Ya?l: Today on A Moment of Science, global warming . . .

D: Uhm, right . . .

Y: That's your cue, Don.

D: I know, Ya?l . . . but . . .

Y: What's wrong?

D: Well, we're a science program, right? And I know that the accepted scientific wisdom is that human activity is causing the Earth to get warmer. But I can't help but think that the melting of polar ice and everything else we're seeing is at least partly caused by natural warming. I mean, weather patterns do change naturally over time.

Y: Of course. And we know that the earth was going through heating and cooling cycles long before people started pumping carbon dioxide into the atmosphere. And, in fact, scientists have observed a strange weather pattern over the past twenty or so years. Normally, cold winds from the Arctic blow south. But during the past few decades, warmer air and clouds from the south have blown north toward the Arctic. All that warm air is a big reason why so much Arctic ice has melted.

D: But warming caused by human activity still plays a role, right?

Y: A big role. Natural warming combined with man-made warming has melted so much Arctic ice that it's created a sort of feedback loop. Snow and ice reflect sunlight. But the darker ocean water absorbs light and heat, causing even more ice to melt.

D: So natural warming or man-made warming in isolation would cause some ice to melt...

Y: But not nearly as much as the two forces combined. That's what's really put us where we are today.