Sunday May 20, 2012

To see climate change, watch the sea

By MASAE HONMA

THE Earth turns white when a change in large-scale ocean circulation triggers a sudden worldwide shift toward freezing temperatures. You may remember this apocalyptic scenario as the climax of the 2004 US movie The Day After Tomorrow. But how many of us are aware that the ocean can dramatically effect our climate in reality?

In addition to well-known currents near the surface of the sea, such as the Kuroshio current around the coast of south east Asia, Japan and China, there is a massive global current that flows unseen in the deep, thousands of metres below the surface, called oceanic general circulation.

Ocean water becomes heavier when it is colder and when it contains more salt. Around the polar regions, ocean water is cooled down by air and forms ice. Because the ice does not contain salt, the salinity of the surrounding sea water rises, which results in ocean water near Antarctica or the North Atlantic sinking to join oceanic general circulation.

The oceans are said to be able to hold about 1,000 times more heat than the atmosphere can. If all the oceanic water in the world released enough heat to reduce its own temperature by 0.01°C, the temperature of the atmosphere would be 10°C higher. A small change in the sea can profoundly effect our climate.

As oceanic general circulation delivers heat around the world, a change in it could affect the climate. Thus, detailed observation of the sea is necessary to detect climate change.

But observing the sea is not easy at all. Satellites can only observe the surface of the sea. To observe conditions underwater, the surveying devices need to be in the water, making it extremely difficult to cover the entire sea.

Therefore, a worldwide observation system dubbed Argo, a global array of deep-sea probes to measure temperature and salinity, began operation in 2000. Japan is one of the many nations participating. About 3,500 automatic measuring probes – called Argo floats – now drifting in the world’s oceans dive to about 2,000m below the surface every 10 days to measure water temperature and salinity.

Research so far has already shown the possibility of salinity falling in northern regions – near the North Pole, off Alaska and off the Chishima (Kuril) Islands. Such salinity changes may cause changes in sinking or rising currents in the oceanic general circulation.

The research also reveals the tendency of salinity to rise in areas with little rain, including Hawaii, and of salinity to fall in areas with heavy rain, including some northern areas. Widening gaps in the amount of rainfall from one area to another, like similar gaps already observed in areas on land, may be occurring at sea, experts said.

Observations using floats equipped with oxygen density measuring instruments have started as well, to study the environment that maintains the marine ecosystem.

The level of oxygen is relatively high near the surface of the sea, where many fish live, thanks to the abundance of phytoplankton that produces oxygen. But this ecosystem could not be maintained if the phytoplankton did not get sufficient nutrients from deeper layers. The new observations attempt to find out how the nutritious water from deeper levels with little oxygen moves up to the surface.

Observations in the sea around Japan by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and others since last year have found that large-scale gyres measuring hundreds of kilometres in diameter sometimes occur, stirring up water and revitalizing biological activity. The Argo floats will also study this mechanism in detail.

“We’ve only just started the observations, and there are many mysteries. We hope to unravel oceanic climate change on a global scale,” said Shigeki Hosoda, deputy team leader of JAMSTEC. – Yomiuri Shimbun / Asia News Network