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 study from Yale University.

Writing in the journal Nature Climate Change, professor Nadine Unger of the Yale School of Forestry & Environmental Studies reported 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% 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 C. During the same period, the global climate warmed by about 0.6 degrees C, mostly due to increases in fossil fuel-related carbon dioxide emissions.

According to her findings, the climate impact of declining BVOC emissions is of 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 carbon storage capacity lost due to 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 announcement said.

(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, the announcement explained.)

Unger said 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.

"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, an assistant professor of atmospheric chemistry. "Croplands are not strong emitters of these BVOCs; often, they don't emit any BVOCs.

"Without doing an Earth-system model simulation that includes these factors, we can't really know the net effect on the global climate, because changes in these emissions affect both warming and cooling pollutants," she noted.

Unger said the findings do not suggest that increased forest loss provides climate change benefits but, rather, underscores the complexity of climate change and the importance of better assessing which parts of the world would benefit from greater forest conservation.

Since the mid-19th century, the percentage of the planet covered by cropland has more than doubled, from 14% to 37%. Since forests are far greater contributors of BVOC emissions than crops and grasslands, this shift in land use has removed about 30% of the Earth's BVOC sources, Unger said.

Reductions in the potent greenhouse gases methane and ozone — which contribute to global warming — have helped deliver a net cooling effect. These emissions are often ignored in climate modeling because they are perceived as a "natural" part of the earth system, Unger explained.

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

These impacts have also been ignored in previous climate modeling, she said, because scientists believed that BVOC emissions had barely changed between the pre-industrial era and today. However, a study Unger published last year found that emissions of these volatile compounds have indeed decreased. Studies by scientists in Europe have produced similar results.

The effects of ozone and organic aerosols are particularly strong in temperate zones, she said, while methane's effects are more globally distributed.

The sensitivity of the global climate system to BVOC emissions suggests the importance of establishing a global-scale, long-term monitoring program for BVOC emissions, Unger noted.

Global deforestation

On a related topic, a new study from Purdue University found that productivity-boosting agricultural innovations in Africa could actually lead to an increase in global deforestation rates and carbon emissions.

Historically, improvements in agricultural technology have conserved land and decreased carbon emissions at the global level: Gaining better yields in one area lessens the need to clear other areas for crops, sidestepping a land conversion process that can significantly raise the amount of carbon dioxide released into the atmosphere, Purdue said.

Agricultural advances in Africa, however, could have the reverse effect, increasing globally the amount of undeveloped land converted to cropland and raising greenhouse gas emissions, said Thomas Hertel, a distinguished professor of agricultural economics at Purdue.

"Increasing productivity in Africa — a carbon-rich region with low agricultural yields — could have negative effects on the environment, especially if agricultural markets are highly integrated," he said. "This study highlights the importance of understanding the interplay between globalization and the environmental impacts of agricultural technology. They are deeply intertwined."

There is much debate surrounding the effects of agricultural innovation on the environment, Hertel noted. Some researchers suggest that increasing the profitability of farming will amplify its negative environmental effects, raising greenhouse gas emissions and accelerating tropical deforestation. Others argue that intensifying agricultural production is better for the environment overall because more land can be spared for nature if the same amount of crops can be produced using less land.

"We set out to determine who was right," Hertel said. "We discovered that both hypotheses can be valid. It depends on the local circumstances."

Hertel and fellow researchers Navin Ramankutty and Uris Baldos developed a novel economic framework to analyze the effects of regional improvements in agricultural technology on global rates of land use and carbon emissions.

Their analysis showed that historical "green revolutions" in regions such as Latin America and Asia — in which better varieties of cereal grains produced dramatic gains in harvests — helped spare land and diminish carbon emissions compared with an alternative scenario without crop innovations.

The global effects of a green revolution in Africa, however, are less certain, Hertel said.

"If the future global economy remains as fragmented as it has been historically — a world of very distinct agricultural markets — then a green revolution in Africa will lower global carbon emissions," he said. However, "if markets become more integrated, faster agricultural innovation in Africa could raise global carbon emissions in the coming decades."

In an integrated world market scenario, the researchers' analysis showed that ramping up agricultural productivity in Africa between 2025 and 2050 could increase global cropland expansion by 1.8 million hectares and global carbon emissions by 267 million metric tons.

The sharp differences between the global effects of a prospective African green revolution and those of previous green revolutions can be traced to several factors, Hertel said.

The relatively lower yields of African croplands would require more area to be converted to agriculture to make up for the displaced crop production in the rest of the world. The area converted would likely be carbon intensive and have a low emissions efficiency — that is, crop yields would be low relative to the carbon emissions released by converting the land to crops.

However, the potential negative effects of an African green revolution will diminish over time, Hertel said. If sustained over several decades, agricultural innovation in Africa would eventually conserve land and decrease carbon emissions, especially if yields improved quickly. The most carbon-rich land, however, should be immediately protected from conversion to cropland, he said.

"We need to prevent regions in Africa that are rich in carbon and biodiversity from being cleared for agriculture to avoid increasing emissions," he said. "Boosting yields brings many benefits, but increasing global food supplies while minimizing the environmental footprint of agriculture remains a major challenge."

The paper was published Sept. 8 in Proceedings of the National Academy of Science.

Volume:86 Issue:38