9 February 2006
Climate Change Tackled With Constructal Theory
by Kate Melville
Researchers believe that constructal theory - a physics principle that describes design in nature - could provide a new approach for forecasting environmental change. Research to date indicates that the theory can predict the global circulation of air that determines the boundaries between desert and tropical forests, as well as between temperate zones and the poles. The theory also predicts other climate characteristics, such as average wind speed and the average temperature difference between night and day. The climate models created using constructal theory proved to be very accurate, despite the simplicity of the rules governing them.
Constructal theory is based on the idea that every system will be imperfect and the final geometry (or shape) of the system will be defined by the optimal distribution of these imperfections. The researchers behind the new climate modeling technique, who include the father of constructal theory, Duke University's Adrian Bejan, report their results in the International Journal of Heat and Mass Transfer. The findings may lead to a new understanding of the factors that drive global circulation patterns of the atmosphere and ocean.
Predicting the characteristics of global circulation has been a complex business in the past. "While other very complicated empirical models predict the same basic features, constructal theory does this in a much simpler way," said researcher A. Heitor Reis, of the University of Evora. "This is an entirely new kind of approach to climate."
In applying constructal theory to global climate, the researchers modeled 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 Earth with its solar heat input, heat rejection, and wheels of atmospheric and oceanic circulation, is a heat engine without [a] shaft," Bejan explained. "Its maximized mechanical power cannot be delivered, but is instead destined to dissipate through air and water friction and other forms of heat loss. It produces maximum power, which it then dissipates at a maximum rate."
Using this analogy, the researchers predicted the main characteristics of global circulation and climate based on very few inputs - the temperature of the sun, the solar constant, cloud cover and the Earth's greenhouse factor. The solar constant refers to the amount of incoming solar radiation measured on the outer surface of Earth's atmosphere. The greenhouse factor takes into account the concentrations of aerosols and greenhouse gases to determine the amount of heat energy trapped by the atmosphere.
The researchers found that this simple model predicted the latitudinal boundaries of the Earth's three circulation zones - the Hadley, Ferrel and Polar cells - which comprise the main global circulation on Earth. They added that the theory also predicted the average speed of atmospheric and oceanic flow and the average temperature on Earth.
Already, the results are challenging our understanding of the factors that drive climate patterns. Reis said that it was previously thought that the circulation zones arise due in part to the rotation of Earth. "We cast doubt on this idea by showing that the circulation patterns can arise based solely on the optimal structure of global heat flow," he said.
Bejan is bullish about contructal theory's relevance to natural phenomena. "The accumulation of coincidences between theoretical predictions and natural facts adds weight to the claim that the constructal law is a law of nature," he said.
Source: Duke University