Melting Antarctic ice sheets will slow Earth’s strongest ocean current

Melting ice sheets are slowing the Antarctic Circumpolar Current (ACC), the world’s strongest ocean current, researchers have found. 

This melting has implications for global climate indicators, including sea level rise, ocean warming and viability of marine ecosystems. 

The researchers, from the University of Melbourne and NORCE Norway Research Centre, have shown the current slowing by around 20 per cent by 2050 in a high carbon emissions scenario. 

This influx of fresh water into the Southern Ocean is expected to change the properties, such as density (salinity), of the ocean and its circulation patterns. 

University of Melbourne researchers, fluid mechanist Associate Professor Bishakhdatta Gayen and climate scientist Dr Taimoor Sohail, and oceanographer Dr Andreas Klocker from the NORCE Norwegian Research Centre, analysed a high-resolution ocean and sea ice simulation of ocean currents, heat transport and other factors to diagnose the impact of changing temperature, saltiness and wind conditions. 

Associate Professor Gayen said: “The ocean is extremely complex and finely balanced. If this current ‘engine’ breaks down, there could be severe consequences, including more climate variability, with greater extremes in certain regions, and accelerated global warming due to a reduction in the ocean’s capacity to act as a carbon sink.” 

The ACC works as a barrier to invasive species, like rafts of southern bull kelp that ride the currents, or marine-borne animals like shrimp or molluscs, from other continents reaching Antarctica. 

As the ACC slows and weakens, there is a higher likelihood such species will make their way onto the fragile Antarctic continent, with a potentially severe impact on the food web, which may, for example, change the available diet of Antarctic penguins. 

More than four times stronger than the gulf stream, the ACC is a crucial part of the world’s “ocean conveyor belt”, which moves water around the globe – linking the Atlantic, Pacific and Indian Oceans – and is the main mechanism for the exchange of heat, carbon dioxide, chemicals and biology across these ocean basins. 

The researchers used Australia’s fastest supercomputer and climate simulator, GADI, located at Access National Research Infrastructure in Canberra. The underlying model (ACCESS-OM2-01) has been developed over a number of years by Australian researchers from various universities. 

The projections explored in this analysis were conducted by a research team based at UNSW, who found that the transport of ocean water from the surface to the deep may also slow in the future. 

Dr Sohail said it is predicted that the slow-down will be similar under the lower emissions scenario, provided ice melting accelerates as predicted in other studies. 

“The 2015 Paris Agreement aimed to limit global warming to 1.5 degrees Celsius above pre-industrial levels. Many scientists agree that we have already reached this 1.5 degree target, and it is likely to get hotter, with flow-on impacts on Antarctic ice melting,” Dr Sohail said. 

“Concerted efforts to limit global warming (by reducing carbon emissions) will limit Antarctic ice melting, averting the projected ACC slowdown.” 

Published in Environmental Research Letters today, the research reveals that the impact of ice melting and ocean warming on the ACC is more complex than previously thought. 

“The melting ice sheets dump vast quantities of fresh water into the salty ocean. This sudden change in ocean ‘salinity’ has a series of consequences – including the weakening of the sinking of surface ocean water to the deep (called the Antarctic Bottom Water), and, based on this study, a weakening of the strong ocean jet that surrounds Antarctica,” Associate Professor Gayen said. 

The new research contrasts with previous studies that suggested the ACC may be accelerating due to steeper temperature differences in different latitudes of the ocean caused by climate change, he says. 

“Ocean models have historically been unable to adequately resolve the small-scale processes that control current strength. This model resolves such processes, and shows a mechanism through which the ACC is projected to actually slow down in the future. However, further observational and modelling studies of this poorly-observed region are necessary to definitively discern the current’s response to climate change.”