12 December 2001

Strategies To Reduce Global Warming Could Backfire

by Kate Melville

Blocking the sun may not be such a cool way of counteracting climate change, scientists at the University of Illinois say. Potential effects upon the biosphere could be important to agriculture and forest production, and also could create secondary feedback mechanisms that may further change the climate.

A number of engineering schemes have been proposed as mitigation strategies for global warming, such as lofting reflective balloons into the stratosphere or erecting a huge parasol in orbit. By blocking some of the sunlight, these devices would create a cooling effect to offset the warming caused by increasing levels of greenhouse gases.

But, even if such feats become practical, there are concerns about how the biosphere would respond to a reduction in solar radiation. To compensate for the climate effects of doubling the amount of carbon dioxide in the atmosphere, for example, the amount of sunlight striking Earth would need to be decreased by nearly 1.8 percent.

"The biosphere plays a very important role in determining how much carbon dioxide is in the atmosphere," said Donald Wuebbles, a professor and head of atmospheric sciences at the UI. "Through photosynthesis, carbon dioxide is removed from the atmosphere and stored in plants. Decreasing the solar constant by 1.8 percent could impact the amount of biomass produced, and therefore affect how much uptake and storage of carbon dioxide occurs."

To study such effects upon the biosphere, Wuebbles and his colleagues - John Foley, a professor of atmospheric and oceanic sciences at the University of Wisconsin at Madison, and UI graduate student Vaishali Naik - used a dynamic global ecosystem model to simulate the response of vegetation to engineered climate conditions.

First, the researchers doubled the amount of carbon dioxide in the atmosphere. Then they decreased the solar constant by 1.8 percent and compared the results. The researchers found a definite influence on the biosphere. There was a decrease in net production of biomass in tropical forests and in boreal forests located in higher northern latitudes. However, there was a slight increase in biomass production in the mid-latitudes.

"We think this mixed message is coming as a result of feedback mechanisms such as changes in water stress," Wuebbles said. "Without those feedbacks, we would have seen a decrease in net primary production everywhere."

Photosynthesis depends not only on atmospheric carbon dioxide and incident solar radiation, but also on an ample water supply, Wuebbles said. "In water-stressed regions, such as deserts, biomass is heavily dependent on the availability of water, as well as sunlight. If you reduce the solar radiation, there will be less evaporation from the soil, leaving more water for plants to consume. With less water stress, the plants can grow better, creating more biomass."

While previous studies have indicated that elevated levels of carbon dioxide in the atmosphere would stimulate photosynthesis, resulting in increased primary production of vegetation, the situation is not quite that simple, Wuebbles said. "For example, blocking some sunlight would decrease plant growth, but that would also decrease uptake, which would give a positive feedback on the amount of carbon dioxide in the atmosphere, which could lead to more climate change."

More work needs to be done to understand the impacts on the biosphere from an engineered response to climate change, Wuebbles said. "In particular, we need to take into account these various feedback mechanisms that may affect the amount of carbon dioxide in the atmosphere."