What has changed is his doubt about the very existence of global warming. And he is now going a step further: Humans are almost entirely the cause, he says.
But Hurricane Katrina cannot be attributed to global warming, he notes. The number of hurricanes hitting the United States has been going down, not up; likewise for intense tornadoes. Polar bears aren’t dying from receding ice, and the Himalayan glaciers aren’t going to melt by 2035. And it’s possible that we are currently no warmer than we were a thousand years ago, during the “Medieval Warm Period” or “Medieval Optimum,” an interval of warm conditions known from historical records and indirect evidence like tree rings. And the recent warm spell in the United States happens to be more than offset by cooling elsewhere in the world, so its link to “global” warming is weaker than tenuous.
He details it all in his own words. The Conversion of a Climate-Change Skeptic By Richard A. Muller, New York Times