Keeping watch for the Antarctic storm
There’s an old sailor’s expression: ‘below 40 degrees south there is no law, below 50 degrees south there is no God’. No where is more isolated and extreme than the Antarctic. But while it may be out of sight it most certainly shouldn’t be out of our minds. Cut off from civilisation by the storm-ridden Southern Ocean, the Antarctic ice sheets hold around ninety per cent of our planet’s ice and seventy per cent of its fresh water. There’s so much water in the Antarctic that if only a small proportion of it were to melt, global sea level could rise by several feet, threatening the likes of Dhaka, New York, Sydney and London. The big question is just how much will return to the world’s oceans in a warming world? While previous scientific studies have suggested Antarctica should be relatively immune to higher temperatures over the next hundred years, a spate of recent research has suggested the seventh continent is far from being as omnipotent as we had once thought, with some areas already irreversibly melting. Work published in the latest issue of the scientific journal Nature suggests if we stay on the current trajectory of greenhouse gas emissions, Antarctica alone could contribute an estimated three feet of global sea level rise by 2100 (much of it from the relatively low-lying West Antarctic), effectively doubling projections made by the Intergovernmental Panel on Climate Change. This latest research make particularly sobering reading.
|How stable are the Antarctic ice sheets?|
Whilst so-called ‘climate sceptics’ have often used climate changes preserved in the geological record as an argument that nothing of significance is happening today, the past actually provides important insights into how out planet works, independent of what models might say. One of the most important in this regard is a period of sustained warmth known as the Last Interglacial some 125,000 years, when thanks to changes in our planet’s orbit around the Sun, ancient fossils and beach ridges indicate global mean temperature was up to 3˚F warmer than present day and sea levels were over 20 feet higher. Taking into account the expansion of the world’s oceans by higher temperatures and melting as much of the Greenland ice sheet as possible, we’re left with over 12 feet unaccounted for. This is not a trivial amount. And the only suspect left in the room is Antarctica.
Antarctica appears especially vulnerable where ocean waters bathe those parts of the ice sheets sitting on the sea bed. This isn’t generally a problem when temperatures are unchanging. Unfortunately, today the south is warming at four times the rate of the north. Why so much heat is being taken up by the oceans in the Southern Hemisphere is an area of intense research but one contender is the strengthening westerly winds across the mid-latitudes of the Southern Ocean – best known by their colourful monikers the ‘roaring forties’, ‘furious fifties’ and ‘screaming sixties’. These winds can have a profoundly significant impact on the way the Southern Ocean behaves, changing the uptake and distribution of heat, both at the surface and far deep below. While the latest Nature study looked at the impact of similar scales of warming around the entire continent, this isn’t necessarily the case. For instance, we recently we found that by modelling changes in the winds, temperatures can vary wildly across the Southern Ocean, with one of the biggest changes a hotspot of ocean warming off the East Antarctic coast that extended 1600 feet below the surface. And this is where the implications of the latest work become even more worrying.
The East Antarctic ice sheet is a behemoth. It doesn’t just cover a larger area than the west, it’s far, far bigger. With an average thickness of 7300 feet, it’s nine times the size of the West Antarctic in terms of ice volume. If temperatures start to rise in the ocean off the East Antarctic rather than the west, all bets are off. Of the 200 feet of global sea level rise locked up in the Antarctic, some 180 feet lies in the east. But because most of the ice sits above sea level, many scientists had considered it to have been relatively stable in the past. The thinking was the ice sheet is so large that without melting warm ocean waters lapping along the edge of the continent, the East Antarctic would be left largely unscathed by higher air temperatures, ignorant of what’s happening offshore. We now know this just isn’t true. With improved mapping of Antarctica, we now know there are many country-sized parts of the continent that sit on the sea bed, making them vulnerable to warming seas. One of the most important is the Wilkes Basin, an area comparable in size to New Zealand and covered by 14,000 feet of ice, enough to raise the world’s sea level by 12 feet. If anywhere can drive up the world’s sea level quickly, it’s the East Antarctic. Satellites can only tell us part of the story, however. While ocean waters may be cooling and experiencing more sea ice, this may perversely be a consequence of less dense melt water capping the surface. We need to find out what’s happening down below. The problem is being so isolated and extreme, it’s hard to know what precisely is going on.
The latest work shows the southern continent is highly sensitive to what were only subtly warmer ocean temperatures than today. These observations help the reduce the uncertainties in ice sheet and climate models, improving their reliability. The problem is some parts of the Southern Ocean may warm more than others in the future. So what to do? It’s clear we urgently need to reduce our emissions of greenhouse gases if we want to avoid drastic sea level rise. But if we want to be forewarned we desperately need to improve our understanding of what parts of Antarctica melted first during past warm periods like the Last Interglacial. In parallel, we must also step up today’s monitoring of the Southern Ocean. Significant efforts are being made to find out what is happening in the south but we need a fuller picture of where the greatest temperature changes are taking place before melting becomes irreversible. We need to deliver more teams into the Southern Ocean and probe the depths around the entire Antarctic coastline with all the inevitable risks that working in sea ice brings. If melting is happening we need to know where and by how much. Only then can we be best prepared for the storm that is threatening to break.