In Australia, two other biologists, Michael Roderick and Graham Farquhar, were intrigued by another paradoxical result, the worldwide decline in something called the "pan evaporation rate."
PROFESSOR GRAHAM FARQUHAR (Australian National University): It's called pan evaporation rate because it's evaporation rate from a pan. Every day, all over the world, people come out in the morning and see how much water they've got to add to a pan to bring it back to the level it was the same time the morning before. It's that simple.
NARRATOR: In some places, agricultural scientists have been performing this routine daily task for more than a hundred years.
GRAHAM FARQUHAR: The long-term measurements of pan evaporation are what gives it its real value.
DOCTOR MICHAEL RODERICK (Australian National University): And the fact that they're doing the same thing, day in, day out, with the same instrument.
GRAHAM FARQUHAR: Yeah, they deserve a medal, each of them.
MICHAEL RODERICK: Yeah.
NARRATOR: Nobody outside of agriculture took much notice of the pan evaporation measurements, but, in the 1990s, scientists spotted something very strange, the rate of evaporation was falling.
GRAHAM FARQUHAR: There is a paradox here about the fact that the pan evaporation rate's going down, an apparent paradox, but the global temperature's going up.
NARRATOR: This was a puzzle. Most scientists reasoned that like a pan on the stove, turning up the global temperature should increase the rate at which water evaporated. But Roderick and Farquhar did some calculations and worked out that temperature was not the most important factor in pan evaporation.
MICHAEL RODERICK: Well, it turns out, in fact, that the key things for pan evaporation are the sunlight, the humidity and the wind. But really, the sunlight is a really dominant term there.
NARRATOR: They found that it was the energy of the photons hitting the surface—the actual sunlight—that kicks the water molecules out of the pan and into the atmosphere. And so they, too, reached an extraordinary conclusion.
MICHAEL RODERICK: You know, if the pan is going down, then maybe that's the sunlight going down.
NARRATOR: Was the falling pan evaporation, in fact, evidence of global dimming? Somewhere in the journals, they felt, must be the hard numbers that could tie the two things together.
MICHAEL RODERICK: And then one day, just by accident, I had to go to the library to get an article out of Nature. And, as you do, I couldn't find it, and I just glanced at a...through the thing, and there was an article called "Evaporation Losing Its Strength," which reported a decline in pan evaporation over Russia, the United States and Eastern Europe.
And there, in the measurements, they said that the pans had, on average, evaporated about a hundred millimeters less of water in the last 30 years.
NARRATOR: Mike knew how much sunlight was needed to evaporate a millimeter of water, so he put the two sets of figures together, the drop in evaporation with the drop in sunlight.
MICHAEL RODERICK: So you just do the sum in your head: a hundred millimeters of water, less a pan evaporation, two and a half mega joules, so two and a half times a hundred is two hundred and fifty mega joules. And that was, in fact, what the Russians had measured with the decline in sunlight in the last 30 years. It was quite amazing.
NARRATOR: It was the same in Europe and the U.S.A. The drop in evaporation rate matched the decline in sunlight reported by Beate Liepert and Gerry Stanhill. Two independent sets of observations led to the same conclusion. Here, at last, was compelling evidence that global dimming was real.
BEATE LIEPERT: All of a sudden you see, "Oh my god, the world is dimming." And then you, all of a sudden, you see, "Oh my god, this really has an im..., tremendous impact.( Continued at - http://www.pbs.org/wgbh/nova/transcri...)