is this purely nature, or are tthere manmamade, therefore controllable, factors at worork? a warmer, moister environment can intensify storms, creating heavier precipitation. and this scientists say is why human activities may account, at least in part, for the rise in extreme weather we'rere experiencing. >> the debate is setettled. climate change is a fact. and when our children''s children lookok uin the eye a and ask ife didid all we could to leave them a safer, more stable world with new sources of energy, i want us to be able t to say yes, we did. [applause] >> as the obobama admininistratn renews itits commitment to ac the debate over climate change remains polarized. climat scientist michael mann is a central figure in that debate. he was one of the scientists behind the development of the controversial hockey stick chart, which showed how temperature in the late twentieth century was exceptionally warm compared to the previous 900 years. this triggered a tax on dr. mann and the science behind his work, all documented in his book "the hockey stick and the climate wa