Finding Out What Forests Really Do To Climate

To get very far in understanding the effects of forest cover on climate, we need to break down the complex form and behavior of the forest into simple components. These are the building blocks of a model that can include the role of forest in making climate. Several of them have already been talked about in Chapters 4 and 5, but it will do no harm to mention them again (Figure 6.2). One important basic aspect of forests is the proportion of sunlight that they absorb. Known as "albedo" (from a Latin word meaning "whiteness") this is important in determining how easily the forest can heat up in the sun. The darker the forest surface (i.e., the lower the albedo) the more solar energy is absorbed, as opposed to being reflected straight back out into space. When it is absorbed, this energy tends to heat up the leaves. Some of the heat then goes to warming the air around the top of the forest canopy. But, in fact, much of the heat energy that is in the leaves just "vanishes"; the leaves stay much cooler than you would expect from all the sun's energy that they are absorbing. The missing heat has not really vanished—it has just been stored for a while in the water vapor that evaporates from the leaves. This is known as latent heat. It is a strange thing that even though the dark forest cover is absorbing more heat from the sun—compared with a more sparsely vegetated environment—it does not show up in terms of temperature! The more open non-forested environment will nearly always be hotter during the day.

So, this brings us to a second important aspect of how forests affect climate. Transpiration, the evaporation of water out of tiny pores in the leaf surface, takes up heat. This is water that has fallen from the sky, soaked into the soil, been sucked up by roots and carried up within the tree all the way to the cells of the leaves. A single

Figure 6.1. Forest loss in tropical lands.

(b) Vietnam

(c) Madagascar. Source: Redrawn from Groombridge (1993).

Pristine forest

Pristine forest

Figure 6.1. Forest loss in tropical lands.

(b) Vietnam

(c) Madagascar. Source: Redrawn from Groombridge (1993).

large tree can take up as much heat in evaporation as you'd get from one hundred 100 W lightbulbs burning continuously. This stored latent heat will eventually be released somewhere up in the atmosphere, thousands of meters up and perhaps hundreds or even thousands of kilometers away. What enables the heat of evaporation to be released again is the condensation of water into droplets, forming clouds and eventually rain. In addition to this, there is what is known as "physical evaporation'': rainwater evaporating from the surface of the leaves or from the soil surface, without having passed through the tree. In forested areas, the greatest part of the evaporation of water going on is through transpiration from the leaves, rather than physical evaporation.

Climate scientists refer to the proportion of the heat absorbed by the forest that goes into evaporation—rather than just heating up the air—as the "Bowen ratio''. This is something that varies between different forest and vegetation types, but also according to season and even with the most recent weather conditions.

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■ Dense forest cover Does not include coastal mangrove forests and open pine savanna

Figure 6.1 (cont.). Forest loss in tropical lands. (d) Central America. Source: Redrawn from Groombridge (1993).

In essence, forests pump heat and water out into the air above them. They do this more effectively than most other vegetation types, and far more so than bare soil. Something that also helps forests act as water pumps to the atmosphere is that they store a lot of rainwater amongst the root mass of the forest, which is rich in spongy organic matter from the decay of leaves, roots and branches of the trees. Water that would otherwise run straight off the land surface and down to the sea is instead held in the soil, to be sucked up by roots and then evaporated from leaves in the canopy. The deepest roots of many trees reach tens of meters down into the ground, and this also helps them to sustain a good rate of evaporation long after the surface soil has dried out, because the trees can continue to tap into groundwater in pores in the rocks below.

How do these two processes—heat transfer and water transfer—affect climate? On the local scale, evaporation from all the leaves in a forest canopy makes the

Darker forest surface absorbs light

Lighter open arid vegetation reflects light

Figure 6.2. Some of the ways in which forests modify temperature. (a) Albedo: the dark forest surface absorbs sunlight, warming the air. (b) Latent heat uptake in evaporation cools the air. (c) Roughness helps to feed heat and water vapor to the atmosphere above, cooling the forest. Source: Author.

Evapotranspiration from leaves

Evapotranspiration from leaves

H2° u^ H2° H2° Water in soil r^U
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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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