GLOBAL warming may be creating a “dead zone” in the Sea of Japan. Rising
temperatures are shutting down a circulation process that is crucial to life
there, say researchers in Japan, and the same problem could affect oceans across
the planet.
In winter, oxygen-rich surface water in the Sea of Japan becomes colder than
the water below and sinks, taking oxygen with it. The oxygen encourages the
growth of bacteria that break down organic matter falling to the seabed. At the
same time, the current brings inorganic matter up from the depths, which plant
plankton feed on.
When this convection current was first measured in the 1930s, its effects
were felt more than 2500 metres below the surface. But Yoon Jong-Hwan, a South
Korean marine physicist at the Research Institute for Applied Mechanics on the
Japanese island of Kyushu, says the current is now so weak that it doesn’t reach
down beyond a few hundred metres.
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Meanwhile, the oxygen level at 2500 metres is falling at a rate that would
reduce it to zero within 350 years. This would choke off life at the bottom of
the food chain and extinguish species higher up.
Yoon believes the culprit is global warming. During the past 50 years the
average temperature around the northern Sea of Japan has increased by between
1.5 and 3 °C. The surface water stays warmer in winter, which weakens
convection currents. Oceans might be affected too, says Yoon. “I suspect the
same problem is happening in the open ocean, but we can’t find evidence
[elsewhere] of convection currents being stopped at the bottom,” he says.
Yoon warns that if ocean waters start to circulate less efficiently, the
problem will rapidly get worse. As the convection system weakens, fewer
inorganic nutrients are brought up from the bottom, cutting off a key food
supply for plankton. Not only do plankton form the base of the food chain for
the entire ocean, they are also a major carbon dioxide sink. Without them,
global warming would accelerate, winding down circulation systems even
further.
Other researchers say more work is needed to find out how ocean currents are
affected by global warming. “We’re now initiating a major programme in Britain
to study this,” says Adrian New of the Southampton Oceanography Centre. With his
colleague Jochem Marotzke, he plans to investigate whether rising temperatures
around the North Atlantic are affecting the ocean’s circulation.