Global warming reaches the Antarctic abyss

by Catherine Brahic, Copenhagen, New Scientist, March 11, 2009

Even the deepest, darkest reaches of the Antarctic abyss are feeling the heat, according to new results presented at the climate change congress in Copenhagen, Denmark, on Tuesday.

Gregory Johnson, of the US National Oceanic and Atmospheric Agency, says even he was surprised by the findings. He says the changes could be responsible for up to 20% of the observed global sea-level rise.

As part of the CLIVAR project, Johnson and a team of international colleagues have been spending weeks at a time at sea, tracing straight lines across all of the world's oceans. As they make these traverses, they measure the temperatures of the water from the very bottom right up to the surface.

The team takes its measurements along the same routes as expeditions carried out in the 1990s, which provides a picture of how things have changed in roughly one decade.

Global influence

The researchers are particularly interested in the masses of cold water that sink down to the abyss along the shores of Antarctica before moving north along the ocean floor into the Atlantic, Pacific and Indian Oceans.

These three flows of Antarctic abyssal water overwhelmingly influence the deep waters of the world, says Johnson. Water sinks off the coast of Greenland too, but the Antarctic abyssal water volume is twice that of the north Atlantic.

Early results from CLIVAR show that abyssal water is warmer now than it was in the 1990s. The water that travels from Antarctica into the south-eastern Indian basin is roughly 0.1 °C warmer. The deep ocean current travelling from Antarctica into the Pacific is 0.03 °C warmer.

In the northern hemisphere, the deep abyssal Atlantic water, which sits between the ocean floor and the layer of deep water that sinks off the coast of Greenland and travels south, is 0.04 °C warmer.

What surprises Johnson most is that the warmer deep Antarctic water is apparently carried all the way to the north Pacific, too. Other vessels that have monitored what happens to the abyssal water as it moves north have also noticed a warming, albeit a smaller one.

Diluted oceans

The researchers have also looked at the salinity -- important because it affects water buoyancy -- of the deep Antarctic waters. They found that here, too, there is change: in both the southeast Indian Ocean and in the Pacific, the water is less salty today than it was in the 1990s. Most likely, says Johnson, this is a direct result of dilution from the melting Antarctic ice.

He is very reluctant, however, to say what is warming the abyss. Two possibilities present themselves: either the water is being warmed more at the surface near Antarctica before sinking into the abyss, or it is taking longer to sink and therefore has a longer time to soak up the surrounding temperatures.

As for whether human-driven climate change has anything to do with it: "It's just too early to say," Johnson says.

Either way, the changes are significant. On average, over the last decade, water at the surface of the oceans has gained 0.35 watts per square metre -- a measure of the amount of heat absorbed from the warming atmosphere. Johnson's measurements in the abyss are, in some regions, nearly three times that.

Johnson estimates that the warming and consequent expansion of the deep water flows may be responsible for between 10% and 20% of the global sea-level rise seen during that time.

Link to article: http://www.newscientist.com/article/dn16740-global-warming-reaches-the-antarctic-abyss.html