The conversion of forests into cropland worldwide has triggered an atmospheric change that, while seldom considered in climate models, has had a net cooling effect on global temperatures, according to a new Yale study.
Writing in the journal Nature Climate Change
, Professor Nadine Unger
of the Yale School of Forestry & Environmental Studies (F&ES) reports that large-scale forest losses during the last 150 years have reduced global emissions of biogenic volatile organic compounds (BVOCs), which control the atmospheric distribution of many short-lived climate pollutants, such as tropospheric ozone, methane, and aerosol particles.
Using sophisticated climate modeling, Unger calculated that a 30-percent decline in BVOC emissions between 1850 and 2000, largely through the conversion of forests to cropland, produced a net global cooling of about 0.1 degrees Celsius. During the same period, the global climate warmed by about 0.6 degrees Celsius, mostly due to increases in fossil fuel carbon dioxide emissions.
According to her findings, the climate impact of declining BVOC emissions is on the same magnitude as two other consequences of deforestation long known to affect global temperatures, although in opposing ways: carbon storage and the albedo effect. The lost carbon storage capacity caused by forest conversion has exacerbated global warming. Meanwhile, the disappearance of dark-colored forests has also helped offset temperature increases through the so-called albedo effect. (The albedo effect refers to the amount of radiation reflected by the surface of the planet. Light-colored fields, for instance, reflect more light and heat back into space than darker forests.)