... newly tree-free ground “represents huge amounts of solar energy changed into sensible heat, i.e. hot air.”
Scientific American
Clearing Forests May Transform Local—and Global—Climate
By Judith D. Schwartz*
March 4, 2013
Researchers are finding that massive deforestation may have a profound, and possibly catastrophic, impact on local weather
In the last 15 years 200,000 hectares of the Mau Forest in western Kenya have been converted to agricultural land. Previously called a “water tower” because it supplied water to the Rift Valley and Lake Victoria, the forest region has dried up; in 2009 the rainy season—from August to November—saw no rain, and since then precipitation has been modest. Whereas hydropower used to provide the bulk of Kenya’s power ongoing droughts have led investors to pull out of hydro projects; power rationing and epic blackouts are common. In a desperate move to halt environmental disaster by reducing population pressure, the Kenyan government evicted tens of thousands of people from the land.
Severe drought, temperature extremes, formerly productive land gone barren: this is climate change. Yet, says botanist Jan Pokorny of Charles University in Prague, these snippets from Kenya are not about greenhouse gases, but rather the way that land-use changes—specifically deforestation—affect climate; newly tree-free ground “represents huge amounts of solar energy changed into sensible heat, i.e. hot air.” Pokorny, who uses satellite technology to measure changes in land-surface and temperatures, has done research in western Kenya for 25 years, and watched the area grow hotter and drier. The change from forest cover to bare ground leads to more heat and drought, he says. More than half the country used to be forested; it's now less than 2 percent.
Each year Earth loses 12 million to 15 million hectares of forest, according to the World Wildlife Fund, the equivalent of 36 football fields disappearing per minute. Although forests are ebbing throughout the world, in Africa forest-climate dynamics are easily grasped: according to the United Nations Environmental Programme, the continent is losing forests at twice the global rate. Says Pokorny, the conversion of forest to agricultural land, a change that took centuries in Europe, “happened during one generation in western Kenya.” Pokorny's work, coupled with a controversial new theory called the “biotic pump,” suggests that transforming landscapes from forest to field has at least as big an impact on regional climate as greenhouse gas–induced global warming.
After all, de-treeing the landscape alters the way ecosystems function and self-regulate. For Pokorny, the key is evapotranspiration, whereby plants continuously absorb and emit water in the form of vapor. Evaporation consumes heat and thus has a cooling effect. He calls this "the perfect and only air-conditioning system on the planet." On a moderately sunny day, a tree will transpire some 100 liters of water, converting 70 kilowatt-hours of solar energy into the latent heat held in water vapor. When soil is bare and dry—paved over or harvested—the process comes to a halt. The sun hits and warms the ground directly.
This past November found Pokorny flying a small Cessna from Lake Naivasha up the hills to the Mau Forest, where land surface temperatures in woodlands measured 19 degrees C; agricultural land that until recently had been forest hovered close to 50 degrees C.
A photo taken from the air shows the dark green of forests diminish along the slope to the lowlands; the valley has clusters of deep forest green among the broad, pale, geometric shapes of cultivated land. His team measures surface temperature as opposed to the usual air temperature metric, two meters above. The surface “is what you are in contact with, and creates the dynamic movement of air,” he says—and ground temperature “indicates the way solar radiation is transformed at the Earth's surface.” His surveys from above combine three measures: “a normal camera, Thermovision [thermal infrared sensors] and our eyes. In putting the pictures together, we see the high temperatures are where there is no vegetation,” which includes swaths of land where forest has been cut.
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.”
Then there is the need to better understand the specific function of forests and even individual tree species. For example, can planted trees have the same effect on hydrology as intact natural forest? “A practical question is whether we're able to mimic effects of forests,” Pokorny says. “For we can't just have forests—we need agricultural land. Can other land types, like savanna and high biomass grassland, serve some functions of trees? Are there pioneer tree species that create microclimates that help other trees grow?”
Deforestation has numerous untoward environmental consequences, including the release of carbon: about one sixth of global emissions are due to cleared or degraded forests. Such deforestation also destroys the habitat for vast variety of species, and threatens the welfare of the more than a billion people who rely on forests for their livelihoods. It may also have unforeseen impacts on the water cycle—which can in turn alter climate patterns. "If we don't pay attention to this now, we could lose our forests," Makarieva told me by phone. "This would make disaster come faster by destroying the water cycle."
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