Clouds exhibit schizophrenic tendencies with respect to Earth’s energy budget. They reflect incoming solar radiation and thereby promote global cooling. Simultaneously, they absorb longwave radiation from Earth’s surface and radiate some of it back to the surface, thereby promoting global warming. On balance, however, clouds reflect more solar energy than they emit back to the surface. Therefore, the net forcings or radiative emissions that transfer energy from clouds to the planet’s surface—although they vary with location and season—are negative on average: Clouds generally cool Earth more than they warm it. 
As global temperatures rise, evaporation of water from the oceans increases exponentially. This is partly responsible for the heavier cloud cover that has resulted over oceans and has decreased net forcing from clouds, counteracting to some degree global warming, although warming still has occurred. General trends with clouds are apparent, yet mathematically describing cloud formation remains a major challenge in developing accurate computer models to predict global climate change.
 Ramanathan, V., R. D. Cess, E. F. Harrison, P. Minnis, B. R. Barkstrom, E. Ahmad, and D. Hartmann (1989) Cloud-radiative forcing and climate: Results from the earth radiation budget experiment. Science 243:57-63.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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