Mature forests - the planet's natural air conditioners

Everyone thinks of rain forests as important to the stability of the climate because they convert carbon dioxide into carbon, thereby sequestrating the greenhouse gas. So they are, but rain forests also play another equally important role in the climate of the planet, and this role is often overlooked. Rainforests are places which degrade energy, and this energy degradation is as important as the role rain forests play in sequestrating carbon. If this was understood better we would go to much greater lengths to preserve mature forests in all places. I shall explain how it works.

Virtually all of the energy that the planet receives comes from the sun in the form of radiation. Some is bounced back into space and the atmosphere, covering the planet like a blanket, keeps the some energy around the planet to keep us warm. As you know carbon dioxide emissions add to the blanket effect, making the planet warmer and warmer.  

When solar radiation (or light) strikes the planet some of it is degraded by ecosystems. It has been found that the more mature and diverse the ecosystem, the greater the solar radiation degradation. If you think about places at about the same latitude on the planet some are hotter than others. The Sahara desert is hotter than the Amazon rain forest because the Amazon degrades solar radiation at a very high level. A less mature forest degrades solar radiation less and if you chop down part of the forest and farm the land, then the part that you farm will degrade the solar radiation even less, making the farm much hotter than the neighbouring rain forest.

Places like cities are almost always hotter than the neighbouring countryside by a degree or two. This is not attributable just to the human activity in cities, but rather more attributable to the fact that the neighbouring countryside can degrade the solar radiation better than the city. The city has hardly any ecosystem to absorb the radiation; the countryside has much more. The countryside absorbs radiation but the city reflects it back into space. The rain forest, with its mature rich and ecological diverse make up absorbs more solar radiation than any other part of the planet.

The effect of the role of forests in cooling the atmosphere is has been measured in the 1989 Lavall-Holbo experiments. They measured the different types of ecosystems – a quarry, a clear cut forest, a Douglas Fir plantation, a natural forest and a 400 year old mature Douglas forest. I have listed these in the order of their eco-complexity, starting with the simplest. The readings showed that the greater complexity of ecosystem the (a) more radiation was absorbed (b) the cooler the place was and (c) the more the energy degraded from radiation (into molecular motion).

What rainforests do very well (an un-lauded quality) is to reduce the solar gradient by converting water into latent heat. The Amazon reradiates only 17% of the energy it receives from the sun, but the Sahara reradiates 41% of it. What happens in the rainforests is that the leaves of the trees and shrubs capture energy and build structures with it, rather than dumping the energy back into space. The process involves the transpiration of plants. The Amazon has high transpiration – around 71% whereas the Sahara has very low transpiration – around 2%.

When there is a high amount of energy reradiated more water evaporates. You need 580 calories to evaporate one gram of water, which through the process of evaporation carries those calories as latent heat from the surface into the atmosphere. When the water vapour returns to the surface as rain or snow the calories return. If you create conditions which allow more latent heat into the atmosphere you will create more energy in the atmosphere and hence more storms stronger hurricanes and all the changed patterns of climate which scientists have been observing and which we in our everyday lives have also noticed.

In this context plans to make London a greener place by Mr Johnson, the Mayor, will make it a cooler more pleasant place in very hot weather, as well as absorbing some of the carbon that London emits. His idea of “greening London” by planted in roofs and open spaces is very important, but will not make a huge difference. On the other hand, preventing mature forests from being cut down, whether for biomass or farmland or for the trees themselves, will make the farmlands hotter and more importantly the planet will lose an important device which keeps the planet at a cool, habitable temperature.

Cutting down mature forests is a triple whammy. We lose a place where carbon is sequestrated, we emit carbon dioxide into the air by cutting the trees and disturbing the soil, and we lose the best natural air conditioner that the planet has.

People cut down forests in order to make money. They sell the wood, or grow crops on the land they clear. Many, although not all of the mature forests are in places which are not the world’s richest, so telling Brazilians in the Amazon to stop cutting down their trees will not go very far. We are prepared to pay for air conditioners in our offices homes and cars. Why should not we pay the nations with forests for simply allowing them to remain, uncut, unfarmed, natural air conditioners of the planet.