Converting Forests to Cropland Actually Cools Our Climate

There's a lot of talk about what human impacts are doing to warm the climate, but very little about what might cool the climate. Now, scientists have found that, surprisingly, the conversion of forests into cropland worldwide can cause a net cooling effect on global temperatures.

Over just the last 150 years, there have large-scale forest losses. This, in turn has 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.

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

In order to get a better sense of how disappearing forests are impacting climate, the researchers used sophisticated computer modeling to calculate BVOC declines. They found that there has been a 30-percent decline between 1850 and 2000, largely through the conversion of forests to cropland. This, in turn, has produced a net global cooling of about .1 degrees Celsius. That said, the globe also warmed by about .6 degrees due to fossil fuel carbon dioxide emissions.

Since the mid-19^th century, the percentage of the planet covered by cropland has more than doubled from 14 percent to 37 percent. Because forests are far greater contributors of BVOC emissions than crops and grasslands, this has removed about 30 percent of Earth's BVOC resources.

That's not to say that increased forest loss provides climate change benefits. Instead, it underscores the complexity of climate change, and shows that it's important to assess which parts of the globe would benefit from greater forest conservation.

The findings are published in the journal Nature Climate Change.