In order to predict the climate several decades into the future, we need to understand many aspects of the climate system, one being the role of clouds in determining climate sensitivity. Clouds affect climate, and are in turn affected by changes in the climate. The relationship is a complicated system of feedbacks, in which clouds modulate Earth's radiation balance.
- Clouds cool Earth by reflecting incoming sunlight.
- Clouds warm Earth by absorbing heat emitted from the surface and reradiating it back down (blanket effect).
- Clouds are themselves changed by the warming or cooling of Earth.
What is important is the net cooling or warming effect of all clouds on Earth. If Earth should warm due to the greenhouse effect, weather patterns would change. It is not known whether a change in the clouds would have a net effect of slowing the warming or of speeding the warming. Improving our understanding of the role of clouds in climate is crucial to understanding the effects of global warming.
Atmospheric scientists have learned a great deal in the past several decades about how clouds form and move in Earth's radiation-driven system. Investigators now realize that traditional computer models of global climate have taken a rather simplistic view of clouds and their effects, partly because detailed global descriptions of clouds have been lacking, and partly because in the past the focus has been on short-term regional weather prediction rather than on long-term global climate prediction. To address today's concerns, we need to increase the accuracy of our models and accumulate more and better data.
A major effort is under way at GISS, under the direction of Dr. William B. Rossow, to gather better information about clouds and their radiative effects. Since 1983 the International Satellite Cloud Climatology Project (ISCCP), as part of the World Climate Research Program, has been collecting observations from weather satellites to assemble a global multiyear dataset. GISS serves as the Global Processing Center for ISCCP, in cooperation with institutions in several other countries. The datasets give the measured values of some of the key variables that determine the interaction of clouds and radiation.
There are also a number of other cloud datasets available, especially from the field experiments performed to supplement them. A thorough study of all these data will take many years and will lead, of course, to new experiments, but the investigations have already provided fresh insights into how clouds might change with climate and provided us with some global statistics about the global distribution and character of clouds.
Data collection and model development proceed at GISS in parallel, with the goal of formulating an increasingly precise understanding of how sensitively the clouds change in response to external forces and what effect those changes would have on global climate change.