Snow and sea-ice reflect the sun's energy very effectively so, as they melt, more energy is absorbed at the earth's surface. Feedbacks feed back upon feedbacks, which is why climate modelling is so difficult. In most models cloud cover increases in a warmer climate. This affects the energy budget in two opposing ways. Clouds reflect sunlight, reducing the amount of energy reaching the surface.
Clouds are a major source of uncertainty.
If the effects of cloud feedbacks are eliminated, this range is reduced to 1.7-2.3 C.2 Many other feedbacks, particularly those involving chemistry and biology, may also be important. The most important chemical feedbacks are interactions among greenhouse gases in the atmosphere. Biological feedbacks can, like clouds, work both ways. Other important feedbacks include ocean-atmosphere interactions and the possibility that methane will be released from melting tundra. Improving the representation of feedbacks in climate models, and checking them against observations, is probably the most important area of What is Climate modelling research at present.