Environmental activists often bemoan cutting down forests for development. They would be shocked to find out that cutting down forests to make way for croplands has cooled the planet over the past century.

A study by Professor Nadine Unger with the Yale School of Forestry & Environmental Studies found that converting forests to croplands has had a net cooling effect of 0.1 degree Celsius since the 1850s — which was outpaced by 0.6 degrees Celsius of warming from fossil fuel emissions.

Forests contain large amounts of biogenic volatile organic compounds (BVOCs), organic chemicals that impact things like tropospheric ozone, methane, and aerosols.

The Earth's surface dedicated to cropland has roughly doubled since the 1850s, which has contributed to a 30 percent reduction in BVOCs since that time. Unger says this has helped stem some global warming.

While deforestation does release carbon dioxide into the atmosphere, causing a warming effect, the reduction in forests has increased what is called the albedo effect — referring to radiation that reflected by light-colored fields. Forests are dark, while croplands are lighter and reflect more radiation into space, producing a net cooling effect.

The combination of reduced BVOCs and an increased albedo effect have led to net global cooling from deforestation over the last 150 years.

"Land cover changes caused by humans since the industrial and agricultural revolutions have removed natural forests and grasslands and replaced them with croplands," said Unger. "And croplands are not strong emitters of these BVOCs — often they don't emit any BVOCs."

But Unger is careful to note in her study that deforestation has helped spur some warming due to a reduction in aerosols (which have a cooling effect). Unger says a 50 percent reduction in aerosols has spurred warming since the 1850s.

"So they don't get as much attention as human-generated emissions, such as fossil fuel VOCs," Unger said. "But if we change how much forest cover exists, then there is a human influence on these emissions."

Most of the debate surrounding global warming has revolved around man-made greenhouse gas emissions from power plants and transportation, like trucks and airplanes. But an increasing amount of focus has been put on natural sources of greenhouse gases and aerosols, like soil and plant life.

A study from Northern Arizona University from earlier this year found that soil microbes were causing the world to warm faster. This is because increasing carbon dioxide levels were causing microbes in the soil to release more of their own carbon dioxide, spurring more warming.

But a more recent study from the Carnegie Institution found that higher temperatures and carbon dioxide levels have little impact in the long-term on the rate that tropical forest soil releases carbon dioxide.

"If these findings hold true in other tropical regions, then warmer temperatures may not necessarily cause tropical soils to release their carbon to the atmosphere at a faster rate," said Carnegie scientist Greg Asner. "On the other hand, we cannot expect that the soil will soak up more carbon in places where vegetation is stimulated by warmer temperatures. Unlike tropical trees, the soil seems to be on the sidelines in the climate adaptation game."

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