i if wee toto reduce ouour greenhououse , it's not going to be this runaway effect. we can stop some of these changes that are happening in the arctic, but that does meanan we're goioing o have to cocommit to reducing the amount of fossil fuels we're putttting in t the atmososphered that's the real challenge. >> most experts say fossil f fus are the leading cause of climate change. the climate change deniers often associated with the fossil fuel lobby and industry-supported think tanks downplay the connection between fossilil fuels and the warming f our planet. climate scientist michael mann on how a changing climate may affect our future. >> sometimes you''ll hear from the e itics. thehey'll say, "we demand proof that humanans are causing thihis." and my response is that t proof is reserved for alcoholic beverages andd mathematitical theorems."tt doesn't characterize how scienee works. science works thrhrough weights of evidence, through likelihoods, through confidence levels, and so we're about as certain that humans are causing global warming as we are about any s