Scientific American takes a whack at explaining the Biotic Pump

Clearing Forests May Transform Local—and Global—Climate; Judith D. Schwartz, March 4, 2013, Scientific American

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The "biotic pump" theory argues that natural forests act as a “pump” that draws moisture inland. According to this concept, first described in a 2007 paper by Russian physicists Victor Gorshkov and Anastassia Makarieva of the Saint Petersburg Nuclear Physics Institute in the peer-reviewed Hydrology and Earth System Sciences, condensation, rather than temperature differential, is a primary driver of weather.

Here's a snapshot of the concept: The concentration of trees in wooded areas means a high rate of transpiration. This moist air cools as it ascends and the water vapor condenses, producing a partial vacuum. This creates an air pressure gradient, whereby the forest canopy sucks in moist air from the ocean. According to Gorshkov and Makarieva, forests don't merely grow in wet areas, they create and perpetuate the conditions in which they grow.

Without forest cover—specifically mature, natural forest to ensure sufficient biomass and resilience—moisture is no longer pulled in, the physicists say. Rain becomes erratic and ultimately stalls.

The Russian scientists associate the unprecedented heat and drought in their country over the last few years with rapid deforestation in western Russia.

The theory is controversial; indeed, it challenges the viability of the climate models currently in use. The theory “explains why in forested regions precipitation does not decrease with distance from the ocean, even thousands of kilometers, while the interiors of deforested parts of continents become dry already a few hundred kilometers away from the oceanic coast,” they wrote in an e-mail. 

"Condensation of water vapor over forests creates pressure gradients that have been shown to be sufficient to drive winds that bring moisture from ocean to land."

Should the biotic pump be confirmed by further research, it brings new urgency to the need to protect forests. 

“Most climate models recognize the role of 'precipitation cycling' in forests, but not moisture transport by forests,” Makarieva and Gorshkov say. The difference is significant: if deforestation means simply reduced evaporation, the decline of precipitation would be significant but not catastrophic, around 15 percent; however, rains depend on imported moisture. If the vehicle for transport—an intact forest—is impaired, that's a different story.

The physicists say: “In our theory, imported moisture will decline if the forest is destroyed, especially in the inland portion of the continent. If there is no imported moisture there is nothing to be evaporated, so the water cycle will undergo a dramatic—not minor—reduction of intensity.” 

In the Amazon, they add, this could be up to 90 percent.

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Even more fun is watching Dr Makarieva explain air to non-theoretical physicists.  "You know, some people think when it rains the air goes down. This is not so." 

(8:53 minute mark, YouTube) 

I'm hoping if I keep this up long enough I'll get an email from Dr Makarieva asking, "What part don't you understand?" 

What I really want to know is whether greenhouse gases become more of a temperature determinant the more that biotic pumps fail; i.e., the more that mature coastal forests are wiped out.     

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