A Climate Theory Goes with the Flow

Global circulation: map of air and wind currents. Copyright © Michael Halbert 2000

Constructal" theory is proving to be the Swiss army knife of fundamental theories. The thermodynamic theory, which enables mathematical description and minimization of flow in a system, has long been used to efficiently design heat exchangers, heating-distribution networks, and electronics-cooling systems. Recently, its developers, Adrian Bejan, J.A. Jones Professor of mechanical engineering, and his colleagues, even used it to create a unified theory of locomotion for animals from flying fish to insects.

Now they have found that the theory can be used to predict the basic features of global circulation and climate. They say the new approach to climate may have important implications for forecasting environmental change.

The researchers found that the theory can predict the global circulation that determines the boundaries between desert and tropical forests, as well as between temperate zones and the poles. Based only on the optimal flow of heat from the sun, the theory also predicts other climate characteristics, such as average wind speed and the average temperature difference between night and day.

The team first reported its findings online in the International Journal of Heat and Mass Transfer.

Constructal theory is founded on the principle that configurations evolve in time so as to optimize the flow of matter or energy. Atmospheric circulation is the large-scale movement of air that distributes heat on the surface of the Earth, the researchers explain. These flows develop as air and water move from hot to cold regions, a result of variation in the heating of the Earth's surface by the sun. To apply the constructal theory to global climate, the researchers treated the Earth as if it were a heat engine that, rather than doing work, dissipates all the power it produces through air and water currents.

The findings may lead to a new understanding of the factors that drive global circulation patterns of the atmosphere and ocean, the researchers say. The theory might also prove useful for predicting the consequences of environmental change, such as shifts in the atmospheric concentrations of greenhouse gases, for broad weather patterns. "If the properties of the atmosphere change as people say they will," Bejan says, "we could anticipate what that might mean for global climate."