Forest producing rain

How reforestation can help to produce more rain

The two biggest worries concerning our changing climate are about raising sea levels and drought. But what if planting trees doesn’t only help with absorbing CO2 from the air, but can also provoke more rain? In the Amazon rainforest, the rainy season begins two to three months before everywhere else in Latin America. This phenomenon has scientists puzzled. Now one team of researchers from the US believe they have the answer: “trees can produce their own rain.” 

Transpiration is a well-known part of photosynthesis, where moisture is drawn up by plant roots, gathers on leaves and then evaporates into the atmosphere. However, academics at the University of California believe that the Amazon rainforest goes one step further. The sheer amount of moisture released into the atmosphere by transpiration from trees in the Amazon Rainforest helps to make it rain. These trees pump so much moisture into the air that it changes the atmosphere, triggering a shift in wind patterns that brings in more moisture from the ocean. This means it can start raining in the Amazon rainforest as much as three months before the arrival of the weather system known as the Intertropical Convergence Zone (ICZ), which is responsible for the region’s normal rainy season.

Climate scientist Rong Fu and her colleagues from the University of California used NASA’s Aura satellite to observe water vapors over the Amazon rainforest. She told Science magazine that the satellite showed vapors consistent with transpiration, rather than evaporation. This is because the differences in the processes means the water from transpiration is heavier than that from evaporation. Another clue that transpiration was behind the heavier vapors was that they were most prevalent at the end of the dry season, when photosynthesis is at its strongest. As the tree-induced rain clouds release their own rain, they warm up the atmosphere. This encourages circulation, which in turn brings in more moisture from the ocean.

Planting extra trees to combat climate change across Europe could also increase rainfall. A new study found that converting agricultural land to forest would boost summer rains by 7.6% on average. The authors of the study believe that extra rain could partially offset the rise in dry conditions expected with climate change. The researchers also found that adding trees changed rainfall patterns far downwind of the new forests. The findings about increasing rainfall are partly based on observations of existing patterns. But the underlying reasons are less clear. They are probably related to the way the forests interact with cloudy air.

A number of studies have looked at the range of impacts, both positive and negative, that the boom in planting trees is likely to bring. This new paper considers the impact of converting agricultural land across Europe to sustainable forests. The authors use an observation-based statistical model to estimate how changes to forest cover would impact rainfall across the continent. They found out that if there was a 20% increase in forest, uniformly across Europe, then this would boost local rainfall, especially in winter and with greater impacts felt in coastal regions. But as well as local rain, the planting of new forests causes impacts downwind. The scientists found that rainfall in these locations was increased particularly in the summer months. Taking the two impacts together, in what the team describe as a realistic reforestation scenario, they found that precipitation overall went up by 7.6% in the summer. That’s quite a significant finding, according to lead author Ronny Meier from ETH Zurich. It also has implications for climate change. “Probably the most threatening climate change signal that we expect in relation to precipitation, is this decrease in summer precipitation that is expected in the southern parts of Europe like the Mediterranean,” he told BBC News. “And there, according to our study, forestation would lead to an increase in precipitation. So the forestation would probably be very beneficial in terms of adapting to the adverse effects of climate change.”

The new forests tend to evaporate more moisture to the atmosphere than agricultural land and this extra supply is the main reason behind increased rainfall downwind. They point out that the cloudy air that produces rain tends to stay longer over forested areas. And the rougher nature of these forests may trigger the rain. “A forest is a much rougher surface than agricultural land,” said Ronny Meier.” So, it induces more turbulence at the land-atmosphere interface, and also, the forest exerts more drag on to the atmosphere than agricultural land. We think that this drag, this higher turbulence over the forests is probably the main reasons for the fact that we find more precipitation in regions with more forests.”

But the authors of the study also point out that the increased rainfall could have potentially negative impacts by boosting rainfall patterns that have already been affected by climate change, particularly in the Atlantic region. The authors say that the reasons and consequences behind these local and distant impacts on rainfall are uncertain. For the authors, the fact that trees planted in one country may have implications in another means that the world should really consider all the impacts of how we use land. It also shows once again, that the idea of solving climate change with trees is not as simple as it is often portrayed. “Planting trees is certainly not a quick fix for climate change,” said Prof Wim Thiery, from the Free University of Brussels, Belgium, who was not involved in the new study. Ideally we strongly cut back on our greenhouse gas emissions and use a well-organized way of planting new trees to help in slowing down climate change.

The research has been published in the journal Nature Geoscience.