ChangeAs relentlessly bad as the news about global warming seems to be, with ice at the po

Change

As relentlessly bad as the news about global warming seems to be, with ice at the poles melting faster than scientists had predicted and world temperatures rising higher than expected, there was at least a reservoir of hope stored here in Canada's vast forests.

The country's 1.2 million square miles of trees have been called the "lungs of the planet" by ecologists because they account for more than 7 percent of Earth's total forest lands. They could always be depended upon to suck in vast quantities of carbon dioxide, naturally cleansing the world of much of the harmful heat-trapping gas.

But not anymore. In an alarming yet little-noticed series of recent studies, scientists have concluded that Canada's precious forests, stressed from damage caused by global warming, insects and persistent fires, have crossed an ominous (危险的) line and are now pumping out more climate-changing carbon dioxide than they are sinking. Worse yet, the experts predict that Canada's forests will remain net carbon sources, as opposed to carbon storage "sinks", until at least 2022, and possibly much longer.

"We are seeing a significant distortion of the natural trend," said Werner Kurz, senior research scientist at the Canadian Forest Service and the leading expert on carbon cycles in the nation's forests. "Since 1999, and especially in the past five years, the forests have shifted from being a carbon sink to a carbon source."

Translation: Earth's lungs have come down with emphysema(肺气肿). Canada's forests are no longer our friends.

So serious is the problem that Canada's federal government effectively wrote off the nation's forests in 2007 as officials submitted their plans to abide by the international Kyoto Protocol, which obligates participating governments to reduce their greenhouse gas emissions.

Under the Kyoto agreement, governments are permitted to count forest lands as credits, when calculating their national carbon emissions. But Canadian officials, aware of the scientific studies showing that their forests actually are emitting excess carbon, quietly omitted the forest lands from their Kyoto compliance calculations.

"The forecast analysis prepared for the government ... indicates there is a probability that forests would constitute a net source of greenhouse gas emissions," a Canadian Environment Ministry spokesman told the Montreal Gazette.

Canadian officials say global warming is causing the crisis in their forests. Inexorably rising temperatures are slowly drying out forest lands, leaving trees more susceptible to fires, which release huge amounts of carbon into the atmosphere.

Higher temperatures also are accelerating the spread of a deadly pest known as the mountain pine beetle, which has destroyed pine forests across British Columbia and is threatening vital wood in the neighboring province of Alberta. More than 50,000 square miles of British Columbia's pine forest have been stricken so far with the markers of death: needles turn bright red before falling off the tree.

Bitter cold Canadian winters used to kill off much of the pine beetle population each year, naturally keeping it in check. But the milder winters of recent years have allowed the insect to grow rapidly. "That's what's causing some of our forests to switch from a carbon sink position to a source position," said Jim Snetsinger, British Columbia's chief forester. "Once those infested trees axe killed by the pine beetle, they are no longer taking in carbon -- they are giving it off. "

Snetsiuger noted that eventually, over the course of a generation, some of the dying forests will begin to regenerate and once again begin storing more carbon than they release. But for the foreseeable future, experts say, their models show that Canada's forests will stay stuck in a bad global-warming c

A．they balance the world temperatures

B．they are abundant to cleanse the earth's atmosphere and play an important role in cleansing the earth's atmosphere

C．they could always suck in vast quantities of carbon dioxide

D．they naturally cleanse much of the harmful heat-trapping gas