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 cycle, both succumbing to the effects of climate change and, as they decay and release more carbon, helping to accelerate it. That grim reality is stoking a new debate over commercial logging, one of Canada’s biggest industries. Environmentalists contend that the extreme stresses on Canada’s forests, particularly the old-growth northern forest, mean that logging ought to be sharply cut back to preserve the remaining trees--and the carbon stored within them- for as long as possible. Moreover, they argue that the process of logging releases even more carbon stored in the forest peat (泥炭), threatening to set off what they describe as a virtual "carbon bomb" --the estimated 186 billion tons of carbon stored in Canada’s forests, which is equivalent to 27 years worth of global carbon emissions from the burning of fossil fuels(矿物燃料). "There’s only one thing which hauls all that carbon out of the forest, and that’s logging," said Merran Smith, director of the climate program at the environmental group ForestEthics. "What we need to do is maintain as much biodiversity(生物多样性)us we can, so we are prepared to adapt as temperatures change, so we have resilience(恢复力)." But Kurz and other government scientists contend that logging less is no solution to the global warming problem and would in fact increase carbon emissions over the long term. That’s because, they argue, essential wood products for construction, furniture and other uses would have to be replaced with other man-made materials, such as plastic, steel or concrete, which require the burning of even more fossil fuels― and therefore carbon emissions― during their manufacturing process. "It’s not as simple as saying, ’Log less and therefore have more carbon sequestered in the forests,’ " Kurz said. "That is true, but if in order to do that you have more fossil fuel emitted elsewhere, your impact on the climate may be negative. " Instead, some scientists argue for more extensive logging of the remaining commercial forests so that older forests, which are most vulnerable to insect infestations and have nearly reached their carbon-storage capacity, can he replanted with younger trees that will take in even more carbon during their growing years.Ecologists call Canada’s 1.2 million square miles of trees "lungs of the planet" for ______. A．they balance the world temperaturesB．they are abundant to cleanse the earth’s atmosphere and play an important role in cleansing the earth’s atmosphereC．they could always suck in vast quantities of carbon dioxideD．they naturally cleanse much of the harmful heat-trapping gas