Politicians in the nation's capital have been reluctant to set limits on the carbon dioxide pollution that is expected to warm the planet by 4 to 7 degrees Fahrenheit during the next century, citing uncertainty about the severity of the threat. But that uncertainty may have shrunk somewhat with the release last week of two scientific reports suggesting that global warming is not just a hypothetical possibility, but a real phenomenon that has already started transforming especially sensitive parts of the globe. Overall, the reports say, Earth's climate has warmed by about 1 degree Fahrenheit since 1900. In the Arctic,...