托福阅读素材（六）

More missing carbon could be hiding in the West. Fire once regularly swept the grasslands, rejuvenating them while killing off woody shrubs like mesquite, juniper, and scrub oak. Decades of firefighting policies called for dousing the smallest blaze and allowed the brush to thrive. The practice disrupted the grasslands' natural cycle and led to bulkier, woodier brush that fueled larger, more destructive fires. But it may also have created a major storehouse for carbon. All told, forest and scrub across the 48 states could be taking in half a billion tons of carbon, balancing out more than a third of the emissions from U.S. cars and factories. It's a huge gift, says Wofsy: "That's at least four times what they were trying with Kyoto"—the climate treaty that the U.S. refused to ratify—"and it hasn't hurt anyone."

　　That leaves more than 1.5 billion tons (1.4 billion metric tons) of missing carbon to account for in the Northern Hemisphere. Mature forests, such as tropical rain forest and the great belt of coniferous forest across Alaska and Canada, probably can't help because they're in a steady state, taking in no more carbon dioxide for growth than they give off (plants breathe too). But Europe's managed woodlands, new forests planted in China, and forests regrowing in Siberia after decades of logging could account for another half billion tons (.45 billion metric tons), researchers say.

　　Then there is a change in the far north, where satellite measurements over the past 20 years have shown that vegetation is getting lusher and enjoying a longer growing season. Natives of the North American Arctic report a new luxuriance on the tundra, where once stunted plants, such as dwarf birch, willow, and alder, are growing taller. The reason is simple, says Princeton's Pacala: "You go to the far north, and it's just palpable how much warming there is."

　　Indeed it is. While the world as a whole has warmed by about 1 degree Fahrenheit (0.56 degree Celsius) since 1900, parts of Alaska have warmed by 5 degrees Fahrenheit (2.8 degrees Celsius). Brad Griffith studies caribou at the University of Alaska Fairbanks, where he has noticed a change in the winters. He remembers clear, cold days and powder snow. "It was never slick, never cloudy; you never had to clean your windshield." Now the winters arc warmer, wetter, and slushier. The shrubs on the North Slope seem to love the change, and Griffith has found that the lusher forage gives newborn caribou a better shot at survival.

　　That's the good news from the north: Right now global warming, ironically, may be helping forestall even more warming, by speeding the growth of carbon-absorbing trees. But balanced against that are warning signs—hints that northern ecosystems could soon turn against us. Eventually, warming in the far north may have what scientists call a positive feedback effect, in which warming triggers new floods of carbon dioxide in the atmosphere, driving temperatures higher.

　　Worrisome signs begin on the aircraft approach to Anchorage. As the route skirts the hundred-mile-wide (161-kilometer-wide) Kenai Peninsula, ugly gray gaps appear in the dark green canopy of spruce below. Since the early 1990s bark beetles have been on the rampage in the Kenai, killing spruce on more than 2-million acres (809,000 hectares) there. Farther south in the Kenai, says Glenn Juday, a forest ecologist at the University of Alaska, skeletal trees stretch from horizon to horizon. "It's the largest single area of trees killed by insects in North America," says Juday. "No outbreak this size has happened in the past 250 years."