hurricanes, floods, droughghtsraraging wildfirires, snowstorms, and tornadoes. is this purely nature, or are thehere manmadede, therefore controllable, factors at work?k? 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're e experiencing. >> the debate is settltled. climate change is a fact. and when our children's children look u us the eye andnd ask ife didd all we could to leave them a safer, more stable world with new sources of energy, i want us to be able to o say yes, we did. [applause] >> as the obamama administstratn renews its s commitme t to ac the debate over climate change remains polarized. climate 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 th