Though much has been discussed over the years about the accelerated melting of the world’s biggest ice sheet in Antarctica, the situation continues to aggravate. New research shows warm waters rushing towards the East Antarctic ice sheet, about the size of the US, revealing a potential new driver of global sea-level rise. The research, published last week in Nature Climate Change, shows changing water circulation in the Southern Ocean may be compromising the stability of the ice sheet. The researchers are Laura Herraiz Borreguero and Jess Melbourne-Thomas with the Commonwealth Scientific and Industrial Research Organisation, an Australian government agency responsible for scientific research, and Alberto Naveira Garabato, a Professor at the National Oceanography Centre of the University of Southampton.
The changes in water circulation are caused by shifts in wind patterns, and linked to factors including climate change. The resulting warmer waters and sea-level rise may damage marine life and threaten human coastal settlements. The research findings underscore the urgency of limiting global warming to below 1.5 degree Celsius so as to avert the most catastrophic climate harms. Ice sheets comprise glacial ice that has accumulated from precipitation over land. Where the sheets extend from the land and float on the ocean, they are known as ice shelves. It’s well known that the West Antarctic ice sheet is melting and contributing to sea-level rise. But until now, far less was known about its counterpart in the east.
The research focused offshore a region known as the Aurora Subglacial Basin in the Indian Ocean and this area of frozen sea ice forms part of the East Antarctic ice sheet. How this basin will respond to climate change is one of the largest uncertainties in projections of sea-level rise this century. If the basin melted fully, global sea levels would rise by 5.1m. Much of the basin is below sea level, making it particularly sensitive to ocean melting, because deep seawater requires lower temperatures to freeze than shallower seawater.
The researchers examined 90 years of oceanographic observations off the Aurora Subglacial Basin and found unequivocal ocean warming at a rate of up to 2C to 3C since the earlier half of the 20th century. This equates to 0.1C to 0.4C per decade. The warming trend has tripled since the 1990s, reaching a rate of 0.3C to 0.9C each decade. To understand how this warming is linked to climate change, one should look to a belt of strong westerly winds over the Southern Ocean. Since the 1960s, these winds have been moving south towards Antarctica during years when the Southern Annular Mode, a climate driver, is in a positive phase. The phenomenon has been partly attributed to increasing greenhouse gases in the atmosphere. As a result, westerly winds are moving closer to Antarctica in summer, bringing warm water with them.
Previous work on the effects of climate change in the East Antarctic has generally assumed that warming first occurs in the ocean’s surface layers. The latest study’s findings — that deeper water is warming first — suggest a need to re-think potential impacts on marine life. Antarctic krill, for example, breed by sinking eggs to deep ocean depths. But, warming of deeper waters may affect the development of eggs and larvae. This, in turn, would affect krill populations and dependent predators such as penguins, seals and whales.
The researchers have expressed hope that their results will inspire global efforts to limit global warming below 1.5C. To achieve this, global greenhouse gas emissions need to fall by around 43% by 2030 and to near zero by 2050. Warming above 1.5C greatly increases the risk of destabilising the Antarctic ice sheet, leading to substantial sea-level rise. But staying below 1.5C would keep sea-level rise to no more than an additional 0.5m by 2100.
