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; rains depend on imported moisture, however. 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.
Such ideas are not new. In his 1864 book Man and Nature (original title: Man the Disturber of Nature's Harmonies), George Perkins Marsh catalogued numerous examples of changing climate conditions on losing forests and wrote, “When the forest is gone, the great reservoir of moisture stored up in its vegetable mold [humus] is evaporated, and returns only in deluges of rain to wash away the parched dust into which that mold has been converted. The well-wooded and humid hills are turned to ridges of dry rock[.]”
Back in Kenya Sarah Higgins, a conservationist who runs the Little Owl Sanctuary for injured birds near Lake Naivasha, says she's seen weather patterns change with the forest's fortunes. When she started farming 30 years ago “we were almost guaranteed sufficient rainfall for our crops.” Then came the destruction of the Mau Forest, and the area above and on either side of the farm was “denuded of trees and overgrazed, down to bare Earth. Our regular rainfall started to fail and we were seeing dry years, poor yields and more droughts.”