Adaptation – Part III
By Ranil Senanayake –December 14, 2016
Dr Ranil Senanayake
Preparing for the future by looking back.
Understanding the issues and options before us.
Understanding the issues and options before us.
It has been calculated that a one-degree increase in global temperature would eliminate fresh water from a third of the world’s land surface by 2100, mostly in the tropics. This is supposed to happen incrementally, as the number of record hot days increase so does the water evaporated from a hot land surface. Satellite imagery has long demonstrated that the heat signature from open or exposed lands is much higher than tree covered lands. Keeping the land cool, is another confirmation of the value of our mountain forests.
Recent studies demonstrate a temperature difference between cleared and non-cleared regions with similar incident solar radiation to be as high as 15 degrees centigrade. As the CO2 concentrations are the same in both regions the cooling effect is directly associated with evapotranspiration from leaves, changed water and cloud dynamics.
The cooling effect of 1 large tree is about the same obtained by running 10 room sized AC units during daytime, which is equivalent to 1,200,000 BTU day or 94,000,000,000 BTU/day per hectare of forest. As an example of this type of loss, when we consider that, between 1990 and 2000 Sri Lanka lost an average of 26,800 ha of forests per year, it translates to a loss of daily cooling by factor of 5,267,000,000,000,000 BTU as a consequence.
If designed well, this cooling effect can be harnessed in the design of agricultural lands, especially for crops sensitive to high temperature stress. The traditional pattern of rice fields that follow a contour bordered by tall trees is a design feature that might lend to modern application. Tree boundaries have a cooling effect and help to maintain a lower ambient temperature over the agricultural field.
The other advantage of developing forested hillsides is their role in cloud creation. Clouds are formed by water vapour condensing around some microscopic nuclei, the greatest contribution to Cloud Condensation Nuclei (CCN) coming from aerosols of biological origin. The first understanding of this phenomenon came from the study of Di-Methyl Sulfide (DMS). DMS comes in huge quantities from the oceans of the world and gives oceans their characteristics smell. It is produced by a variety of marine organisms from phytoplankton to coral reefs. DMS influences the global climate by producing the particles that promote cloud formation, and increases cloud albedo reflecting much of solar radiation back into space and thereby controlling the rate of planetary heating.
The cloud condensing nuclei in terrestrial ecosystems are produced by the forests. Forests represent some 48% of all terrestrial evapotranspiration, where groundwater cleaned by a tree is released into the atmosphere. This massive amount of water release is accompanied by huge quantities of plant aerosols and bacteria such as Pseudomonas and Areogenes. These bacteria live on the leaves of plants and their stomatal cavities and are convected into the atmosphere with the water vapour and aerosols released by these leaves. Once in the atmosphere they to act as cloud condensation nuclei (CCN) around which water condenses as droplets which go on to form clouds. Recent studies estimate that over one billion tonnes of such organic nuclei are released into the upper atmosphere annually. The effect is easily seen by the increase of cloud cover, cloud albedo and rainfall over forested regions. For tropical regions, the effect is significant, the forested areas being up to 15 degrees cooler than in cleared areas.
