An intriguing study from Yale University shows that large-scale forest losses over the last 150 years – largely through the conversion of forests to cropland – produced a net global cooling effect 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.
Detailing her work in the journal Nature Climate Change, Yale’s Nadine Unger calculated that a 30-percent decline in biogenic volatile organic compound (BVOC) emissions between 1850 and 2000 was responsible for the cooling effect.
“Land cover changes caused by humans since the industrial and agricultural revolutions have removed natural forests and grasslands and replaced them with croplands,” explained Unger. “And croplands are not strong emitters of these BVOCs – often they don’t emit any BVOCs.”
BVOCs emitted from forested areas are players in both the formation and distribution of other climate pollutants, such as ozone, methane, and aerosol particles. Not all of these compounds affect atmospheric chemistry in the same way. Aerosols, for example, contribute to global cooling since they generally reflect solar radiation back into space. Therefore, a 50 percent reduction in forest aerosols has actually spurred greater warming since the pre-industrial era. However, reductions in the potent greenhouse gases methane and ozone have helped deliver a net cooling effect.
Unger says the climate impact of declining BVOC emissions is on the same magnitude as two other consequences of deforestation: 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 albedo effect (the amount of light and heat reflected by the surface of the planet).
She believes the combined effects of reduced BVOC emissions and increased albedo may have entirely offset the warming caused by the loss of forest-based carbon storage capacity.
She is quick to stress that the findings do not suggest that increased forest loss provides climate change benefits, but rather underscore the complexity of climate change and the multitude of factors involved. “These emissions are often ignored in climate modeling because they are perceived as a natural part of the Earth system,” explained Unger. “So they don’t get as much attention as human-generated emissions, such as fossil fuels. But if we change how much forest cover exists, then there is a human influence on these emissions. The sensitivity of the global climate system to BVOC emissions suggests the importance of establishing a global-scale long-term monitoring program.”
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