It is a well known fact that forests help maintain rainfall patterns over large parts of the world. The thinking goes that if we cut down our forests then we will end up with droughts. It is a belief not only held by western eco-warriors but also by many forest communities and indigenous peoples around the world who strongly believe that without their forests the rain will dry up.But what proof is there that this is really the case? It’s a problem that has been intriguing scientists for over 100 years (I just found an article in the journal Nature dating from August 1912 on the topic of “Forests and Rainfall”). But proving this link is not an easy task. We know that there are good physical reasons why it should happen: forests soak up water through their roots and release moisture into the atmosphere through their leaves in a process called transpiration. This process moistens air currents which should help maintain rainfall. But because rainfall is so variable and driven by so many different processes it is hard to convincingly attribute any change in rainfall to cutting down or planting a specific forest.
At Leeds, we are trying a new approach to explore this question. We are combining rainfall data from a NASA satellite called TRMM (the Tropical Rainfall Monitoring Mission) with data from an atmospheric back trajectory model that calculates where the air has come from. For every time it has rained over the tropical continents for the last 10 years we are running our back trajectory model to calculate where the air came from. We are looking at whether it does in fact rain more when air has come from over forested regions. It is the first time that anyone has looked at the problem from a global perspective. Our initial results seem to confirm the theory but we need to do more work to be sure that this is the case. Check back in a few months. We are hoping that this might finally give some hard evidence to back up this 100 year old theory and provide another proven reason why it’s good to protect our forests.
Dr Dominick Spracklen works in the Institute for Climate and Atmospheric Science in the School of Earth and Environment at the University of Leeds, and leads the screening of the NGO projects. He is an Advanced Research Fellow, funded by Natural Environment Research Council (NERC).