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First published: 15th February, 2023
The polar regions are well known for their ice caps. In Antarctica, the land is covered in kilometers thick ice sheets, while the ocean surrounding the landmass freezes to form a large sea ice field extending to about 20 million kilometers every winter. Sea ice forms when the cold polar air freezes the surface seawater during winter. Sea ice is an important component of the Earth’s climate system. Ice is highly reflective, hence sheltering the seawater below from solar radiation. Ice rejects brine as it forms and makes the surrounding waters salty, hence making the polar regions very weakly stratified. This allows for efficient mixing of deep warm waters with cooler surface waters and is an important part of the global thermohaline circulation. Sea ice is a critical component of the planet’s climate and understanding how sea ice will respond to the changing climate is essential if we want to to accurately represent and forecast climate in our computer models.
Both the poles have seen summer sea ice declining over the last few years, with the Arctic having seen a decline for multiple decades. Sea ice extent during summer have fallen at about -8% per decade since the 1950s. The decline of sea ice around Antarctica began more recently, around the year 2016. February 2023 may break all records for the lowest sea ice extent in the Southern Ocean! This decline is mainly driven by the excess heat that we have put into the climate system by emitting greenhouse gases.
Unintuitive though it may be, the warming climate did not result in a reduction of sea ice in the Antarctic until 2016! In fact, sea ice concentration increased by about 12% in the period from 1979 – 2011. During the same period, the surface air has been warming over the Southern Ocean. The response of sea ice to a warming climate is quite complex as it is affected by the radiation in the atmosphere, the winds, the precipitation, the ocean currents, and the melting of glaciers along the coastline. Further, the overall trend of increasing sea ice hides regional differences. The West Antarctic seas saw a decline in sea ice over this period of time. But the increase in sea ice in other regions was larger, thus leading to an overall increasing trend.
The reasons are still a matter of scientific inquiry, but we have good candidate answers that can explain the trends we have seen this far. I will explain the current state of the science around this crucial question – why did the Southern Ocean sea ice field take so long to start melting under a warmer climate?
Before considering candidate answers to our question, we need to understand the factors that affect the sea ice field. Heat can come from above or from below. The atmosphere controls the radiative heat transfer to the ocean, the winds can carry heat away and also control the rate of evaporation, and finally precipitation also results in heat transfer. The winds push sea ice, and depending on the pattern of winds, sea ice can be bunched together or pushed apart. Coastal winds act to push sea ice away, making coastal regions a site of production and export of sea ice.
From below, the ocean circulation affects how heat moves around the global ocean. In the polar regions, surface waters are colder than waters at depth. If the deep water can make it to the surface, it can elevate the temperatures there. This upwelling of deep waters is controlled by the global thermohaline circulation – the large scale overturning of waters from the equator to the poles. Ocean currents and the westerly belt of winds control the upwelling of these deep waters (click here to see a video of deep waters spiralling upwards around Antarctica). Additionally, the surface stratification of the ocean controls the amount of upwelled waters that can mix with the surface waters. The amount of freshwater that flows into the ocean from melting glaciers and from atmospheric precipitation determine how strongly or weakly stratified the surface ocean is. Weaker surface stratification allows more of the warm deep water to mix with the surface, whereas, fresher waters are lighter and they tend to inhibit the mixing of upwelled deep waters.
Now that we understand that sea ice is found within a complex web of interactions between the atmosphere, ocean, and the cryosphere, we can start exploring the factors that led to sea ice growth from the 1970s to 2016, and the eventual decline since then.
One of the leading theories suggests that the ocean played a major role in our story. A study based on computer models was able to replicate the trends that satellites have observed and found that increasing air temperatures first acted to suppress sea ice growth. The amount of sea ice is determined by how much ice grows in a season versus how much is melted away. Increasing air temperatures slowed down the growth of sea ice. Sea ice does not retain much of the salt and tends to reject brine as it grows, thus dumping a lot of salt into the surface water. The suppression of sea ice growth freshened the surface waters. Fresher waters are more strongly stratified, losing their connection with the warmer waters that lie at depth. With lesser heat making it to the surface from the deep ocean, less heat was available to melt the sea ice. So we had a situation where sea ice growth slowed down due to warmer air temperatures, but the melting of sea ice due to warm water supply from deep below slowed down even more, hence leading to an overall buildup of sea ice!
But then, what led to the reduction in sea ice since the year 2016? Once again, our answer seems to lie with the warm deep waters. Deep waters rise to the surface in giant ocean gyres. Liping Zhang and their co-authors used computer simulations of the Earth’s climate to find that there was a buildup of heat in the subsurface ocean, largely due to stronger winds. Winds in the Southern Ocean are responsible for driving a vertical pump (Ekman pump) that pulls warm waters from deep below. They do this by pushing surface waters apart horizontally and creating a drop in height of the surface of the sea. The spinup of the winds drove a stronger vertical pump which led to the buildup of heat below the surface layer of the ocean. Warmer waters are lighter, making the subsurface light enough to eventually break the stratification which allowed greater mixing of warm waters with surface waters. This process may be the reason why sea ice reduced since 2016 in the Southern Ocean.
The question though is not yet settled! There are many changes occuring to the atmosphere, our climate, the cryosphere, the winds and the major gyre systems that we don’t fully understand. These uncertainties are some of the largest uncertainties in climate model simulations that inform the IPCC reports. It is becoming more and more clear to people across the world that our climate is undergoing rapid change, and governments would like more fine-grained forecasts of our future climate. Settling these uncertainties in the polar regions would go a long way in addressing that need!