Albedo is The Measure

How leaves can affect global warming

From planting a million artificial trees capable of absorbing atmospheric carbon dioxide to growing an algal carpet in the ocean turning it into a carbon storehouse, various attempts at mitigating global warming are on. Andy Ridgewell and his team from the University of Bristol have suggested that agricultural practices can also help lower air temperatures depending on a plant's albedo-the ability of leaves to reflect sunlight back into space.

Forests reflect about 13 per cent of sunlight. Crop leaves have an albedo of 20 per cent and therefore can reflect more. Albedo also varies between different strains of crop species, mainly due to variations in canopy morphology and leaf-surface properties. Thus, instead of changing the type of crop growing in an area, one can simply change the strain of the same crop to increase the albedo of crops, suggested the researchers.

The study also emphasized the presence of waxy cuticles on aerial portions of the plant. Previous studies showed that glaucous leaves reflect more UV radiation compared to leaves without the coating.

Ridgewell and his group created a computerized model simulating croplands in a latitude encompassing North America and Eurasia. Using data on the albedo levels of crops growing in this belt, the team simulated a very high level of atmospheric CO2 (700 ppm) as compared to the present day level (350 ppm). They also increased the albedo level of crop plants by 10, 20 and 40 per cent. They found that 20 percent increase in albedo in the entire latitude could lead to a reduction of 1°C in summertime surface air temperatures.

This does not, however, mean that forests should be cut down and turned into farmlands. The study showed that little change occurs in India. The authors said this may be because India is bordered by oceans on each side within the same latitude. Hence the approach has low potential in tropical countries.

Richard Betts, head of climate impacts in the Met Office, UK, said even if this model succeeds in offsetting global warming of the earth's surface by increasing the reflection of sunlight back to space, it will not have any impact on greenhouse gas emissions.


Down to Earth February, 2009