Around half of all carbon dioxide produced by humans since the industrial revolution has dissolved into the world's oceans-with adverse effects for marine life-according to two new studies.

Scientists who undertook the first comprehensive look at ocean storage of carbon dioxide found that the world's oceans serve as a massive sink that traps the greenhouse gas.

 "The oceans are performing a great service to humankind by removing this carbon dioxide from the atmosphere," said Christopher Sabine, a geophysicist with the National Oceanic and Atmospheric Administration in Seattle, Washington. "The problem is that this service has potential consequences for the biology and ecosystem structure of the oceans."

Of all the carbon dioxide (CO2) emitted into the atmosphere, one quarter is taken up by land plants, another quarter by the oceans. Understanding these natural mechanisms is important in forecasting the rise of atmospheric CO2 because even though plants and bodies of water now absorb surplus greenhouse gas, they could become new trouble spots. The ocean absorbs CO2 from the atmosphere in an attempt to reach equilibrium by direct air-to-sea exchange. This process takes place at an extremely low rate, measured in hundreds to thousands of years. However, once dissolved in the ocean, a carbon atom will stay there, on average, more than 500 years, estimates Michael McElroy, Butler professor of environmental science.

Another process, called "the biological pump," transfers CO2 from the ocean's surface to its depths. Warm waters at the surface can hold much less CO2 than can cold waters in the deep. "This is the 'soda bottle on a warm day' effect," says Agassiz professor of biological oceanography James McCarthy, "and is not unique to carbon dioxide; it applies to all gases dissolved in water. There is a higher capacity to hold a gas with a lower temperature than with a higher temperature." This means that when deep ocean waters rise to the surface as part of normal ocean-circulation patterns, the water heats up and actually releases CO2.

A greenhouse gas, carbon dioxide traps solar heat in Earth's atmosphere. The gas is known as the biggest contributor to global warming of the planet.

Since mass consumption of fossil fuels began with the industrial revolution around 1800, the concentration of carbon dioxide in the atmosphere has grown from an estimated 280 parts per million to around 380 parts per million.

Today's current level of atmospheric carbon dioxide is only around half of what scientists have predicted atmospheric levels should be, based on estimates that humans have contributed 244 billion metric tons of carbon dioxide to Earth's atmosphere.

According to the study by Sabine and his colleagues, the amount of carbon dioxide the oceans have currently taken up is about a third of what they can hold. After that, the researchers warn, the rate of global warming could accelerate.

In the second, related study, scientists found that while the oceans are helping to mitigate global warming, the dissolved carbon dioxide is already having a detrimental effect on marine life.

Based on these and other studies, we need to be careful about using oceans as a carbon dioxide sequestion vehicle.  We may do much more harm than good.