Climate Change and Feedbacks

      For centuries, mankind has engaged in activities that alter the environment and, potentially, the global climate. Fossil fuel burning and release of other trace gases and aerosols may well have significant long-term consequences. Since 1800, atmospheric carbon dioxide has increased by 25% and methane concentrations have more than doubled. Agriculture and deforestation alter the Earth's surface in ways that have the potential to change the climate. In these and many other examples, the immediate impacts of man's activities are understood, yet the long-term consequences on the Earth-atmosphere system cannot easily be predicted. For example, one of the major sources of uncertainty in predicting climate change lies in the impact of clouds upon the radiative energy flow through the Earth-atmosphere system. The largest uncertainty in climate prediction models is how to correctly account for the effects of clouds. Because of the importance and uncertainties of clouds and radiation fields, they have become one of the top scientific priorities in the U. S. Global Change Research Program.

      As the Earth undergoes changes in its climate, the amount of cloud cover as well as the physical properties of clouds may well change in ways that are not yet understood. The complex interaction between a changing climate system and the changing cloud conditions is called cloud-climate "feedback." Do clouds decrease or increase global warming? Will a warmer climate result in fewer or more clouds? Can a "runaway greenhouse effect" occur as it did on the planet Venus? While we cannot as yet provide definite answers to such questions, the sensitivity results shown below indicate that relatively small changes in global cloudiness can have a large impact on our climate system. For example, a 50% increase in carbon dioxide may warm the Earth much less than a 50% increase in the amount of high cirrus clouds.