How Forests Regulate the Global Climate

Forests cover a substantial share of the world's land and do far more than provide timber and shelter for wildlife. They are among the most important regulators of the planet's climate, quietly influencing the temperature, moisture and chemistry of the air across whole continents. Understanding how they perform this role has become increasingly urgent, because the health of the world's forests is closely bound up with the stability of the climate that human societies depend upon.

The best-known way in which forests shape the climate is through their handling of carbon. Trees draw carbon dioxide out of the air during photosynthesis, using the energy of sunlight to combine it with water and build the sugars from which they grow. The carbon becomes locked into wood, leaves and roots, while much of it is also passed into the soil as leaves fall and roots decay. A growing forest therefore acts as a store, or sink, holding vast quantities of carbon that would otherwise remain in the atmosphere as a heat-trapping gas. When a forest is cleared or burned, however, this stored carbon is released back into the air, and the same trees that once cooled the planet can become a source of warming instead.

Forests also influence the climate through their effect on water. Trees draw moisture up from the soil through their roots and release it into the air from their leaves in a process called transpiration. Over a large forest this adds enormous amounts of water vapour to the atmosphere, which later falls again as rain. In some regions the moisture released by one stretch of forest is carried by winds and falls as rainfall over land far away, so that clearing trees in one place can reduce the rain that reaches another. In this sense a great forest behaves almost like a pump, moving water through the landscape and helping to keep distant regions green.

The colour and texture of a forest matter too. A dense canopy of dark leaves absorbs a large proportion of the sunlight that strikes it, whereas bare ground, and especially snow, reflects much more of that light back into space. The fraction of sunlight that a surface reflects is known as its albedo. Because forests generally have a low albedo, they tend to absorb heat and warm the surface locally. This creates a subtle balance: a forest cools the planet by storing carbon and releasing moisture, yet its dark surface can have a warming influence, and the overall effect depends on where the forest grows.

These competing effects mean that forests in different parts of the world do not all influence the climate in the same way. Tropical forests, which are warm and wet and grow quickly, are especially powerful at storing carbon and pumping moisture into the air, and clearing them is widely regarded as damaging to the climate. Forests in the far north, by contrast, grow in regions where snow would otherwise reflect a great deal of sunlight, so their dark canopy can have a stronger local warming effect that partly offsets the benefit of the carbon they hold. Scientists therefore stress that the climate value of a forest cannot be judged by its size alone.

Human activity has placed great pressure on these systems. When land is cleared for farming, timber or building, the carbon held in the trees is lost and the cooling services the forest provided are weakened. A hotter, drier climate can in turn make forests more vulnerable, as prolonged drought stresses the trees and increases the risk of large fires that release still more carbon. There is a danger that warming and forest loss can reinforce one another, each making the other worse in a cycle that is difficult to break once it gathers pace.

Because of this, protecting existing forests and allowing damaged ones to recover are seen as some of the most practical steps available for limiting climate change. Mature forests that have grown undisturbed for a long time hold especially large amounts of carbon, which is why conserving them is often considered more valuable than planting new trees to replace them. Restoring forests takes time, and a newly planted area may need many years before it stores as much carbon as the woodland it replaced. The message emerging from research is a simple one: the forests that remain are a resource the world can ill afford to lose, and keeping them standing is among the surest ways of keeping the climate in balance.