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Today,
farming often involves transporting crops long distances so consumers
from Maine to California can enjoy Midwest corn, Northwest cherries and
other produce when they are out of season locally. But it isn’t just the
fossil fuel needed to move food that contributes to agriculture’s
carbon footprint.
New
research published in the journal Biogeosciences provides a detailed
account of how carbon naturally flows into and out of crops themselves
as they grow, are harvested and are then eaten far from where they’re
grown. The paper shows how regions that depend on others to grow their
food end up releasing the carbon that comes with those crops into the
atmosphere.
“Until
recently, climate models have assumed that the carbon taken up by crops
is put back into nature at the same place crops are grown,” said the
paper’s lead author, environmental scientist Tristram West of the
Department of Energy’s Pacific Northwest National Laboratory. “Our
research provides a more accurate account of carbon in crops by
considering the mobile nature of today’s agriculture.”
West
works out of the Joint Global Change Research Institute, a partnership
between PNNL and the University of Maryland. His co-authors are
researchers at PNNL, Oak Ridge National Laboratory and Colorado State
University.
Carbon, carbon everywhere
Carbon
is the basis of life on Earth, including plants. During photosynthesis,
plants take in carbon dioxide and convert it into carbon-based sugars
needed to grow and live. When a plant dies, it decomposes and releases
carbon dioxide back into the atmosphere. After eating plants, animals
and humans release the plants’ carbon as either carbon dioxide while
breathing or as methane during digestion.
But
the geography of this natural carbon cycle has shifted with the rise of
commercial agriculture. Crops are harvested and shipped far away from
where they’re grown, instead of being consumed nearby. As a result,
agriculturally active regions take in large amounts of carbon as crops
grow. And regions with larger populations that consume those crops
release the carbon.
The
result is nearly net zero for carbon, with about the same amount of
carbon being taken in as is released at the end. But the difference is
where the carbon ends up. That geography matters for those who track
every bit of carbon on Earth in an effort to estimate the potential
impacts of greenhouse gases.
Digging into data
Agricultural
carbon is currently tracked through two means: Towers placed in farm
fields that are equipped with carbon dioxide sensors, and computer
models that crunch data to generate estimates of carbon movement between
land and the atmosphere. But neither method accounts for crops
releasing carbon in areas other than where they were grown.
To
more accurately reflect the carbon reality of today’s agricultural
crops, West and his co-authors combed through extensive data collected
by various government agencies such as the Department of Agriculture,
the U.S. Census Bureau and the Environmental Protection Agency.
Looking
at 17 crops—including corn, soybeans, wheat and cotton—that make up 99%
of total U.S. crop production, the researchers calculated the carbon
content of harvested crops by county for each year from 2000 to 2008.
Next
they used population numbers and data on human food intake to estimate,
by age and gender, how much carbon from crops humans consume. On the
flip side, the co-authors also calculated how much carbon humans release
when they exhale, excrete and release flatulence. They did the same
analysis on livestock and pets.
But
not all food makes it to the dinner table. The researchers accounted
for the crops that are lost due to spoilage or during processing, which
ranges from 29% of collected dairy to as much as 57% of harvested
vegetables. Beyond food, they determined the amount of carbon that goes
into plant-based products such as fabric, cigarettes and biofuels. And
they noted how much grain is stored for future use and the crops that
are exported overseas.
National crop carbon budget
Combining
all these calculations, the researchers developed a national crop
carbon budget. Theoretically, all the carbon inputs should equal the
carbon outputs from year to year. The researchers came very close, with
no more than 6.1% of the initial carbon missing from their end
calculations. This indicated that the team had accounted for the vast
majority of the carbon from America’s harvested crops.
The
team found overall that the crops take in—and later return—about 37% of
the U.S.’s total annual carbon dioxide emissions, but that amount
varies by region. Carbon sinks, or areas that take in more carbon than
release it, were found in the agriculturally active regions of the
Midwest, Great Plains and lands along the southern half of the
Mississippi River. Regions with larger populations and less agriculture
were found to be carbon sources, or areas that release more carbon than
they take in. The calculations indicated the Northeast, Southeast and
much of the Western U.S. and Gulf Coast were carbon sources. The
remaining regions—the western interior and south-central
U.S.—flip-flopped between being minor carbon sinks or sources, depending
on the year.
Informing policy decisions
Next,
West would like his team’s methods applied to forestry, which also
involves the movement of carbon-containing products from one locale to
another. Comprehensive carbon calculations for agriculture and forestry
could be used in connection with previous carbon estimates that were
based on carbon dioxide sensor towers or carbon computer models.
“These
calculations substantially improve what we know about the movement of
carbon in agriculture,” West said. “Reliable, comprehensive data like
this can better inform policies aimed at managing carbon dioxide
emissions.”
This research was funded by NASA through the North American Carbon Program.
Regional uptake and release of crop carbon in the United States
