Last week I reported on the wide and growing partisan divide in U.S. public opinion over global warming: self-identified Democrats are 39 percentage points more likely than their Republican counterparts to rate climate change a serious problem.

But what puzzled me most was the 13-point drop in concern among Republicans since 1999. Call me naïve, but with all the scientific evidence that's been piling up on the issue -- accompanied by increasing media attention -- I guess I expected slow (though perhaps reluctant) increases in concern all across the political spectrum. Years of rising global temperatures, melting sea ice, and solidifying scientific consensus ought to have converted at least some honest skeptics, right?

A big report released last week by Pew, charting two decades of American political values and core attitudes, provides some clues about what's going on.

Typical Republicans, circa 1999, haven't necessarily found their belief in global warming shaken over the years. Instead, for whatever combination of reasons, people who believe in global warming are drifting away from the Party.