Deforestation is messing with our weather and our food

Today, the National Socio-Environmental Synthesis Center (SESYNC) at the University of Maryland published new research in Nature Communications providing insight into how large-scale deforestation could impact global food production by triggering changes in local climate. In the study, researchers zero in on albedo (the amount of the sun's radiation reflected from Earth's surface) and evapotranspiration (the transport of water into the atmosphere from soil, vegetation, and other surfaces) as the primary drivers of change in local temperature.

"Understanding the precise mechanisms of forest-generated warming or cooling could help regional management agencies anticipate changes in crop yields," said Safa Motesharrei, co-author of the paper and a systems scientist SESYNC. "Together, with the knowledge of other ecological factors, this information can help decision makers and stakeholders design policies that help to sustain local agricultural practices."

Agriculture—specifically, converting forest cover to plantations for oil palm, soy, rubber, coffee, tea, rice, and many other crops—is widely believed to be one of the main causes of deforestation. Such change in land cover could drive a rise or fall in local temperature by as much as a few degrees. This kind of fluctuation could substantially impact yields of crops that are highly susceptible to specific climate conditions, resulting in harvests that are less productive and less profitable.

The path to understanding these local impacts, researchers say, is through albedo and evapotranspiration, which cause warming, and cooling, respectively. Forests have a darker surface than, for example, an agricultural field—forests therefore have a lower albedo, which means less solar radiation is reflected and more is absorbed. On the other hand, forests absorb more rainwater and transpire it as water vapor later.

"These two competing biophysical effects could determine whether—at a specific location or during a specific time of the day or season of the year—a forest could cause local cooling or warming," said Yan Li, lead author of the study and visiting climate scientist at the University of Maryland. "And, by extension, whether clearing a forest could lead to a rise or fall in local temperature."

For example, the researchers found that tropical forests, which occur closest to the equator, have a strong cooling effect year-round. Boreal forests, which occur furthest from the equator, and temperate forests, which occur between tropical and boreal forests, show a seasonal variation. Boreal forests have strong warming in winter and moderate cooling in summer with net warming annually, and temperate forests show moderate cooling in summer and moderate warming in winter with net cooling annually. The scientists say this difference in cooling or warming can be largely explained by whether albedo or evapotranspiration is the dominant effect.

The study addresses questions that have been previously impossible to answer without this global satellite data. Earlier research has studied the effects of forest cover on temperature using field observations or global climate models. Because field work can be expensive, time-intensive, and logistically difficult, field measurements are generally available for only limited areas. These data are therefore difficult to scale up to develop a global picture. And because climate models require immense computational resources to run, they're often unable to provide focused local information with reliable precision.

The satellite data used in the study—collected by NASA's Moderate Resolution Imaging Spectroradiometer, or MODIS—provides the best of both worlds: information that is rich in detail and global in coverage. As a result, the researchers could effectively zoom in and back out again to analyze the same phenomena everywhere around the world.