托福阅读素材（六）

　　The vast tracts of dead trees will ultimately send their carbon back to the atmosphere when decay or fire consumes them. A warming climate is likely to blame, Juday and others believe. Warmth favors the beetle by speeding up its life cycle and improving its chance of surviving the winter. And as Juday has found in his study area, warming also stresses the hardy northern trees, making them less able to fight off infestation.

　　Two hundred seventy miles (434 kilometers) north of the Kenai, on a hillside just west of Fairbanks, the Parks Loop Stand appears to the unschooled eye to be thriving. But Juday, who has worked in this grove of hundred-foot-tall (30-meter-tall) white spruce for 15 years, knows practically every tree's biography—and he is concerned. Heavier, wetter snowfalls have broken off branches and crowns. The trees have also been assaulted by a pest new to northern Alaska, the spruce budworm.

　　The first outbreak of spruce budworm in this region was recorded in 1989, and Juday thinks the warmer climate is again to blame. Sickly orange branches high in the trees and ragged spruce seedlings festooned with black pupae show that the budworm is still at work. "This was a healthy, beautiful white spruce stand," says Juday. But so many trees have died that the formerly dense canopy has opened up, and the moss that carpeted the shadowy floor has given way to sun-loving grasses.

　　It's not just the snow and the pests. On the jagged stump of a recently fallen tree Juday points to another fingerprint of warming. The 200-year-old tree's growth rings are thick at the core of the stump, but the outermost rings, representing the tree's last few decades of life, are as thin as puff pastry layers. Juday believes the tree's growth has been slowing because of hotter summers. Thin rings are a sign that the trees are undergoing stress, running short of water in the heat.

　　Since that finding, Juday's group has examined cores from black spruce, another major tree type in interior Alaska. It too grows more slowly in warmer years because of moisture stress. The future of the northern forest could be bleak. Assuming that Alaska continues to warm at the rate some climate models predict, Juday's analysis points to "zero white-spruce growth" by 2090. If that happened, the boreal forest as we know it would be no more. A smaller carbon storehouse could take its place—perhaps a grassy parkland dotted with aspen groves, Juday suggests. Substantial amounts of carbon dioxide could be released into the atmosphere from the corpse of the old forest.

　　Across the far north a still bigger pulse of greenhouse gas could come from the soil. In a somber grove of black spruce on the broad floodplain of the Tanana River south of Fairbanks, Jamie Hollingsworth, who manages an ecological research site at the University of Alaska, sinks a 4-foot (1.2-meter) steel probe into a damp carpet of moss. It slips in easily at first, then stops abruptly about three feet (one meter) in. Hollingsworth digs through a foot-thick (0.3-meter-thick) layer of moss, roots, and decaying needles, then scoops aside the silty soil below until his shovel grates on the hard permafrost that defeated the probe. Chipping off a clod or two, he reveals silvery veins of ice.

　　That eternal ice is in jeopardy across much of the far north. Near Fairbanks, at the heart of Alaska, the soil has warmed as much as 3 degrees Fahrenheit (5.4 degrees Celsius) over the past 40 years, putting large tracts of permafrost in danger of thawing. Here and there—even at spots on the university campus—it has already crossed the threshold, and melting has left the ground unstable and boggy. Farther north there's a larger margin of safety.

　　Fires can speed up the melting. In the summer of 2001 a fire raced through a hundred thousand acres (40,000 hectares) of floodplain forest along the Tanana. The charred snags now stand on bare sand and silt, in many places burned clean of the usual thick moss carpet. The moss is critical to the permafrost: It insulates the soil, keeping it at subfreezing temperatures and helping preserve the ice through the summer. Any permafrost in the fire zone is now in danger of thawing—and hotter summers have made fires more common in many parts of the north, including Siberia and western Canada.

　　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.