is now moving to depths of 80 m, where it is influencing and altering the circulation and heat transfer in Arctic water masses (see also the study  by Paul Vossen, Climate, Environment).

Some time in the next days, we expect to see this year’s summertime sea-ice minimum in the Arctic. On 16 September the sea-ice extent (sea-ice concentration > 15 %) was 3.84 million km², one of the lowest values recorded this year (Fig. 1).  We’ll have to wait and see if the number drops any farther (see Table 1). As a rule, the summertime sea-ice minimum comes in mid-September, though sometimes not until the second half of the month; it largely depends on the prevailing weather and wind conditions.

According to our data, the sea-ice extent is developing between the years 2012 and 2019 and thus probably represents the second lowest ice extent since 1979. Figure 5 shows the curve of the September mean until 2019 and the possible mean ice extent in 2020 predicted by Sea Ice Outlook. Other algorithms, e.g. that used by the National Snow and Ice Data Centre (NSIDC), can vary slightly. “These minor differences are due to the higher resolution in our data, and to the slightly different methods, which use different data centres to calculate the ice concentration. They show the uncertainties that even modern satellite-based observations of the sea ice can contain,” explains Dr Gunnar Spreen from the University of Bremen’s Institute of Environmental Physics (IUP), who is currently participating in the last leg of the MOSAiC expedition. For example, sensors on board different satellites are used; the respective sensors scan at different wavelengths, resulting in different spatial resolutions. In this regard, the IUP’s algorithm uses a very high resolution: 6.5 km x 6.5 km. Further, how coastlines are handled in the analysis is an important factor in calculating the area covered by ice, since the satellite data naturally also includes stretches of land, not just the ocean (using ‘retrieval algorithms’; for more information, see the Nature Correspondence article here).

This year’s summertime sea-ice extent in the Arctic, the second-lowest ever recorded, makes one thing very clear: “Eight of the ten lowest sea-ice extents ever observed came after 2010! Global climate warming is especially pronounced in the Arctic, where it is affecting the entire climate and ecosystem. The current trend in summertime ice loss indicates that we might see ice-free summers in the Arctic by mid-century, perhaps even sooner, which could have dramatic effects on the Arctic habitat, but also on weather and climate trends in the Northern Hemisphere and especially Europe” (see article here). The scientifically based recommendations are clear: only efforts to massively reduce greenhouse-gas emissions, and rapidly implemented, can help to avoid an unchecked rise in temperatures, and all the negative consequences that will accompany it,” says Dr Gunnar Spreen, head of the Research Group Remote Sensing of Polar Regions at the IUP/University of Bremen

Dr Renate Treffeisen (AWI)
Dr Klaus Grosfeld (AWI)
Dr Christian Melsheimer (IUP Bremen)
Dr Monica Ionota-Scholz (AWI)
Prof Dr Christian Haas (AWI)
Dr Gunnar Spreen (IUP Bremen)