Is Warm January a Sign of Good Luck, or Bad Times?

According to weather forecasts, by Friday thermometers will read more than 50 degrees in Boston, and nearly 60 in Washington, D.C. And alll along the corridor between the two cities, people will be shaking their heads — hatless in the warm winter sun — about global warming.

The Web site RealClimate sums up the current dialogue in America this way: We now commonly attribute warm weather to global warming. But then we are commonly told by the meteorological authorities, that that analysis is wrong, that it's really El Nino at work.

"There's no way to explain the changes we've seen in terms of any of these natural factors," says Michael Mann, a professor of meteorology and geosciences at Penn State University who works with the site.

"So in that context, when we talk about recent climate change, we are talking about humans."

Who do we have to blame — or to thank — for the weather? To help us sort that question out, Robert Siegel talks with Mann.