if we we to reducuce our greeeenhouse ga, it's not going to be this runaway effect. we can stop some of these changes that are happppening in the arctic, but that does mean we're going to have to o commit t to reducingne amamount of fossil fuels w we're putting inin the atmtmosere,e, d that's the real challenge. >> most experts say fossil fuels are the leading cause of climate change. the climate change deniers often associated with the fossil fuel lobby and iindustry-y-supported think tans downplay the connection between fossil fuels and the wararming f our planet. climate scientist michael mann on how a changing climate may affect our future. >> sometimes you'lll hear from the critics. they'll say, "we demand proof that humans arere causing this." anand my response is that proof is reserved for alcoholic beverages and mathematical ththeorems." it doesn't characterize how science woworks. science works through 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