托福阅读材料:缺失碳的情况

　　Climate experts keep a worried eye on the permafrost because vast reserves of peat and other carbon-rich organic material are frozen into it—a global trove of carbon estimated at 200 billion tons (181.4 metric tons). For hundreds, perhaps thousands, of years low temperatures entombed it. Now, says Terry Chapin of the University of Alaska, "it's potentially a very large time bomb."

　　The permafrost's full megatonnage isn't certain. Some of the subterranean ice would create bogs when it melted, and the oxygen-poor waters of bogs can inhibit decay and keep the carbon locked up. But northern warming could well bring a drier climate, and that could open the way to a worst-case scenario, says NOAA's Tans. "If, due to warming in the Arctic, the permafrost warmed up and dried out, most of that carbon could be released." The atmospheric level of carbon dioxide could jump by a hundred parts per million as a result, he says—more than 25 percent above current levels.

　　So where in nature can we look for salvation? Until recently climate scientists hoped it would come from farther south. In temperate and tropical vegetation, they thought, a negative feedback effect called carbon fertilization might rein in the carbon dioxide rise. Plants need carbon dioxide to grow, and scientists have found that in laboratory chambers well-nourished plants bathed in high-carbon dioxide air show a surge of growth. So out in the real world, it seemed, plants would grow faster and faster as carbon dioxide built up in the atmosphere, stashing more carbon in their stems, trunks, and roots and helping to slow the atmospheric buildup. Such a growth boost could, for example, turn mature tropical forests—which normally don't soak up any more carbon than they give off—into carbon dioxide sponges.

　　Alas, it appears not to work. At Duke University's forest in North Carolina, William Schlesinger and his colleagues have been giving hundred-foot-wide (30-meter-wide) plots of pines a sniff of the future. Over each plot a ring of towers emits carbon dioxide at just the right rate to keep the concentration in the trees at 565 parts per million, the level the real atmosphere might reach by midcentury. When the experiment started seven years ago, the trees showed an initial pulse of growth.

　　"These trees woke up to high carbon dioxide and were able to make good with it for a couple of years," says Schlesinger. But then the growth spurt petered out, and the trees' growth has slipped most of the way back to normal. That's not to say that high carbon dioxide didn't have some long-term effects. Poison ivy, for some reason, "is one of the winners," says Schlesinger, with a sustained growth rate 70 percent faster than normal. And allergy sufferers will not be pleased to learn that the carbon dioxide-fertilized pines produced extravagant amounts of pollen.

　　To take advantage of a carbon dioxide bonanza, it seems, most plants also need extra nitrogen and other nutrients. Schlesinger's experiment is one of many to show lately that in the real world, more carbon just means plants will probably run short of something else essential. Resurgent forests are soaking up plenty of carbon now, but we owe that mainly to our ax-wielding forebears, who cleared the land in centuries past. That land sink is not likely to increase by much, say scientists. And it will eventually saturate as today's young forests mature. "We can expect this sink to disappear on the order of a hundred years," says Princeton's Pacala. "You can't count on it to keep getting larger, like manna from heaven, the way a carbon-fertilization sink would."

　　The outlook for an increased ocean sink is no brighter. Taro Takahashi of Columbia University's Lamont-Doherty Earth Observatory has spent decades on oceanographic research ships, making thousands of carbon dioxide measurements just above and just below the water surface to track the exchange of gas between the ocean and the atmosphere.

　　The North Atlantic and the southern oceans have cold, nutrient-rich waters that welcome carbon dioxide, Takahashi has found. Carbon dioxide dissolves easily in cold water, and the nutrients foster marine-plant growth that quickly uses up the dissolved carbon dioxide. When the plants and the animals that feed on them die and sink into the abyss, their remains carry away the carbon and make room for more.

　　The traffic mostly goes the other way in warmer, less biologically rich seas. But the global balance is favorable, for now at least. More carbon dioxide dissolves in the oceans than is given off. Takahashi's measurements confirm that the oceans take up nearly as much carbon as the regrowing forests and thickening brush on land: an average of 2 billion tons (1.8 billion metric tons) a year. "One-half of the missing carbon is ending up in the ocean," Takahashi says.

　　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.