evidence: MICHAELS: Well, surface temperature of the planet is warmer than it was a hundred years ago about 9/10th of a degree Celsius. LEVIN: Nine-tenth degree of a degree Celsius. [...] So that means that probably about half, maybe half of that nine-tenths of the degree might be caused by greenhouse gases because when the planet warmed beginning in 1976, the temperature of the stratosphere started to drop and that's the prediction of greenhouse theory that's not intuitive. The great philosopher of science Karl Popper said, if you can meet a difficult prediction with your theory, you can continue to entertain your theory.