For over a decade, the largest scientific uncertainty about how the planet will respond to warming temperatures hasn't come from how much carbon dioxide will be soaked up by the ocean or absorbed by the trees. It's come, instead, from clouds.
The fluffy, whimsical collections of water droplets floating in the air have, for some time, confounded climate scientists and models alike. Scientist have long known that depending on how clouds respond to warming temperatures, the world could become even warmer or a little bit cooler. They just haven't known which.