Given that a doubling of carbon dioxide would change the surface heat flux by only two watts per square meter, it is evident that a small change in cloud cover can strongly affect the response to carbon dioxide. The situation is complicated by the fact that clouds at high altitudes can also supplement the greenhouse effect. Indeed, the effects of clouds in reflecting light and in enhancing the greenhouse effect are roughly in balance. Their actual effect on climate depends both on the response of clouds to warming and on the possible imbalance of their cooling and heating effects. Similarly, factors involving the contribution of snow cover to reflectivity serve, in current models, to amplify warming due to increasing carbon dioxide.
Snow is associated with winter when incident sunlight is minimal, however.
Moreover, clouds shield the Earth's surface from the sun and minimize the response to snow cover. Indeed, there is growing evidence that clouds accompany diminishing snow cover to such an extent as to make that feedback factor negative. If, however, one asks why current models predict that large warming will accompany increasing carbon dioxide, the answer is mostly due to the effect of the water vapor feedback. Current models all predict that warmer climates will be accompanied by increasing humidity at all levels. As already noted, such behavior is an artifact of the models since they have neither the physics nor the numerical accuracy to deal with water vapor.