托福阅读素材（七）

　　That may be as good as it gets," he adds. "My major question is whether this ratio is going to change" as global warming raises the temperature of surface waters and carbon dioxide continues to build up in the atmosphere. "The prognosis is not particularly bright," Takahashi says. A warm soda fizzing over the rim of a glass illustrates one effect: carbon dioxide is less soluble in warmer water. What's more, dissolved carbon dioxide can easily slip back into the atmosphere unless it is taken up by a marine plant or combines with a "buffer" molecule of carbonate.

　　But the ocean's supply of carbonate is limited and is replenished only slowly as it is washed into the ocean by rivers that erode carbonate-containing rocks such as limestone. In absorbing those two billion tons of carbon from the atmosphere year after year, the ocean is gradually using up its buffer supply. Jorge Sarmiento, an oceanographer at Princeton University, has been trying to predict the impact of such changes on the ocean's ability to act as a carbon dioxide sponge. He expects that over the next century, its carbon appetite will drop by 10 percent—and it may ebb much further in the long run.

　　With no new help from nature in sight, perhaps it is time for us to think about creating our own carbon sinks. Scientists have dreamed up plenty of possibilities: planting new forests, for example, which the Kyoto climate treaty would encourage. The approach has already taken root on a grand scale in China, where the government has planted tens of millions of acres since the 1970s. The bureaucrats set out to control floods and erosion, not stem global change, but the effect has been to soak up nearly half a billion tons (.45 billion metric tons) of carbon.

　　Steve Wofsy sees another possibility in his forest studies. Young forests like his study plot are hungry for carbon right now because they are growing vigorously. So why not try to keep a forest young indefinitely, by regular thinning? "You manage it so that every year or every ten years you take out a certain amount of wood" to be used in, say, paper, housing, and furniture, Wofsy says. "You might have a situation where you could make the landscape continue to take up carbon for a long time—indefinitely."

　　Then there's the siren call of the sea. Although as Sarmiento points out the ocean's natural uptake is dwindling, scientists have tried to find a way to give a boost to its carbon appetite. In the 1980s oceanographer John Martin suggested that across large tracts of ocean, the tiny green plants that are the marine equivalent of forests and grasslands are, in effect, anemic. What keeps them from flourishing—and perhaps sucking up vast quantities of carbon dioxide—is a lack of iron. Martin and others began to talk of a "Geritol solution" to global warming: Send out a fleet of converted oil tankers to sprinkle the oceans with an iron compound, and the surge of plant growth would cleanse the air of industrial emissions. As the plants and the animals that grazed on them died and sank, the carbon in their tissues would be safely locked away in the deep ocean.

　　Reality has not been quite so elegant. Experiments have shown that Martin was partly right: A dash of iron sulfate does cause the ocean's surface waters to bloom with patches of algae tens of miles long, so vivid they can be seen by satellites. But oceanographers monitoring what happens in the water have been disappointed to find that when the extra plants and the animals they nourish die, their remains mostly decay before they have a chance to sink and be buried. The carbon dioxide from the decay nourishes new generations of plants, reducing the need for extra carbon from the atmosphere. Nature is just too thrifty for iron fertilization to work.

　　Perhaps carbon can be deep-sixed without nature's help: filtered from power plant emissions, compressed into a liquid, and pumped into ocean depths. Ten thousand feet (3,000 thousand meters) down, water pressure would squeeze liquid carbon dioxide to a density great enough to pool on the seafloor, like vinegar in a bottle of salad dressing, before dissolving. At shallower depths it would simply disperse. Either way environmentalists and many scientists are wary of the scheme because injecting vast quantities of carbon dioxide would slightly acidify the deep ocean and might harm some marine life. Last year protesters forced scientists to cancel experiments meant to test the idea, first near Hawaii and then off Norway.

　　But Peter Brewer, who is studying the scheme at the Monterey Bay Aquarium Research Institute, says it's too early to write it off. Rising carbon dioxide in the atmosphere will acidify the ocean's surface waters in any case, he points out, and pumping some of the carbon into the ocean depths could slow that process. "Why would you want to take this off the table before you know what it does?" he asks.