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Sugarcane: Climate’s double-edged sword

STANFORD (US) — Depending on the comparison, farming sugarcane for biofuel is either better for the environment or worse.

Sugarcane, a principal crop for biofuel, reduces the local air temperature when compared to pasturelands or fields growing soybeans or maize, but raises it when compared with natural vegetation.

Large areas pf the Brazilian Cerrado, a vast tropical savannah lying south of the Amazon basin have been converted from natural vegetation to sugarcane for use in biofuel production, but the effect on the global climate if sugarcane farming were to expand further is not clear.

Brazil is currently second only to the United States in ethanol production, much of which is used domestically.

“The temperature changes are happening locally, where the land-use change is happening,” says David Lobell, assistant professor in environmental earth system science at Stanford University. “It does not seem to spill over into other countries, for example, at least as far as we can tell right now.”

But because sugarcane growing in the Cerrado is expanding and the region encompasses approximately 1.9 million square kilometers (733,000 square miles) – an area larger than Alaska, the potential exists for a globally significant effect.

Using maps and data from hundreds of satellite images, researchers calculated the temperature, the amount of water given off and how much light was reflected rather than absorbed for each of the different types of vegetation.

Compared to land cultivated with other annual crops, sugarcane reduced the local air temperature by an average of 0.93 degrees Celsius (1.67 F), but compared to natural vegetation of the Cerrado, mainly grass and shrubs, the sugarcane fields warmed the ambient air by 1.55 C (2.79 F).

The bulk of the temperature difference is due to evapotranspiration, the moisture released to the air through the leaves of the plants and the soil.

Most of the land put into sugarcane has been previously converted from natural vegetation to pastureland, says Scott Loarie, a postdoctoral researcher of global ecology.

“If someone has a farm that once was natural vegetation, that transition to pasture and annual crops caused local warming. So now as the farm is going to sugarcane, by comparison it is cooling temperatures locally.”

The research is published in the journal Nature Climate Change.

This local cooling does not necessarily mean that the global climate is cooling as a result. It depends in part on what happens with the agriculture that was displaced by the sugarcane, Loarie says.

For example, if cattle used to graze on a tract of land and some Amazon forest is cut down to provide new pasture for them, net carbon emissions will actually increase.

“You might not make any difference as far as cooling the world globally at all; in fact, you might make the world marginally warmer.”

“The global implications of these local effects were not a part of this study, and any discussion of mitigating global climate should consider the potential for these land use cascades.”

The study demonstrates how satellite data can be used in real time to understand the effects of environmental changes. Most research studying the impact of biofuel use on climate has been done with computer modeling.

“I think the coolest thing about this study is you actually can see these temperature effects happening already,” Lobell says. “In terms of the more general point about bio energy, I think it is another good example of why looking only at greenhouse gases is not the full picture.”

The temperature findings support the existing rule of thumb that biofuel crops are best located on land that is already used for agriculture, Lobell says, because there is less carbon released to the atmosphere by converting land where the existing vegetation contains low amounts of carbon, such as pasture or crops, than by cutting down the dense, carbon-rich forests in the Amazon.

While planting sugarcane moves the temperature closer to what it would have been if the natural vegetation had not been removed from the land, it doesn’t mean the land is any closer to its natural state in other respects, Loarie says.

“Converting pasture to sugarcane is definitely not ecological restoration,” says Chris Field, professor of biology and of environmental earth system science, who was involved in the research.

“Still, the direct effect on climate is potentially important enough to play a role in future decisions about land use and land management in large parts of the tropics.”

The study was funded by the Stanford University Global Climate and Energy Project.

More news from Stanford University: http://news.stanford.edu/

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