A recent article in New Scientist by Fred Pearce entitled “Rainforests may pump winds worldwide” describes a new meteorological theory wherein vast forests play a critical role in generating winds that pump water around the world through the atmosphere. Here is how Pearce summarizes this theory:
“How can forests create wind? Water vapour from coastal forests and oceans quickly condenses to form droplets and clouds… the gas [from this evaporation] takes up less space as it turns to liquid, lowering local air pressure. Because evaporation is stronger over the forest than over the ocean, the pressure is lower over coastal forests, which suck in moist air from the ocean. This generates wind that drives moisture further inland. The process repeats itself as the moisture is recycled in stages, moving towards the continent’s heart. As a result, giant winds transport moisture thousands of kilometres into the interior of a continent.
The volumes of water involved in this process can be huge. More moisture typically evaporates from rainforests than from the ocean. The Amazon rainforest, for example, releases 20 trillion litres [20 cubic kilometers] of moisture every day.
‘In conventional meteorology the only driver of atmospheric motion is the differential heating of the atmosphere. That is, warm air rises,” Makarieva and Gorshkov told New Scientist. But, they say, “Nobody has looked at the pressure drop caused by water vapour turning to water.’”
This theory, which is somewhat out of the mainstream, is nonetheless compatible with well known but less ambitious connections between forests and precipitations. In our November 9th, 2006 post entitled “Reforesting Brings Rain,” we reference an MIT study “Deforestation, Desertification and Drought,” wherein the authors conclude “deforestation along the southern coast of West Africa (e.g., in Nigeria, Ghana and Ivory Coast) may result in complete collapse of monsoon circulation, and a significant reduction of regional rainfall.”
|The role forests play in alleviating droughts and moderating weather
may have more to do with their ability to store and transpirate
massive volumes of water than their role as “carbon sinks.”
In our post of November 30th, 2007 entitled “Hydraulic Redistribution” we reference a UC Berkeley study “Deep-rooted plants have much greater impact on climate than experts thought,” which contends that rainforest trees, through hydraulic lift (energy provided by evaporation of water out of the leaves) “transpirate” sufficient volumes of water into the atmosphere to increase and moderate precipitation, which impacts the climate globally. Transpiration from rainforests add moisture to clouds blowing in from the ocean, giving them critical mass to release evaporation from the ocean as rainfall, adding to the reserves of land based fresh water and reducing incidence of droughts.
It is important to understand that climate change, such as it is, refers to three distinct and only somewhat overlapping phenomenon; global warming, extreme weather, and droughts. And in all three cases, particularly in the case of droughts, these studies are all suggesting the cause of undesirable climate change, and the cure, may have more to do with the health and extent of our global forest canopy, and less to do with anthropogenic CO2 emissions.