The Ocean Absorbs Carbon

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In addition to absorbing and distributing heat, the ocean also affects global warming by absorbing carbon dioxide. Carbon dioxide (CO2) is the main way in which humans contribute to global warming. The burning of fossil fuels puts a great deal of carbon dioxide into the atmosphere, trapping the sun's heat and raising temperatures on the earth. By taking carbon dioxide out of the atmosphere, therefore, the oceans slow the process of global warming. According to David Adam, the environmental correspondent for The Guardian, "The world's oceans soak up about 11bn [billion] tonnes of human carbon dioxide pollution each year, about a quarter of all produced."4

There is some evidence, however, that as the earth warms, the ocean may become less effective at absorbing CO2 from the atmosphere. There are numerous reasons for this. In the first place, warming of the oceans seems to disrupt the circulation and mixing of surface and deep water. Carbon in deep water is more likely to remain captured, and if carbon is not dragged down to deep water, water at the surface will become saturated

Fight Global Warming by Putting Crop Residue in the Sea

Plants such as corn contain a great deal of carbon. Keeping that carbon from entering the atmosphere as carbon dioxide could greatly reduce greenhouse gas emissions. Two scientists, Stuart Strand and Gregory Benford have a simple suggestion for how to do that. Simply take crop residue—corn husks, plant stalks, and other plant waste—bundle it up, and drop it into the ocean.

Traditionally, environmentalists have tried to find other green uses for crop residue—using it for fuel, for example. But Strand and Benford argue that dumping it in the ocean is the easiest and best solution. Even with the cost of bundling and transporting, Strand and Benford found, dropping the crops in the ocean reduces global carbon dioxide emissions by 15 percent.

There are a couple of problems with the proposal. The most serious is probably that Strand and Benford assume that the crop residue will be dropped into deep water, where it would keep the carbon from turning into carbon dioxide for millennia. However, other studies suggest bacteria and other organisms may break down the plant matter long before then, with carbon dioxide as a byproduct.

In addition, many researchers still hope that as fossil fuels run out or become more expensive, biofuels—fuels made from plant matter—will eventually become popular. If this happened, it might then be much more productive to use a corn stalk to power a car than it would be to sink that same stalk to the bottom of the sea.

Nonetheless, as carbon dioxide levels rise, and if other plans to reduce emissions fail to gain traction, some nations may try dropping stalks in water, however silly it may sound.

more quickly and unable to absorb more carbon dioxide. Thus, when ocean circulation is weakened, less carbon is deposited in deeper waters, and the amount of carbon absorbed by the ocean overall is reduced. A study led by Kitack Lee of South Korea's

Pohang University of Science and Technology found that carbon uptake (that is, the amount of carbon absorbed) in the Sea of Japan from 1999 to 2007 was half that of carbon uptake from 1992 to 1999. The scientists attributed this effect to circulation changes caused by global warming.5

The chemistry of the ocean may also change as CO2 in the atmosphere increases, and this reaction may, in turn, limit carbon dioxide absorption. The ocean currently is filled with charged particles called carbonate ions. These carbonate ions react with CO2 in the atmosphere and turn it into the molecule bicarbonate, which cannot evaporate again into the air. David Archer argues that as more CO2 is pumped into the atmosphere, the ocean will start to run out of carbonate ions. The result will be that the more carbon there is in the atmosphere, the more slowly the ocean will absorb carbon. "The bottom line," Archer says, " . . . is that the natural world takes up fossil-fuel CO2 more slowly than we might have expected given how much CO2 is dissolving in the oceans today."6

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