Deforestation in the tropics affects climate around the world, study finds
“The effects of tropical deforestation on climate go well beyond carbon,” says Professor Deborah Lawrence, “[it] causes warming locally, regionally, and globally, and it changes rainfall by altering the movement of heat and water.”
These are the conclusions of a worldwide study into the deforestation of tropical rainforests, which shows that cutting down trees can have immediate impacts on the climate and put agricultural productivity at risk.
Rainforests are more than just a carbon store
Deforestation and land use change account for approximately 11 per cent of global carbon dioxide emissions. But the new research finds that cutting down trees doesn’t only affect the carbon they lock up.
The research, published in Nature Climate Change, reviews academic studies on deforestation of tropical rainforests in the Amazon basin, central Africa, and southeast Asia. Many of the studies use climate models to simulate what happens if you remove these forests completely, and they suggest that deforestation in the tropics can affect the climate on the other side of the world.
The map below shows how far-reaching some of these potential impacts are. The triangles show areas where rainfall is expected to decrease because of tropical deforestation, and the circles show areas of increase. The colours indicate the link to where the deforestation occurs.
So the models suggest deforestation in the Amazon, for example, can reduce rainfall over the US Midwest and even in northeast China. Deforestation in central Africa can cause a drop in rainfall in southern Europe, and loss of trees in southeast Asian can bring wetter conditions in southern Europe and the Arabian Peninsula.
Image - Lawrence & Vandecar (2014) Fig1 (note)
Global impact of tropical deforestation on rainfall. Projected increases (circles) and decreases (triangles) in rainfall due to complete deforestation of either the Amazon (red), central Africa (yellow) or southeast Asia (blue). Boxes indicate the area where the forest was removed in the models. Numbers show the study the results relate to. Source: Lawrence & Vandecar (2014)
Lead author Professor Deborah Lawrence tells us in an email:
“These are physical effects from removing trees that are not simply related to the loss of carbon dioxide stored inside them. Tropical deforestation results in immediate climate impacts independent of, and in addition to, its contribution to the greenhouse effect.”
Tropical deforestation is a global problem
So how does it work? How can cutting trees down in the Amazon affect rainfall in China?
First you have to bear in mind that rainforests cool the air above them by turning water from the soil into moisture in the air. Chop the trees down, and you remove the cooling effect from this additional moisture. The effect is so pronounced, the study finds, that if all the trees in the tropics were cut down global temperature could increase by as much as 0.7 degrees.
With the trees gone the air warms up, creating large, rising masses of warm air. When these air masses hit the upper reaches of the atmosphere, they create ripples called teleconnections that flow towards the mid- and higher latitudes.
Lawrence compares it to boiling water:
“Imagine steam rising off a pot of boiling water, hitting the ceiling in your kitchen and flowing outward, along the ceiling, out the door to your hallway.”
So these changes to the atmosphere in the tropics can flow out to the atmosphere of temperate regions and alter their climate, Lawrence says.
Worst-case scenario
It seems unlikely we’ll ever cut down an entire rainforest. So why do scientists run these experiments? Lawrence explains:
“We want to understand just how important rainforests are to sustaining the life support system on earth, and we start with a worst case scenario. Large-scale tropical deforestation is the outcome of business as usual economic development, it is the path we took in the US and Europe during the course of our development.”
Scientists also use more realistic scenarios, Lawrence tells us, which help identify potential tipping points if deforestation continues at current rates. For example, some studies show that clearing 30 to 50 per cent of the Amazon would trigger a drop in rainfall that could cause a significant decline in how the rainforests functions as an ecosystem.
The study also looks at changes that have already happened as forests have been cleared. In Brazil, for example, the rainy season starts 11 days later in deforested areas, and scientists think that the loss of trees in central Africa may have caused a more than 20 per cent decline in rainfall from the Congo basin to the east coast.
Implications for agriculture
Scientists have also modelled the impact climate changes from deforestation could have on farming in the tropics.
Warmer and drier conditions caused by deforestation could put agricultural productivity at risk, the study finds. Yields of soy in the Amazon, for example, are projected to drop by up to 60 per cent if more trees are cut down, and cattle production may not be viable in some areas as the quality of pasture declines. Adaptation measures might reduce the impact of these effects to some extent, of course.
Deforestation can also cause longer dry seasons and delays to the start of the rainy season, the study suggests. Because forests help moderate high daytime and low night-time temperatures, cleared land is more susceptible to temperature extremes, which some crops may not tolerate.
Limiting deforestation is therefore important for farming as well as tackling climate change, Lawrence argues:
“Agriculture and forestry need to be considered together. Maintaining large tracts of tropical forest is essential for maintaining a climate that sustains tropical agriculture. Forest conservation is an essential aspect of planning for agricultural development.”
And alongside climate change, cutting down trees will make growing crops and raising livestock even harder:
“We are already anticipating worldwide challenges to food security because of what we are doing to the atmosphere. Now we have to worry about additional climate challenges because of what we are doing to the surface of the earth.”