Fewer biofuels, more green space
Growing and harvesting bioenergy
crops—corn for ethanol or trees to fuel power plants, for example—is a poor use
of land, which is a precious resource in the fight against climate change, says
a University of Michigan researcher.
Untampered green areas like forests
and grasslands naturally sequester carbon dioxide, and they are one of
society’s best hopes for quickly reducing the greenhouse gas in the atmosphere,
says John DeCicco, research professor at the U-M Energy Institute.
DeCicco and William Schlesinger, president emeritus of the Cary Institute of Ecosystem Studies have authored an opinion piece in the current edition of Proceedings of the National Academy of Sciences.
The researchers call for
policymakers, funding agencies, fellow academics and industry leaders to
urgently shift their focus from bioenergy to what they call “terrestrial carbon
management,” or TCM. That strategy emphasizes planting more trees and
conserving more wild areas that feed on carbon dioxide.
“The world needs to rethink its priorities
about how to use the biosphere given the urgency of the climate problem and the
risks to biodiversity,” DeCicco said.
The biosphere encompasses all life
on Earth, and for climate protection, it particularly refers to trees, plants
and the living carbon—microorganisms—in soils.
“Current policies advancing
bioenergy contribute to the pressure to convert natural land into harvested
forest or cropland,” DeCicco said. “But high quality land is a limited
resource. For reducing atmospheric CO2, the most efficient use of ecologically
productive land is to leave it alone, or reforest it. Let it act as a natural,
long-term carbon sink.”
The new opinion piece expands on
DeCicco’s earlier findings that biofuels are not inherently carbon-neutral, as
they are widely purported to be, and Schlesinger’s long-time research as a
leading ecologist and biogeochemist.
The assumption that bioenergy simply
recycles carbon—which DeCicco and Schlesinger call a major accounting error—is
built into the lifecycle assessments used for energy policy as well as the
protocols for international carbon accounting. And it has fostered major
R&D investments in biofuels, which, in turn, have been assigned a key role
in many climate stabilization scenarios.
The core of that assumption is the
idea that producing a biofuel and then burning it for energy moves a given
amount of carbon from the biosphere to the atmosphere, and back again in an
unending and stable cycle. That’s in contrast to the current one-way flow of
fossil-fuel carbon from the Earth to the atmosphere.
But here’s where DeCicco sees a
problem: For bioenergy to be actually carbon neutral, harvesting the biomass to
produce it would have to greatly speed up the net flow of carbon from the
atmosphere back into vegetation. Otherwise, many decades can pass before the
“carbon debt” of excess carbon dioxide in the air is repaid by future plant
growth.
“All currently commercial forms of
bioenergy require land and risk carbon debts that last decades into the future.
Given the urgency of the climate problem, it is puzzling why some parties find
these excess near-term CO2 emissions acceptable,” the researchers write.
In 2016, DeCicco published a study
finding that just 37 percent—rather than 100 percent—of the carbon dioxide
released from burning biofuels was balanced out by increased carbon uptake in
crops over the first eight years of the U.S. biofuel mandate.
To reduce the concentration of
carbon dioxide in the atmosphere, DeCicco and Schlesinger point out, requires
increasing the rate at which trees and other plants remove it from the air.
Although they don’t rule out possible breakthroughs in algae or other futuristic
bioenergy options, they say that for now the best biologically based carbon
dioxide reduction strategy is to protect and restore carbon-rich natural
ecosystems.
“By avoiding deforestation and by
reforesting harvested areas, up to one-third of current carbon dioxide
emissions from fossil fuels could be sequestered in the biosphere,” the
researchers write. “Terrestrial carbon management can keep carbon out of the
atmosphere for many decades.”