Forests and Global warming

Greg Samdon, environmentalist, writes for Science and the News magazine.

'The interaction between forests and global warming is turning out to be more complicated than was originally assumed. Some of the basic assumptions which have underpinned theories of how deforestation affects the atmosphere are having to be re-examined.

First of all, it needs to be said that forests are an asset to the environment of the planet. They provide a haven for wildlife, and are both beautiful and economically productive in their own right. Also, there can be no doubt that trees are essential 'carbon sinks'. That is, a forest takes global-warming carbon dioxide out of the environment and converts it into solid carbon in the form of wood. Forests also have a role to play as 'green lungs' for the planet. That is, in the process of photosynthesis, carbon dioxide is converted to oxygen without which humans would not be able to breathe. The process of photosynthesis requires water, which is released into the atmosphere through the plant's leaves, cooling the air in the process. Every tree may cause only a tiny drop in temperature, but when taken across forests which make up a considerable percentage of the earth's surface, the effect is noticeable.

That said, the role of forests as 'green lungs' for the planet must not be over-estimated. It has been found that a huge amount of the oxygen in the atmosphere comes not from forests but from other plants on the earth's surface (including crops planted by humans), and even more comes not from the land but from plants such as algae, seaweed and plankton in the sea.

Nor does the absorption of carbon dioxide come without a price. Recently scientists discovered that growing plants emit methane, a gas usually associated with swamps and decay. It has now been shown that growing plants put out hundreds of times more methane that would be emitted if the plants were simply dead and rotting away. Since methane is a greenhouse gas, this means that forests are actually helping with some global warming. In fact some scientists believe that the deforestation which has been happening in some parts of the world has helped to slow methane build-up in the atmosphere.

Another way in which plants help to warm the atmosphere is because the leaves of trees absorb sunlight, and this makes them rather warmer than the air around them. Therefore though the overall effect of trees is cooling, both because they put out evaporated water and because leaves reflect some light back into space (and of course all the green light in the spectrum that falls on them), this effect is less than was originally thought.

So should we consider reducing the number of forests on the earth's surface? Of course we should not. It is not the forests which are the problem, it is the human race. Recent decades have seen the fastest amount of warming that has ever been recorded, and almost all of it is due to human activity. Even methane emitted by all the forests on earth comes to less than 10% of the methane in the atmosphere, while humans have released so much carbon dioxide into the atmosphere that the air now has 25% more than it had a century ago.

Nevertheless, the results of the recent scientific studies show that we must always be careful to avoid the law of unintended consequences, which says sometimes what you try to do has the opposite effect to what was intended. For example, it has been shown that the removal of pollutants from the atmosphere has had two effects. Firstly, as the air in some countries has become cleaner, both humans and the rest of the environment have benefitted from the improved air quality. But at the same time, many of these airborne pollutants were reflecting sunlight back into space. With them gone, the earth is absorbing more sunlight and becoming warmer.