Antarctic sea ice extent has been declining since 2016 but this year the bottom dropped out. A new record low minimum extent (for the satellite era when measurements are accurate) happened in February. Ice extent recovery started out on a normal trajectory in March and April but then faltered. Now the winter maximum extent plateauing far below the previous low maximum. What has happened to the ice?
Postdoctoral climate scientist Zach Labe wrote an extended blog post in July explaining we just don’t know, but pointed to a combination of an amplified weather pattern — the Amundsen low — and increased heat in the oceans as possible drivers. There’s no doubt that specific weather patterns are affecting this winter’s sea ice distribution and extent but Antarctic sea ice extent has been in decline since 2016. Is this decline caused by natural variability or is something else going on?
A new open access Nature Communications paper by two Australian scientists gives the best explanation I have seen for what’s happening. Record low sea ice coverage indicates a new sea ice state. Weather patterns help explain the details of this year’s record low extent, but they do not explain the changes that have happened since 2016. The abstract of the paper is concise and accessible to the general reader.
In February 2023, Antarctic sea ice set a record minimum; there have now been three record-breaking low sea ice summers in seven years. Following the summer minimum, circumpolar Antarctic sea ice coverage remained exceptionally low during the autumn and winter advance, leading to the largest negative areal extent anomalies observed over the satellite era. Here, we show the confluence of Southern Ocean subsurface warming and record minima and suggest that ocean warming has played a role in pushing Antarctic sea ice into a new low-extent state. In addition, this new state exhibits different seasonal persistence characteristics, suggesting that the underlying processes controlling Antarctic sea ice coverage may have altered.
The weather over Antarctica is still very cold, despite an increasing frequency of extreme warm events. Warming in the ocean, below the surface is causing the decline of Antarctic sea ice, but how exactly can the subsurface be warming if it’s not directly caused by the weather?
The Australian scientists go back to a 2015 paper that modelled the impacts of the Antarctic ozone hole on the southern ocean. The Antarctic ozone hole causes a cooling over Antarctica because ozone interacts with ultraviolet light, heating the atmosphere. The cooling of the upper atmosphere leads to a tightening of the jet stream around Antarctica and an increase of the westerly winds — the roaring fifties — that ring Antarctica. Climate change caused by increasing levels of CO2 and other greenhouse gases causes a tightening of the jet stream and westerly winds by increasing the tropical and subtropical convection cells in the atmosphere and pushing the temperate zones towards the poles. So the model of the impacts of the ozone hole is amplified by the human caused addition of greenhouse gases.
What the model study found was a two step impact on surface temperatures, sea ice and ocean heat. The first step was an increase in the cold winds blowing northwards off of Antarctica over the southern ocean as violent storms whipped close to the continent pulling cold air off of the frigid interior. This first step causes the surface ocean to cool and sea ice extent to expand northwards. This is what was observed from 2007 to 2016 following a relatively stable period from 1978-2007.
However, the spinning up of the winds around Antarctica causes the ocean currents to spin up as well. One of the impacts of low pressure areas is that they spin up cold water from below as they spin water out of the region of low pressure. It’s a process that ocean scientists call Ekman upwelling. The whole southern ocean south of the roaring fifties is a low pressure area when averaged over months or years. Therefor the tightening of the winds and currents around Antarctica has led to increased rates of upwelling of water around Antarctica.
Rapid warming of the upper 300m — one thousand feet — of the southern ocean has been observed around since 2016. Antarctic sea ice has been in precipitous decline since 2016.
The intermediate water that is welling up under the sea ice is a mixture of relatively warm water that originated in the north Atlantic’s Atlantic Meridional Overturning Circulation (AMOC) with a melange of water mixed by the eddies and currents ringing Antarctica. The rising up of these waters is heating the southern ocean and Antarctic sea ice from below.
There is good news in this. The rising up of these waters that originated in the far north Atlantic completes the upper ocean overturning circulation that helps to warm Europe. This completion of the cycle helps prevent the collapse of the “Gulf Stream” and the AMOC because it prevents the stagnation of the southern half of the circulation.
However, the bad news is that this warms the waters around Antarctica, melting both glaciers and sea ice.
The Australian scientists go through an extensive analysis to show that the 3 periods in sea ice and ocean temperature state are statistically significant. They explain in detail, with detailed figures why it is unlikely that the decline in sea ice extent is caused by natural variability. However, because the changes are happening so rapidly, the statistics involve short segments and therefore there’s pretty high statistical uncertainty in any conclusions. What we know for certain is that sea ice extent has declined rapidly around Antarctica since 2016 while the top thousand feet of the ocean has warmed. That involves the uptake of enormous amounts of heat by the earth to melt ice and warm water.
The implications are staggering. The Australian scientists coldly discussed the impacts. They include the decline of the formation of the coldest water in the world oceans — Antarctic bottom water. They include increased melting from below of Antarctica’s glaciers and subsequent sea level rise. They include the collapse of colonies of penguins that depend on sea ice. They include the stagnation of the waters in the deepest parts of the ocean, sequestering nutrients from marine life.
The current extremely low sea ice will have a range of impacts. Changed ocean stratification and circulation will alter basal melting beneath ice shelves48. Greater coastal exposure will increase coastal erosion and reduce ice-shelf stability49. Changes in dense shelf water production will alter bottom water formation and deep ocean ventilation50. Sea ice changes will also have contrasting influences on Adélie and emperor penguin colonies51,52, and substantially alter human activities along the Antarctic coastline.
Anthropogenic greenhouse gas emissions have been attributed as the primary cause of Southern Ocean warming28,29, and here we suggest a potential link to a regime shift in Antarctic sea ice. While for many years, Antarctic sea ice increased despite increasing global temperatures6, it appears that we may now be seeing the inevitable decline, long projected by climate models53. The far-reaching implications of Antarctic sea ice loss highlight the urgent need to reduce greenhouse gas emissions.