JUNGFRAUJOCH, Switzerland -- It's the greatest uncertainty in climate change: Just how fast will the world heat up? The answer, it seems, lies up in the clouds. Despite decades of work and mounting political pressure, scientists have not resolved how clouds respond to humanity's touch. This stubborn question has spurred a diffuse global effort based on satellites, models and measures from remote mountain labs. Warming could be a serious problem. It could be a catastrophic one. It's up to the clouds to decide.