October 09, 2012
DURHAM, N.C. -- While climate scientists design intricate and complex models of global climates that require banks of super computers to run for weeks, a Duke University engineer believes he has developed a much simpler way to predict the Earth’s climate patterns.
Adrian Bejan, professor of engineering at Duke’s Pratt School of Engineering, said that a climate model based solely on mathematical equations is simple enough to be calculated with pencil and paper using readily available data.
“Complex models of the Earth’s thermal behavior are opaque from the point of view of the average person, and contain the uncertainties of the many flows of various scales that are part of these models,” Bejan said. “Our approach is much simpler.”
The idea of flow is an important one for Bejan, who developed the constructal law (www.constructal.org), a Duke-inspired theory of design in nature that explains such diverse phenomena as river basin formation, the anatomy of the capillaries with lungs or branches on a tree. The principle states that flow systems evolve in time to balance and minimize imperfections, reducing friction or other forms of resistance, so that they flow more and more easily in time.
The results of Bejan’s application of the constructal theory to the climate were published online in the International Journal of Global Warming, viewable here.
Two of the main variables in the model involve the reflectiveness of Earth’s surface -- much like the degree of whiteness seen from space -- and greenhouse factors, as measured by how much heat from Earth’s surface passes through the atmosphere, Bejan said.
“We created a simple convection-radiation model to anticipate the response of the Earth over time to changes in its reflectivity and other greenhouse factors,” Bejan said. “The novelty is the simplicity and transparency, along with the use of the constructal law, as the principle that governs the evolution of flow configurations -- in this case air masses -- over time.”
The flows that Bejan’s approach deals with are how and when atmospheric currents flow from the warm regions of the Earth to the colder polar regions. Bejan believes that by determining the sizes and characteristics of these zones, along the reflectiveness and greenhouse factors, he can calculate and predict the flow of air masses across the Earth.
There are three primary zones of atmospheric circulation on Earth -- the Hadley cell, the Ferrel cell and the polar cell. The Hadley cell is tropical and located roughly between 35 degrees north and south latitude. The polar cell, like the Hadley cell, is relatively stable. The Ferrel cell, however, is located between the Hadley and polar cells, and is typically the site of greater instability.
“We found the poleward heat currents reach their maximum close to 35 degrees latitude, therefore showing that between this latitude and the pole the Earth’s thermal ‘budget’ is negative,” Bejan said. “This latitude corresponds to the position of the Ferrel cell in the global circulation, driven by the Hadley cell that exports heat, and the polar cell, which receives the heat.
”This movement from hot to cold is in accord with the constructal law, since the currents follow the most efficient paths in their movement,” he said.
Obviously, Bejan said, this model does not address any man-made causes of global warming, but solely the climate response to expected changes in the composition and radiative properties of the atmosphere -- that is, the changes in the warm and cold air and ocean currents on the globe.
The research was supported by the U.S. National Renewable Energy Laboratory and the Air Force Office of Scientific Research. Other members of the research team were Marc Clausse, Laboratory of the Environment, Energy and Health, Paris; Francis Meunier, ESIEE Paris; and Antonio Heitor Reis, University of Evora, Portugal.