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take in, break down, rely on, feed on |
It is true that quite a few most respected scientific authorities have confirmed that the world is becoming hotter and hotter. There is also strong evidence that humans are contributing to the warming. Countless recent reports have proved the same thing. For instance, a 2012 summary about the climate science by the Royal Society noted that, "The global warming over the last half-century has been caused mainly by human activity."
You may not believe that humans could change the planet's climate, but the basic science is well understood. Each year, billions of tons of greenhouse gases are released into the atmosphere because of human activity. As has been known for years, these gases hold heat that would otherwise escape to space, wrapping the planet in an invisible blanket.
Of course, the earth's climate has always been changing due to "natural" factors(因素) such as volcanic eruption or changes in solar, or cycles concerning the Earth's going around the sun. According to the scientific research, however, the warming observed by now matches the pattern of warming we would expect from a build-up of greenhouse gas in the atmosphere—not the warming we would expect from other possible causes.
Even if scientists did discover another reasonable explanation for the warming recorded so far, that would give birth to a difficult question. As Robert Henson puts it, "If some newly discovered factor is to blame for the climate change, then why aren't carbon dioxide and the other greenhouse gases producing the warming that basic physics tells us they should be?"
The only way to prove with 100% certainty that humans are responsible for global warming would be to run an experiment with two identical Earths—one with human influence and one without That obviously isn't possible, and so most scientists are careful not to state human influence as an absolute certainty.
The polar bear is found in the Arctic Circle and some big land masses as far south as Newfoundland. While they are rare north of 88°, there is evidence they range all the way across the Arctic, and as far south as James Bay in Canada. It is difficult to figure out a global population of polar bears as much of the range has been (poor) studied; however, biologists calculate that there are about 20,000—25,000 polar bears worldwide.
Modern methods tracking polar bear populations have been employed only since the mid-1980s, and are expensive (perform) consistently over a large area. In recent years some Inuit people in Nunavut (report) increases in bear sightings around human settlements, leading to a (believe) that populations are increasing. Scientists have responded by (note) that hungry bears may be congregating(聚集) around human settlements, leading to the illusion(错觉) that populations are (high) than they actually are. Of nineteen recognized polar bear subpopulations, three are declining, six (be) stable, one is increasing, and nine lack enough data.
