The biggest environmental issue of 2011 — at least in the U.S. — wasn't global warming. It was hydraulic fracturing, and these three men helped represent the determined opposition to what's more commonly known as fracking. Anthony Ingraffea is an engineer at Cornell University who is willing to go anywhere to talk to audiences about the geologic risks of fracking, raising questions about the threats that shale gas drilling could pose to water supplies. Robert Howarth is his colleague at Cornell, an ecologist who produced one of the most controversial scientific studies of the year: a paper arguing that natural gas produced by fracking may actually have a bigger greenhouse gas footprint than coal. That study — strenuously opposed by the gas industry and many of Howarth's fellow scientists — undercut shale gas's major claim as a clean fuel. And while he's best known for his laidback hipster performances in films like The Kids Are All Right, Mark Ruffalo emerged as a tireless, serious activist against fracking — especially in his home state of New York.