It is difficult to find out how sensitive the climate is, because after the greenhouse effect makes the Earth a little bit warmer there are a lot of different kinds of feedbacks, which can make the warming faster or slower. For example the warmth evaporates more water from the sea, and this water vapor is itself a greenhouse gas, which makes the Earth even warmer. Scientists are fairly sure that most feedback makes the warming faster: but because the feedback is so complicated they don't understand very well, and sometimes their computers are not fast enough to do enough calculations to get a good answer. The project where they work together to understand better is called the Coupled model intercomparison project (CMIP). One of the things they are trying to understand better is clouds, because they might make a big difference.