Last week I joined twelve leading sea ice scientists at the International Space Science Institute (ISSI) in Bern for a meeting to discuss satellite measurements of Antarctic sea ice. ISSI offers generous support to young scientists like me to attend these meetings. But why is Antarctic sea ice so important but so difficult to measure by comparison to its Northern sibling?
Like that of the Arctic, Antarctic sea ice plays a key role in our climate. However, it’s chronically understudied, partly because nobody lives on Antarctica (apart from scientists). It also is less of a barrier to international shipping, a feature which motivates a significant chunk of Arctic sea ice research. Because of these reasons and others (discussed later), it’s relatively poorly understood.
Unlike the Arctic Ocean, which is a sea almost entirely surrounded by continents, the Southern Ocean surrounds its own continental island. Because of this shape and the fact that the island sits over the pole, winds and ocean currents can swirl unimpeded round the bottom of the earth. This swirling of air and water isolates frozen continent from many of earth’s atmospheric and oceanic processes, keeping it cool and partly shielding it from polar amplification. This shielding has even helped sea ice increase its spatial coverage with recent global warming. Even with this shielding, Southern sea ice has its own challenges to growth. Rather than sitting in a protected ocean, southern sea ice is free to float northwards into warmer water and melt, meaning that southern sea ice rarely survives a summer and is in general young and thin compared to northern sea ice.
While we have a good idea how much area is covered by Antarctic sea ice (on average just less than Arctic sea ice at ~10 million square kilometres), data on its thickness is patchy at best. Because of this, we really don’t know how much sea ice actually exists around Antarctica. In the Arctic we can use radar from satellites to measure how much the ice protrudes out of the water, and from this work out how much ice is submerged. In contrast, in the Antarctic, the snow is often so thick that it weighs down the surface of the ice enough to totally submerge it. When this happens we can’t measure ice thickness directly. Adding to the difficulty, Arctic snow generally permits radar penetration and allows us to measure the ice surface. Antarctic snow is much more prone to reflecting satellite radar and so blocks direct measurment of the sea ice.
In fact, snow is regarded among scientists as the major barrier to a thorough understanding of Antarctic sea ice thickness. Its depth, density and microscopic properties affect both our view of the ice from satellites and the processes that form and melt of the ice itself. Scientists are yet to develop a “snow map” for the Antarctic sea ice, a source of consternation for those wanting to study the ice below.
Despite these barriers to research, a full understanding Antarctic sea ice processes is an attractive prize. Our ability to model and predict the climate depends on our estimation of the fresh-water locked up in sea ice each winter. Extremely salty water is rejected from the ice as it freezes and drives global ocean circulation. Finally, the thickness of the ice (and the overlying snow) regulates the light that reaches the surface ocean, fuelling the Southern Ocean food web. Scientists continue to untangle these interrelated processes, and it was great to be a part of it in Bern last week – thanks again to ISSI and to Petra Heil and Rachel Tilling for inviting me.