MEYRIN, Switzerland -- It's a proposition that has befuddled and charged scientists for more than a decade: What if cosmic rays, not CO2, are behind global warming? Long a darling refuge for climate contrarians, and debunked by scientists, it's a proposal nonetheless seeded by a kernel of truth, one that stretches back to the origins of modern physics. And that's why here, at the heart of Europe's premier atom smasher, there sits a cloud.