This is part 2 of a series on functions that not many people know trees can perform, and how those functions can be utilized in greening the desert. Part 1 was on hydraulic redistribution, in which trees act like pumps moving water from wet to dry areas, from high in the soil profile to down in the water table, and vice versa. Part 2 is on the chemical role that trees play in cloud formation and precipitation.
First, I have to acknowledge that much of the science in this post i have found thanks to Kevin Franck’s site where he posts relevant articles and resources . The other main source for much of this is Bill Mollison’s Permaculture Designer’s Manual, which I have linked to in the resources part of the website.
Trees in large numbers (ie forests!) play a giant role in creating rainfall. You probably already know that trees take in carbon from the air and release oxygen into the atmosphere. This is called evapotranspiration. But oxygen is not the only thing trees are exhaling– they also release volatile organic compounds (VOC’s) such as isoprenes, terpenes, and monoprenes. In 2004 it was discovered that these compounds break down into hygroscopic aerosols. From Rice University’s Office of Earth Science, we learn why that is such an important discovery vis-a-vis the relationship between trees and rainfall.
“Aerosols are very important in the formation of clouds. Often aerosols act as Cloud Condensation Nuclei (CCN’s), around which cloud droplets are formed. Without aerosols in the air, there would be far fewer clouds. Aerosols can also affect the properties of existing clouds. Recent studies have found that in the presence of high amounts of aerosols, clouds will have more droplets than normal, with droplets tending to be smaller than usual. Because the droplets are smaller and more numerous, the clouds last longer and are reflect sunlight better than before. This effect could have significant implications on the climate. As the clouds reflect more sunlight, less of the sun’s energy reaches the surface of the earth, which then cools. This means that by putting more aerosols in the atmosphere, humans have the potential to alter the world’s climate and cause global cooling.”
In other words, forests emit compounds that provide significant amounts of cloud condensing nuclei. In a study from Berkeley (links to pdf), they found that the 36% of all aerosols in the atmosphere came from tree-emitted terpenes and isoprenes.
These are just the microscopic particles emitted by trees. Trees also give off larger particles from debris that also make up a large amount of ice nuclei, as I mentioned in the post about Kenya’s tea region and hail storms. Remember the 3 main conditions driving the cycle of desertification and preventing more rain from falling in the Arabian Peninsula? One of them was the lack of particles that make up cloud-condensing and ice nuclei in the atmosphere.
It seems that almost everything that trees push up into the atmosphere plays a major role in modifying precipitation–and most of it in the area of increasing cloud formation and rainfall. And that brings us to the real punch in the gut:
According to Bill Mollison, Forests are the sources of up to 60% of all clouds and 40% of all rainfall–and from above we know that part of that is due to the VOC’s and dander emitted by forests and breaking down into aerosols that become nuclei for clouds, water drops, and ice.
This should make one thing very clear: To green the Arabian Peninsula, we need to plant forests. If only for their role in creating clouds and increasing precipitation, planting forests would be reason enough. However in the next post we will see how afforestation won’t just increase cloud cover and rain–it will ameliorate every major element that is currently preventing greater precipitation.