How Trees Mitigate Climate Change & Regulate Rainfall

Removal of trees and changes in the way land is used is one of the most important contributors to climate change. It accounts for almost a quarter of humanity’s carbon dioxide emissions.

Trees absorb carbon dioxide from the air as they grow. Through the use of solar energy, the carbon in carbon dioxide is transformed the stem, leaves, branches and the rest of the tree’s body. This is why it is so important to plant trees. A tree is essentially a reservoir of carbon. If the tree did not exist, this carbon would remain in the atmosphere in the form of carbon dioxide. When we plant trees, some of that carbon is held in the tree and the rest enters the soil through decomposition of leaves and branches that are naturally shed by trees. This decomposing material adds to the fertility of the soil.

Currently, about a quarter of the carbon dioxide that humanity adds to the atmosphere is soaked up by trees and plants. If we don’t add more trees soon, this human-induced carbon dioxide emission will accelerate the global warming that has already begun.

Forests are also known to have a cooling effect on the planet. However, they do this job effectively only in the tropics. Planting trees in high latitudes can have a reverse effect of increasing temperatures. However, trees do act as natural carbon sinks at all latitudes.

Plants combat warming not just on a global scale but even on a local level. Urban areas can be up to 12°C warmer than rural surroundings due to the heat given off by buildings, roads and traffic, as well as reduced evaporative cooling, in what is commonly referred to as an “urban heat island”.

An urban heat island not only increases the temperature in the area, but can also increase the intensity and frequency of rainstorms, leading to flooding in cities.

Scientists looking at the effect global warming will have on our major cities say a modest increase in the number of urban parks and street trees could offset decades of predicted temperature rises.

The University of Manchester study has calculated that with just a 10% increase in the amount of green space in built-up centers would reduce urban surface temperatures by as much as 4°C.

This 4°C drop in temperature, which is equivalent to the average predicted rise through global warming by the 2080s, is caused by the cooling effect of water as it evaporates into the air from leaves and vegetation through a process called transpiration.

Additionally, a University of Sheffield study found that rivers can cool their local urban environment. Dr. Abigail Hathway, a lecturer in Computational Mechanics and Design in the University’s Department of Civil and Structural Engineering, has been researching the microclimate effect of the river Don, which flows through Sheffield to investigate how these principles could be put to use in urban planning.

She explains: “We monitored temperatures at a number of sites close to the river Don in Sheffield and discovered that, in hot weather and during the daytime, the river has a significant cooling effect.

Even in non-urban areas, on the scale of continents, trees can play a role in regulating rainfall – augmenting rain where rain is scarce, and reducing it where it is too much.

Increasing Rainfall: One study looking at how a corridor of trees along rivers can “transport” rain to the interior of continents suggests that “In the non-forested region’s precipitation declines exponentially with distance from the ocean. In contrast, in the forest-covered region’s precipitation does not decrease and even grows along several thousand kilometers inland. This indicates that forest cover plays a major role in the atmospheric circulation and water cycling on land. This suggests a good potential for forest-mediated solutions of the global desertification and water security problems.”

For example, the tree cover in the Amazon rainforest is known to create its own rainy season. This happens in India too. A recent study from IIT Bombay showed that deforestation over the last few decades has led to weakening of the summer monsoon. The study author says, “Our study found that there are local factors such as changes in land use and land cover that lead to changes in monsoon rainfall. These local changes are in our hands, and because of them there has been a significant reduction in rainfall over two major regions, the Ganga basin and northeast India. That is really alarming.” Recycled precipitation or precipitation caused by vegetation is estimated to account for 20–25% of the rainfall in North India (Ganga Basin) and Northeast India during August and September.

Trees also release tiny organic particles called aerosols that allow water vapour in the air to condense, bringing rainfall to areas that need them.

Decreasing Rainfall: A decrease in rainfall doesn’t sound like a nice experience. However, as already mentioned, climate change is creating unseasonal rainfall and more extreme wet spells and dry spells – which means more flood and drought. There is increasing amount of evidence pointing towards an increase in the number of heavy rainfall events because nearly the whole of India has experienced very strong warming of between 0.1-1 degree Celsius per year. Essentially, rain falls when it is not supposed to and doesn’t fall when it should. Trees help lower temperatures over land, and therefore mitigate rainfall extremes, keeping rainfall at its normal levels.