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.