By: Holly Ayres
In the Northern Hemisphere, our closest region of sea ice (not to be confused with land ice) is the Arctic, a vast region of frozen ocean at the North Pole. Antarctica, a huge mountainous land mass at the South Pole, is geographically opposite to the Arctic. Surrounded by the Southern Ocean, sea ice, eastward currents and strong westerly winds, it is a unique, remote, and distant location to most of us. Around 90% of humans live in the Northern Hemisphere, which makes sense, since around 70% of land is in the Northern Hemisphere. We would be forgiven for assuming that changes to Antarctic sea ice would not impact us as much as changes to the Arctic.
Antarctic sea ice extent reaches a maximum mean of approximately 18.5 million km2 at its winter peak. In the summer months, Antarctic sea ice is almost completely melted by comparison, at a mean of approximately 3 million km2. Some sea ice remains around coastal areas and regions of the Weddell and Ross Seas and higher latitude regions. Figure 1: (left) Antarctic sea ice extent maximum September 2021, (right) Minimum February 2022. Images from the National Snow and Ice Data Centre, 2022.
How is it changing?
Past trends (pre-2016) in Antarctic sea ice extent show a small but significant decrease. Whereas, Arctic sea ice has been decreasing year on year, in line with the average global temperature increase. Many possible reasons for this contradictory trend in Antarctic sea ice have been proposed by a number of studies, including but not limited to, relationships to the stratospheric ozone hole above Antarctica and the positive trend in the Southern Annular Mode, via intensification of the westerly winds surrounding the region. Other theories involve the ‘ocean asymmetry’ to the Arctic, in addition to interactions with increased glacial melt and local pressures systems such as the Amundsen sea low (e.g. Turner et al., 2009; Polvani et al., 2011; Liu and Curry, 2010; Bintanja et al., 2013; Mackie et al., 2020).
However, in recent years, this trend has almost turned around, with a sea ice minimum in 2016/17, and again in February 2022, changing the significance of this increasing trend (Parkinson 2019). We still do not know a lot about the 2022 minimum, but multiple studies have shown that a combination of conditions caused the 2016 minimum. These include influences from El Nino Southern Oscillation and the Southern Annular Mode, changes to atmospheric wave patterns, and the opening of the Weddell Sea polynya (e.g. Turner et al., 2017; Schlosser et al., 2017; Stuecker et al., 2017; Meehl et al., 2019; Wang et al., 2019; Turner et al., 2020).
It is clear that various aspects of the climate have a big impact on Antarctic sea ice, but what about the other way around?
Figure 2: Arctic (top) and Antarctic (bottom) annual sea ice extent anomalies, showing reduction in Arctic sea ice extent and slight increase in Antarctic sea ice extent, from 1979 to 2022. Images from the National Snow and Ice Data Centre, 2022.
The future and impacts on the climate
Up until recently, it was thought that what happens in the Antarctic stays in the Antarctic- or at least in terms of the climate response to sea ice change. The region is sheltered, protected, and seemingly unaffected by the warming world beyond its reach, so why would sea ice impact anything other than the high latitude Southern Hemisphere?
Sea ice acts as a barrier between the ocean and atmosphere. When that barrier is melted, several things happen to the climate system. The area loses its reflective icy surface, meaning more solar radiation is absorbed by the ocean, causing further warming. This process is called ‘polar amplification’. In the winter months, the ocean is usually a little warmer than the air, due to the high specific heat capacity of water. Heat and gasses can now freely be exchanged between the two, and in the winter months, this means heat is released from the ocean to the atmosphere, that would not usually be released if the winter sea ice barrier were still intact. Sea ice also interacts with the ocean’s deep circulation. When sea ice forms, salt is rejected into the water column in a process called brine rejection. Changes in temperature and salinity control the ocean circulation, therefore sea ice plays a key role.
Recent climate modelling studies have assessed this in detail, being the first to assess the full ocean-atmosphere-ice coupled model impacts to Antarctic sea-ice loss (England et al. 2020a,b; Ayres et al., 2022). Antarctic sea-ice loss first triggers a heat flux response from the ocean to atmosphere, leading to local surface warming over the Southern Ocean. This warming leads to changes in the local pressure and wind systems, namely a negative Southern Annular Mode index and weaker westerly winds. Warming Southern Ocean surface temperatures spread both ways to the Antarctic continent and mid-latitudes oceans, and eventually the equator after several years. The warming changes the wind patterns in the tropical pacific, impacting ocean circulation and the upwelling of cool ocean waters, further warming the tropics. The warming is spread into the Northern Hemisphere through a large atmospheric wave, and eventually reaches the Arctic. Warming in the Arctic leads to sea ice loss, all triggered by that initial Antarctic sea-ice loss, several years before. Meanwhile, the ocean also responds to the Antarctic sea-ice loss, first warming and reducing the salinty of the Southern Ocean and weakening the wind driven easterly currents that surround Antarctica. This leads to further warming and salinty changes globally, across all oceans.
The Arctic plays a huge impact on the climate, observed globally to have connections with the world’s oceans and atmosphere. However, despite the majority of research focusing on the Arctic, it seems that changes to Antarctic sea ice may also play a huge role in the future of the global climate, even in the Northern Hemisphere.
Antarctic sea ice loss would impact the entire global climate, with impacts from the top of the atmosphere to the depths of the ocean, pole to pole.