350
billion
Metric tons of carbon
released by humans
up to 1850
440
billion
Metric tons of carbon
released by humans
from 1850 to 2000
Human climate meddling got start
long before dawn of petroleum era
Clearing forests released greenhouse gases by the gigaton
By Alexandra Witze
SANTA FE, N.M.— People influenced the
planet’s climate for millennia before the
industrial revolution’s fossil fuel–burning
machines began spewing carbon dioxide
and other heat-trapping gases into the
atmosphere, a new study suggests.
Clearing land—first to hunt and
gather, and then to farm—removed
trees that otherwise would have soaked
up carbon dioxide. The new work estimates that humans working the land
added about 350 billion metric tons of
carbon into the atmosphere by the year
1850. (For comparison, between 1850
and 2000 people added 440 billion tons
of carbon, mostly from burning fossil
fuels — more than matching in a century
and a half what humankind had taken
the previous eight millennia to produce.)
“Our data show very substantial
amounts of human impact on the environment over thousands of years,” says team
leader Jed Kaplan of the Federal Polytechnic School of Lausanne in Switzerland.
Kaplan reported the work on March
25 at an American Geophysical Union
conference on past civilizations and climate. He, Lausanne colleague Kristen
Krumhardt and others also describe
the findings in an upcoming issue of
The Holocene.
Climate scientists often select 1850
as the start of industrial impact on the
atmosphere. But the world in 1850 was
not pristine. “I call it the ‘virgin continent myth,’ ” says Kaplan.
People cut down forests and cleared
land early on. Agriculture, for instance,
arose in the Fertile Crescent some 8,000
to 10,000 years ago.
Previous research often assumed that
as the world’s population grew, the proportion of cleared land rose as well. But
ad 1000
8,000
years ago
ad 1850
Over millennia, humans cleared much
of the planet’s land area for farming
and other uses, releasing carbon once
stored in trees into the atmosphere.
fraction of land area under natural vegetation
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
the more people crowded onto a landscape, the more efficient they became at
extracting dinner from it, says William
Ruddiman, a retired geologist from the
University of Virginia in Charlottesville.
Irrigation, fertilizer, multicropping and
new tools allowed farmers to increase
crop yields, and per capita land use fell.
Kaplan and Krumhardt looked at how
growing population and changing land-use trends affected carbon emissions.
The scientists gathered population data
for the past 8,000 years, then cataloged
how land use changed over time. The
result: a dramatic sequence showing
a green-forested world giving way to a
brown spread of deforestation.
The researchers then used a computer
simulation to calculate the amount of
carbon put into the preindustrial atmo-
sphere. The 350-billion-ton estimate is
roughly twice what the scientists found
with a different, widely used computer
simulation, and five times that reported
in Biogeosciences in January by a Uni-
versity of Bern team. The differences,
Kaplan says, trace in part to assump-
tions about how carbon is stored in the
soil when forests give way to grasslands.
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april 23, 2011 | science news | 17
