Oceans Slow Global Warming at a Price, Study Says

Oceans have absorbed nearly half the carbon dioxide humans have released into the atmosphere since 1800 -- reducing the global warming effect but dealing a potentially serious blow to fragile marine ecosystems, according to two studies released today.

Based on a survey of ocean carbon levels at 72,000 locations worldwide, scientists from the National Oceanic and Atmospheric Administration estimated that the oceans have absorbed 118 billion tons of carbon generated by humans from 1800 to 1994 -- 48% of the 244 billion tons released over that period by burning coal, oil and other fossil fuels.

The study, published in the journal Science, suggests that the oceans can moderate the greenhouse effect for centuries by absorbing human-generated carbon dioxide.

But the increasing amount of dissolved carbon can cause marine animals’ shells to weaken and makes it more difficult for them to form new ones, oceanographers said in a companion study in Science.

That damage could destabilize marine ecosystems, many of which depend on shell-building organisms for food and habitats.

“The oceans are performing a great service ... but the consequence is that we may be affecting the biology and the ecosystems in the ocean,” said Christopher L. Sabine, a NOAA oceanographer and lead author of the first study.

Sabine’s study is the first to accurately account for all the carbon dioxide humans have generated since 1800. The ocean has been the only net absorber of carbon over the entire period.

Normally, shallow ocean water is saturated with calcium carbonate -- a material animals use to make shells -- but carbon dioxide reduces its concentration.

Lower calcium carbonate concentrations make it more difficult for corals and other animals to make shells and often lead to shell deformations, said Richard A. Feely of NOAA and author of the companion study.

Based on laboratory studies, he does not expect the effect to become dangerous until the end of this century.

Although trees absorb carbon dioxide, they can’t compensate for the enormous deforestation over the last two centuries.