Science Briefs

Cloud Climatology

Greenhouse Effect and Climate Change

Within the next half-century or so an accumulation of airborne pollutants — notably carbon dioxide (CO2), methane (CH4), nitrous oxides (NOx), and chlorofluorocarbons (CFCs) — will cause noticeable changes in climate. Because these so-called greenhouse gases retard the flow of heat from the surface into space, the whole Earth will warm. This is called the greenhouse effect. This warming is partly reduced by other pollutants that form tiny aerosol particles which reflect some sunlight back to space. The global warming will in turn lead to a variety of other changes throughout Earth's climate system: changes in heat flow, wind and ocean currents, precipitation patterns and the like. Given such a profound potential for an adjustment of the basic climatic elements, an improved understanding of the radiation balance and its dependence on the effects of clouds is crucial.

The threat of climatic change is not primarily in the change itself but in its rapidity. The geological record is replete with climatic changes similar in magnitude to the one now contemplated, but past changes were slow enough to allow most species to adapt. What is unprecendented about the current greenhouse warming is that significant change could come to pass in only a few generations, creating human and economic dislocation. For example, since most people live fairly near oceans, a rise in sea level caused by the melting of glaciers could force most people to move inland. If severe storms became more frequent, they would interfere with airborne and waterbourne transportation of goods from market to consumer. A change in the average temperature and its seasonal variations could alter patterns of energy use and demand. The possible political and economic consequences of such disruptions are suggested by the global concern over maintaining an uninterrupted oil supply from the Middle East.

Yet in spite of the need to forecast climatic changes accurately, current understanding of how the climate works is not detailed enough for climatologists to predict exactly when, where, or to what extent changes will take place. The global climate is such a complex system that no one knows how even a small increase in temperature will alter other aspects of climate — winds, rainfall, ocean currents, ice caps or vegetation — or how such alterations will influence the rate of warming. Moreover, changes in any of these climatic features will also affect the distribution and properties of clouds, and the understanding of clouds is so rudimentary that no one knows whether changes in the clouds will dampen or amplify a warming trend. The possibility that clouds might accelerate global warming brings a special urgency to the ancient problem of understanding the climatic importance of clouds.