Clouds cover an enormous 65% of the planet and are responsible for about half of the sunlight that is reflected back out to space.
The effects of clouds are so strong that most of the differences between IPCC-favoured-models comes from the assumptions the models make about clouds. Cloud feedbacks are the “largest source of uncertainty”.[ii] Numerous studies show models project wildly different results for clouds, and yet few could correctly simulate clouds as recorded by satellites.[iii] One researcher described our understanding of cloud parameters as being “still in a fairly primitive state.” [iv]
Sunlight that travels 150 million kilometers can be blocked a mere 1km away from the Earth’s surface and reflected back to space. The situation is complicated though, because clouds also slow the outgoing radiation — which has a warming effect. In general lower clouds are thicker and have a large cooling effect, while higher clouds are thinner and tend to trap more heat than they reflect (i.e. net warming). Observations show the cooling effect of clouds dominates the warming effect. (Allen 2011[v]) which means that, in general, more clouds means more cooling.