Some glacial periods in the Earth’s more recent geological past have been cooler and more severe than others, despite very similar greenhouse gas concentrations and orbital parameters. What is it that decouples global temperature from carbon dioxide levels and the solar heat?
Changes in ocean circulation, particularly in the climatically crucial North Atlantic region, are the most likely candidate. A paper in Nature (subscription required) now suggests that some of these changes originate more than 10,000 kilometres away in the subtropical Indian Ocean.
Edouard Bard and Rosalind Rickaby analysed an 800,000-year record of sea surface temperature and ocean productivity from an ocean sediment core retrieved off the southeastern coast of South Africa (Editor’s summary). This is the region where a portion of the warm and salty water carried southwards by the Agulhas current, the Indian Ocean equivalent to the Gulf Stream, leaks into the South Atlantic. The inflow compensates for the export of cold Atlantic deep water to other ocean basins. More importantly, it fuels the Atlantic overturning circulation which carries warm tropical surface water towards the poles, and cold deep water back towards the equator.
The strength of this heat conveyor depends on the position of ocean fronts, boundaries between water masses of different temperature and salinity, which are known to intermittently shift northwards and southwards.
Bard and Rickaby suggest that the Agulhas current between Madagascar and the African coast has almost come to a halt during times when the subtropical front in the Indian Ocean migrated northwards by up to 1,000 kilometres. Isotopic data from the sediment record suggest this has happened at least twice, namely during glacial stadials around 340,000 and 420,000 years ago. The closure of the Agulhas ‘valve’ might explain why these glacial periods have been severely colder than most others before and thereafter.
The “persuasive” study makes “a compelling case that, at times in the past, severely reduced water transport between the Indian and Atlantic oceans may have caused climate to cool beyond typical ice-age conditions,” writes Barcelona-based oceanographer Rainer Zahn in a News & Views piece (subscription required).
There’s a tempting afterthought: If in warm climates such as ours more Indian Ocean water leaks into the Atlantic it could help stabilize the Atlantic overturning circulation, safeguarding us against drastic climate disruptions caused by the heat conveyor’s hypothesized failure.
But as is often the case in Earth system studies, it is hard to distinguish the forces that push from those that pull on the causal chain to mode shifts. It is not quite clear, for example, what might have caused the subtropical Indian Ocean front to move this far northwards in the first place.
How the Agulhas current might behave if temperatures continue to rise is even more uncertain. “Let’s keep an eye on what the leakage does next,” says Zahn.
Image: The Agulhas current carries warm Indian Ocean water around the Cape of Good Hope.
Credit: SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE