Climate Fact: Desert Expansion and Vegetation
Vegetation “feedbacks,” changes in vegetation as the climate changes, are an important component of Earth’s climate system. At high latitudes, warming tends to allow trees and shrubs to grow faster and expand into previously inhospitable locations. Because trees and shrubs are generally darker than the grasses they overshadow, they absorb more of the sun’s energy and cause the local environment to warm even more – a scenario that illustrates a “positive” vegetative feedback. Positive vegetative feedback was likely an important component of the deglaciations that occurred at the end of past ice ages. Another example of a positive vegetative feedback occurs around Earth’s arid subtropical regions. When subtropical forests dry, trees become scarcer, grass cover expands and forests turn into park-like savannahs. Savannahs absorb about ten percent less solar energy than forests. When less energy is absorbed, there is less convection and less upward motion of air masses. Less upward motion means that there is less moisture convergence at high altitudes and thus less rainfall and more drying, which can lead to the conversion of savannah to desert. Deserts absorb even less energy than savannahs do, further exacerbating this cycle. Attempts to model Earth’s climate without including positive vegetative feedback underestimate the 1950 to 2005 increase in desert cover, which is estimated to be around one million square miles (a ten percent increase over the 55-year period).
Seasons: Winter, Spring, Summer, Fall
Source: Zeng, N and Yoon, J. “Expansion of the world’s deserts due to vegetation-albedo feedback under global warming.” Geophysical Research Letters 36 (2009): L17401.)
