托福阅读素材（五）

　　Other clues in the air samples hint at what that sink is. Both the waters of the ocean and the plants on land steal carbon dioxide from the atmosphere. But they leave different fingerprints behind. Because plants give off oxygen when they absorb carbon dioxide, a plant sink would lead to a corresponding oxygen increase. But when carbon dioxide dissolves in the ocean, no oxygen is added to the atmosphere.

　　Plants taking in carbon dioxide also change what they leave behind. That's because plants prefer gas that contains carbon 12, a lighter form of the carbon atom. The rejected gas, containing carbon 13, builds up in the atmosphere. The ocean, though, does not discriminate, leaving the carbon ratio unchanged. From these clues, Tans and others have found that while the ocean is soaking up almost half the globe's missing carbon—2 billion tons (1.8 billion metric tons) of it—the sink in the Northern Hemisphere appears to be the work of land plants. Their appetite for carbon dioxide surges and ebbs, but they remove, on average, more than 2 billion tons (1.8 billion metric tons) of carbon a year.

　　Forests like Wofsy's are one place where it's happening. For more than a decade his group has monitored the carbon dioxide traffic between the trees and the air. Instruments on his tower track air above the treetops as wind and solar heating stir it. As each waft of air passes the tower, sensors measure its carbon dioxide content. The theory is simple, says Wofsy: "If an air parcel going up has less carbon dioxide than an air parcel going down, you have carbon dioxide being deposited onto the forest."

　　The amount changes fast. "Sunshine, perhaps the temperature, rainfall over the past week—all those factors affect what the forest does on an hour-to-hour basis," he says. Even a passing cloud can dampen photosynthesis, spoiling the trees' appetite for carbon. In winter, when leaves fall and decay, more carbon dioxide—a by-product of plant respiration and decomposition—seeps back out of the forest and into the atmosphere. Still, over more than ten years, the bottom line of billions of measurements has been positive. On balance, Harvard Forest is sieving carbon from the atmosphere.

　　It shows in the trees and on the forest floor. To check that their high-tech air measurements weren't somehow being fooled, Wofsy's group strapped calibrated steel bands around trees to measure their growth, gathered and weighed deadfall, and set up bins to collect fallen leaves. The idea was to measure just how much carbon-containing wood and other organic matter was building up in the forest, and to see if it matched the gas measurements. It did. Each acre of the forest has been taking roughly 0.8 ton (0.75 metric ton) of carbon out of the atmosphere annually, doing its humble part to counteract greenhouse warming.

　　Other forests at research sites in the eastern U.S. are putting on weight as well. That's no surprise, Wofsy says. "In the eastern U.S., the most common age for a forest is 40 to 60 years. That's the kind of forest that's going to be growing."

　　The current Harvard Forest, in fact, has a precise birth date: 1938, when a hurricane barreled in from the Atlantic and leveled earlier stands of trees. Elsewhere in the U.S. humans were the hurricane, clearing vast stands of forest for farming. Abandoned in the early 20th century as agriculture shifted westward to the plains, the land is yielding to forest again. The trees, still young, are getting taller and stouter and putting on denser wood. Year by year this slow alchemy locks up carbon in thousands of square miles of eastern forest.