Deforestation driving CO2 buildup
Genghis Khan and his Mongol hordes had an impact on the
global carbon cycle as big as today's annual demand for
gasoline. The Black Death, on the other hand, came and went too
quickly for it to cause much of a blip in the global carbon
budget. Dwarfing both of these events, however, has been the
historical trend towards increasing deforestation, which over
centuries has released vast amounts of carbon dioxide into the
atmosphere, as crop and pasture lands expanded to feed growing
human populations. Even Genghis Kahn couldn't stop it for long.
"It's a common misconception that the human impact on climate
began with the large-scale burning of coal and oil in the
industrial era," says Julia Pongratz of the Carnegie
Institution's Department of Global Ecology, lead author of a new
study on the impact of historical events on global climate
published in the January 20, 2011, online issue of The Holocene.
"Actually, humans started to influence the environment thousands
of years ago by changing the vegetation cover of the Earth's
landscapes when we cleared forests for agriculture."
Clearing forests releases carbon dioxide into the atmosphere
when the trees and other vegetation are burned or when they
decay. The rise in atmospheric carbon dioxide resulting from
deforestation is recognizable in ice cores from Greenland and
Antarctica before the fossil-fuel era.
But human history has had its ups and downs. During
high-mortality events, such as wars and plagues, large areas of
croplands and pastures have been abandoned and forests have
re-grown, absorbing carbon dioxide from the atmosphere.
Pongratz decided to see how much effect these events could
have had on the overall trend of rising carbon dioxide levels.
Working with colleagues at the Max Planck Institute for
Meteorology in Germany and with global ecologist Ken Caldeira at
Carnegie, she compiled a detailed reconstruction of global land
cover over the time period from 800 AD to present and used a
global climate-carbon cycle model to track the impact of land
use changes on global climate. Pongratz was particularly
interested in four major events in which large regions were
depopulated: the Mongol invasions in Asia (1200-1380), the Black
Death in Europe (1347-1400), the conquest of the Americas
(1519-1700), and the Fall of the Ming Dynasty in China
(1600-1650).
"We found that during the short events such as the Black
Death and the Ming Dynasty collapse, the forest re-growth wasn't
enough to overcome the emissions from decaying material in the
soil," says Pongratz. "But during the longer-lasting ones like
the Mongol invasion and the conquest of the Americas there was
enough time for the forests to re-grow and absorb significant
amounts of carbon."
The global impact of forest re-growth in even the
long-lasting events was diminished by the continued clearing of
forests elsewhere in the world. But in the case of the Mongol
invasions, which had the biggest impact of the four events
studied, re-growth on depopulated lands stockpiled nearly 700
million tons of carbon absorbed from the atmosphere. This is
equivalent to the world's total annual demand for gasoline
today.
Pongratz points out the relevance of the study to current
climate issues. "Today about a quarter of the net primary
production on the Earth's land surface is used by humans in some
way, mostly through agriculture," she says. "So there is a large
potential for our land-use choices to alter the global carbon
cycle. In the past we have had a substantial impact on global
climate and the carbon cycle, but it was all unintentional.
Based on the knowledge we have gained from the past, we are now
in a position to make land-use decisions that will diminish our
impact on climate and the carbon cycle. We cannot ignore the
knowledge we have gained."
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