Brazilians
are world leaders in using biofuels for gasoline. About a quarter of
their automobile fuel consumption comes from sugarcane, which
significantly reduces carbon dioxide emissions that otherwise would be
emitted from using gasoline. Now scientists from the Carnegie
Institution’s Department of Global Ecology have found that sugarcane has
a double benefit. Expansion of the crop in areas previously occupied by
other Brazilian crops cools the local climate. It does so by reflecting
sunlight back into space and by lowering the temperature of the
surrounding air as the plants “exhale” cooler water. The study is
published in the 2nd issue of Nature Climate Change, posted on-line
April 17.
The
research team,* led by Carnegie’s Scott Loarie, is the first to
quantify the direct effects on the climate from sugarcane expansion in
areas of existing crop and pastureland of the cerrado, in central
Brazil.
The
researchers used data from hundreds of satellite images over 733,000
square miles—an area larger than the state of Alaska. They measured
temperature, reflectivity (also called albedo), and
evapotranspiration—the water loss from the soil and from plants as they
exhale water vapor.
As
Loarie explained: “We found that shifting from natural vegetation to
crops or pasture results in local warming because the plants give off
less beneficial water. But the bamboo-like sugarcane is more reflective
and gives off more water—much like the natural vegetation. It’s a
potential win-win for the climate—using sugarcane to power vehicles
reduces carbon emissions, while growing it lowers the local air
temperature.”
The
scientists found that converting from natural vegetation to
crop/pasture on average warmed the cerrado by 2.79 °F (1.55 °C), but
that subsequent conversion to sugarcane, on average, cooled the
surrounding air by 1.67 °F (0.93°C).
The
researchers emphasize that the beneficial effects are contingent on the
fact sugarcane is grown on areas previously occupied by crops or
pastureland, and not in areas converted from natural vegetation. It is
also important that other crops and pastureland do not move to natural
vegetation areas, which would contribute to deforestation.
So
far most of the thinking about ecosystem effects on climate considers
only impacts from greenhouse gas emissions. But according to coauthor
Greg Asner, “It’s becoming increasingly clear that direct climate
effects on local climate from land-use decisions constitute significant
impacts that need to be considered core elements of human-caused climate
change.”
*Co-authors
on the study are David Lobell of the Program for Food Security and the
Environment at Stanford University, Gregory Asner and Christopher Field
of Carnegie’s Department of Global Ecology, and Qiaozhen Mu of the
University of Montana. The work was made possible through the support of
the Stanford University Global Climate and Energy Project.