Think of the smell of your favorite trees and forests, from the sweet smell of pine forests to the heavy fragrances of tropical jungles. New research tells us that these wonderful fragrances may play an important role in increasing cloud cover and reduce the impacts of climate change.
We have known for a long time that there tends to be more cloud cover over forests, and more rainfall. When I worked in Borneo, it was easy to see that the rapid clearing of forests along the coast was accompanied by reduced rainfall, less cloudy weather, and a dramatic increase in temperatures. Although we have known that there is an association between clouds and forests, a new research method has shown us the relationships between the smell of a forest and the creation of clouds.
The formation of clouds depends on the presence of tiny particles in the atmosphere, known as aerosols. Water vapor condenses on the surface of aerosols and the aggregation of aerosols and water creates clouds. Clouds are obviously a source of precipitation, but they also block the sun’s rays, cooling the earth’s surface. For a long time, scientists that that sulfate aerosols, a form of air pollution, were important in cloud formation. Sulfate aerosols are produced by industrial activity and by volcanoes. Many atmospheric scientists thought that as we reduce air pollution by controlling sulfur emissions, the earth might become less cloudy. If this happened, the current increase in earth’s surface temperature would be even worse.
Forests are also a source of aerosols. That fragrance we love so much consists of organic molecules released by trees into the atmosphere, forming aerosols. Until now, these naturally occurring aerosols were not thought to be an important source of cloud-forming particles.
In three papers published in Nature and Science, a group of atmospheric scientists has shown that trees are a very important source of cloud-forming aerosols, more important than sulfate aerosols. Using a sophisticated chamber that mimics the atmosphere in the laboratory, and a matching instrument that makes measurements in the real atmosphere, the international group of scientists were able to tease apart the chemistry and physics of cloud formation.
Trees release organic chemicals such as isoprene or pinene into the air in daytime. As these chemicals mix with the air and rise into the atmosphere, they are oxidized by contact with ozone molecules that occur naturally in the upper atmosphere. These particles are still too small to create clouds – water vapor needs larger particles on which to condense. The researchers found that it was the combination of organic compounds from trees, ozone and bombardment by cosmic rays that caused larger organic particles to form.
This is very important research on many levels. Most important, it tells us that organic molecules from trees may maintain good cloud cover over the earth even if we reduce the sulfur pollutants from industry. The work, if it is borne out by further studies, also shows that the earth’s climate may be partly protected by clouds from the worst effects of climate change. Of course, this does not mean we should stop trying to reduce our greenhouse gas emissions, but merely that we may have a bit more time to limit our emissions.
The work is further confirmation of the enormous importance of trees and forests in protecting the earth. Trees absorb large amounts of carbon dioxide from the air and store it both in the trees and soils. Trees reduce the urban heat island effect, protect water resources from heat, and produce useful carbon-based products. Now we can add a new dimension to the importance of trees – they make clouds.