The Heat Is Online

Ocean Warming Could Impair CO2 Absorption


Jan. 19,1999

By Michael Kahn

WASHINGTON - Global warming could disrupt the ability of a large portion of the world's oceans to absorb carbon dioxide from the atmosphere, setting off a vicious cycle in which the earth gets even hotter, researchers said on Thursday.

Evidence from a new study indicates that some conditions scientists think will occur with global warming may promote the growth of algae in the Southern Ocean that do not absorb carbon dioxide as well as others.

These waters around Antarctica make up 10 percent of the world's oceans and play a significant role in soaking up carbon dioxide, a greenhouse gas seen as one of the main causes of global warming, Kevin Arrigo, biologist at NASA-Goddard Space Flight Centre in Maryland, who led the study, said.

"The capacity for the Southern Ocean to take up carbon dioxide might be reduced," Arrigo said in a telephone interview.

Since carbon dioxide contributes to global warming, scientists believe a growing buildup of the gas would make the environmental problem increasingly worse.

Plants "breathe" carbon dioxide and help control the balance of the gas in the atmosphere. About half the plants that use this gas are in the oceans, Arrigo said.

But the earth's rising temperatures create an environment where some phytoplankton, called diatoms, begin to dominate over single-celled algae called Phaeocystis antarctica, which are better at absorbing carbon dioxide.

"Given the same amount of nutrients, Phaeocystis antarctica takes up almost two times as much carbon dioxide," Arrigo said.

Writing in the Jan. 15, 1999 issue of the journal Science, the researchers said increasing stratification of the Southern Ocean, or differences in water density at different depths, was the change causing diatoms to dominate the environment.

Normally there is very little stratification in the Southern Ocean because frequent strong winds keep the waters well mixed, Arrigo said.

But the increased precipitation that scientists predict will happen with global warming means more diluted water is sitting on the top, making the waters more difficult to mix. This causes the death of the more efficient carbon-eating phytoplankton.

"If global warming continues with increased precipitation in the Southern Ocean, there will be more stratification and that will favour diatoms," Arrigo said.

The study, which took place during a month-long expedition during the Antarctic summer, was also one of the first to show that some types of algae absorb carbon dioxide better than others.

Arrigo said knowing how much atmospheric carbon dioxide the oceans use is important for scientists when trying to predict climate change.

"We need to understand the capacity of the world's oceans to take up all this carbon dioxide," he said. "At some point the ocean might reach its limit and shut off."

(C) Reuters Limited 1999.