Cool Forests

Trees act as air conditioners, cooling the atmosphere near the ground through evapotranspiration. Mammals make use of the evaporative cooling effect of water when they sweat and pant during physical exertion: evaporation from the skin and tongue cools the blood under the surface, helping the animal to regulate its temperature. The trees of the Amazon release 20 billion tonnes of water to the atmosphere each day. The equivalent energy used by this process is equal to the largest hydrodam in the world (in Brazil) operating at maximum power for 145 years. Old-growth tropical forests are able to maintain transpiration during the dry season because the trees can tap into deep soil water reserves, unlike shallow-rooting crops and pasture grasses.

The huge amount of energy used by evapotranspiration in the forest is released higher in the atmosphere when water vapour condenses into clouds. Tropical rainfall provides three quarters of the energy that drives the atmospheric wind circulation through the release of this energy (called latent heat). The clouds that form over tropical forests reflect incoming sunlight (solar radiation), shading the land and helping to reduce surface heating, although there is still some uncertainty about this process. New research in boreal forests suggests that these forests also play a role in cooling the climate by releasing chemicals such as terpenes that act as cloud condensation particles - making clouds reflect more sunlight.

Carbon Sequestration

Because plants and soils have a high carbon content, intact tropical forests are a giant reservoir or store of carbon. They store about a quarter of the carbon in the terrestrial biosphere. Trees absorb CO₂ from the atmosphere during photosynthesis to create the carbohydrates they need to burn as fuel, to grow and reproduce. Tropical forests appear to be sequestering (i.e. absorbing and storing) large amounts of carbon annually – between about a half and one tonne of carbon per hectare per year. Outside the tropics, old-growth boreal and temperate forests appear to be sequestering even more carbon per hectare -  around 2.5 tonnes per year.

The continual primary productivity of tropical forests can be seen from space. Click on the image to view a global model of the earth's carbon metabolism from the NASA Earth Observatory.

Deforestation released about 1.6 billion tonnes of carbon per year during the 1990s, chiefly in the tropics. Studies of the atmosphere suggest that tropical forests are carbon neutral or carbon sinks, which implies that carbon sequestration by undisturbed tropical ecosystems offsets deforestation emissions. If we assume that about as much carbon is sequestered by forests as is released by deforestation and that carbon emissions have a particular cost, then tropical forests are providing a ‘free offsetting service’ worth about €40-90 billion each year.

In summary, the benefits to human wellbeing of the climate regulation processes of evaporative cooling and carbon sequestration in tropical forests outweigh the global warming effect of deforestation. But more research into forest-atmosphere interactions is needed.

Bibliography:

L. Braat & P. ten Brink (2008). The Cost of Policy Inaction: The case of not meeting the 2010 biodiversity target. Wageningen/Brussels. Available at: http://ec.europa.eu/environment/nature/biodiversity/economics/index_en.htm

 
G. B. Bonan (2008). Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests. Science 320, 1444-1449.

S. Luyssaert, E.-D, Schulze, A. Börner, A. Knohl, D. Hessenmöller, B. E. Law, P. Ciais & J. Grace (2008). Old-growth forests as global carbon sinks. Nature 455, 213-215.

D. V. Spracklen, B. Bonn, & K. S. Carslaw (2008). Boreal forests, aerosols and the impacts on clouds and climate. Philosophical Trans.actions of the Royal Society A, Earth Science Special Issue.