The simple formula we’ve learned in recent years – forests remove the greenhouse gas CO2 from the atmosphere; therefore forests prevent global warming – may not be quite as simple as we thought. Forests can directly absorb and retain heat, and, in at least one type of forest, these effects may be strong enough to cancel out a good part of the benefit in lowered CO2. This is a conclusion of a paper that will be published Friday 22, in Science by scientists in the Weizmann Institute’s Faculty of Chemistry.

…But forests do more than just store CO2, and Yakir, together with Dr. Eyal Rotenberg, decided to look at the larger picture – the ‘total energy budget’ of a semi-arid forest. The first hint they had that other processes might be counteracting the cooling effect of CO2 uptake came when they compared the forest’s albedo – how much sunlight is reflected from its surface back into space – with that of the nearby open shrub land. They found that the dark-colored forest canopy had a much lower albedo, absorbing quite a bit more of the sun’s energy than the pale, reflective surface of surrounding areas. In a cloudless environment with high levels of solar radiation, albedo becomes an important factor in surface heating. 

… Next, the researchers looked at the mechanisms for ‘air conditioning’ within the forest itself. To cool down, trees in wetter areas of the globe use water-cooled systems: They open pores in their leaves and simply let some of the water evaporate, drawing heat away in the process. But the semi-arid pine forest, with its limited water supply, is not built for evaporation. The scientists found that it uses an alternative, efficient, air-cooling system, instead. As semi-arid forests are not as dense as their temperate counterparts, the air in the open spaces between the trees comes into contact with a large surface area, and heat can be easily transferred from the leaves to the air currents. This semi-arid air cooling system is quite efficient at cooling the treetops, and this cooling, in turn, leads to a reduction in infrared (thermal) radiation out into space. In other words, while the semi-arid forest can cool itself well enough to survive and take up carbon, it both absorbs more solar radiation energy (through the albedo effect) and retains more of this energy (by suppressing the emission of infrared radiation). Together, these effects turned out to be stronger than the scientists had expected. ‘Although the numbers vary with location and conditions,’ says Yakir, ‘we now know it will take decades of forest growth before the ‘cooling’ CO2 sequestration can overtake these opposing ‘warming’ processes.’   READ MORE

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