#1. Global Warming

Nobody doubts anymore that climate change is at least in part man-made. And even if the effects of global warming remain at the most benign end of the predicted range, it will be a disaster of unprecedented proportions. For years, that disaster has been unfolding so slowly that it's been invisible. But now you can see it: Mountain glaciers around the world are melting, along with North polar sea ice and the ice cap atop Greenland; droughts are baking the U.S. southwest, Australia and sub-Saharan Africa; floods are devastating Bangladesh; and Central America is reeling from powerful hurricanes. Not all of these events can be tied absolutely to global warming, but all of them will surely become more frequent and intense as the world warms — ultimately threatening the lives and livelihoods of hundreds of millions of people.