Dr Richard Betts, Head of Climate Impacts
In the fight against climate change forests are like an undervalued friend, taken for granted and often overlooked.
While everyone knows that deforestation is one source of carbon emissions, this is only part of the story. Forests actually soak up carbon dioxide, partly offsetting our emissions and slowing the rise in CO2.
Tropical rainforests also help to cool the Earth's surface by enhancing evaporation and cloud cover. Destruction of the forests not only adds to the problem of climate change, it takes away part of the solution.
The forestry sector is the third largest emitter of greenhouse gases, surpassed only by energy production and industry. This may surprise you after all the focus on cutting car use and air miles, but globally speaking deforestation emits more greenhouse gases than transport. Currently about eight billion tonnes of carbon dioxide per year are emitted by deforestation, and about one quarter of the twentieth-century rise in CO2 has been due to this cause.
But removing forests has other effects. Plants extract carbon dioxide from the air as they grow, and extensive measurements of tree trunk widths across the tropics have shown that tropical trees have got fatter as a result.
Over the last 40 years, tropical forests are thought to have soaked up about 200 billion tonnes of carbon dioxide from the atmosphere, which would otherwise have added to the CO2 rise that has been measured by precision instruments around the globe. This is a free service we receive from the world's ecosystems, which has buffered us from the full effects of our carbon dioxide emissions. If tropical rainforests had not been there, the past CO2 rise would have been 10% higher.
Tropical rainforests also affect their own climate by recycling rainwater. Rainforest trees draw up huge amounts of water from the soil and release most of it into the atmosphere. providing more water to the clouds to be rained back on the forest. Evaporation from the world's oceans provides the ultimate source of water, but the continental interiors are kept moist by recycling of rainfall across the land which is significantly enhanced by forests.
Rain water in central Amazonia has gone through several cycles of rainfall and evaporation from the forest on its way from the ocean. Loss of the forest, therefore, means a weakening of this water recycling service, and drier climates inland.
Evaporation also helps keep the forested landscape cool. The analogy with the human body is simple: when you are wet, you feel cold because evaporation of the water takes energy away from your body. The same is true of the landscape — evaporation of water from leaves, the ground and from within the soil extracts energy from the land and keeps it cooler.
The moisture in the air also helps promote cloud cover, which has a further cooling effect by reflecting sunlight back to space. Forests enhance this cooling effect by maintaining high levels of evaporation, so removing the tropical forests warms the world's surface twice — once through CO2 emissions and again through reduced evaporation.
Some forests themselves are under threat from climate change. 2005 saw a major drought in Amazonia, with the Amazon river at record low levels and large numbers of forest fires. Sophisticated computer-based climate model projections from the Met Office Hadley Centre indicate that drought could become more frequent in Amazonia, increasing the risk of forest fires spreading.
Most fires in Amazonia are caused by humans, often as part of the deforestation process as areas of forest are cleared by burning. Drought conditions make it more likely that such fires could run out of control, as was seen in 2005 and 1998 in Amazonia and in 1997 in Indonesia.
So, in addition, to deforestation causing direct damage to the forests, climate change could further enhance this damage by increasing the risk of fires becoming uncontrolled.
With the tropical forests being so crucial to the maintenance of their own moist climate, it may be that removing them is irreversible.
Large-scale forest loss, whether by deforestation, climate change or a combination of the two, could dry out the local climate so much that it would be difficult or impossible for the forests to grow back. Deforestation would, therefore, be a lasting legacy rather than a passing phase.
The carbon absorption service is essentially a free subsidy to carbon emitters. We do not see the full effect of our emissions because significant proportions are removed from the atmosphere by forests at no cost to ourselves. And comparing deforestation with other drivers of climate change solely in terms of carbon emissions or uptake ignores the other effects such as water recycling.
So far we are getting massive benefits from forests, and these are not being fully appreciated. But try slowing climate change without forests there to help — it could be much harder than you think.