A
team of researchers from Arizona State Univ., Stanford Univ.,
and Carnegie Institution for Science has found that converting large
swaths of land to bioenergy crops could have a wide range of effects on
regional climate.
In
an effort to help wean itself off fossil fuels, the U.S. has mandated
significant increases in renewable fuels, with more than one-third of
the domestic corn harvest to be used for conversion to ethanol by 2018.
But concerns about effects of corn ethanol on food prices and
deforestation had led to research suggesting that ethanol be derived
from perennial crops, like the giant grasses Miscanthus and switchgrass.
Nearly all of this research, though, has focused on the effects of
ethanol on carbon dioxide emissions, which drive global warming.
“Almost
all of the work performed to date has focused on the carbon effects,”
said Matei Georgescu, a climate modeler working in ASU’s Center for
Environmental Fluid Dynamics. “We’ve tried to expand our perspective to
look at a more complete picture. What we’ve shown is that it’s not all
about greenhouse gases, and that modifying the landscape can be just as
important.”
Georgescu
and his colleagues report their findings in the early online edition of the Proceedings of the National Academy of Sciences.
Co-authors are David Lobell of Stanford Univ. and Christopher Field
of the Carnegie Institution for Science, both located in Stanford,
Calif.
In
their study, the researchers simulated an entire growing season with a regional climate model. They ran two sets of
experiments
—one with an annual crop representation over the central
U.S. and one with an extended growing season to represent perennial
grasses. In the model, the perennial plants pumped more water from the
soil to the atmosphere, leading to large local cooling.
“We’ve
shown that planting perennial bioenergy crops can lower surface
temperatures by about a degree Celsius locally, averaged over the entire
growing season. That’s a pretty big effect, enough to dominate any
effects of carbon savings on the regional climate,” said Lobell.
The
primary physical process at work is based on greater evapotranspiration
(combination of evaporated water from the soil surface and plant canopy
and transpired water from within the soil) for perennial crops compared
to annual crops.
“More
study is needed to understand the long-term implication for regional
water balance,” Georgescu said. “This study focused on temperature, but
the more general point is that simply assessing the impacts on carbon
and greenhouse gases overlooks important features that we cannot ignore
if we want a bioenergy path that is sustainable over the long haul.”
