Massive burning of forests for agriculture thousands of years ago
may have increased atmospheric carbon dioxide enough to alter global
climate and usher in a warming trend that continues today, according to
a new study that appeared online August 17, 2009, in the journal Quaternary Science Reviews.
Researchers
at the University of Virginia and the University of Maryland-Baltimore
County found that today's 6 billion people use about 90 percent less land
per person for growing food than was used by far smaller populations
early in the development of civilization. Those early societies likely
relied on slash-and-burn techniques to clear large tracts of land for
relatively small levels of food production.
"They used more
land for farming because they had little incentive to maximize yield
from less land, and because there was plenty of forest to burn," said
William Ruddiman, the lead author and professor emeritus of
environmental sciences at the University of Virginia. "They may have
inadvertently altered the climate."
Ruddiman said that early populations likely used a land-clearing method that
involved burning forests, then planting crop seed among the dead stumps
in the enriched soil. They would use a large plot until the yield began
to decline, and then would burn off another area of forest for planting.
This form of rotation farming was continued as their populations grew. Humans engaged in early agriculture possibly cleared five or
more times more land than they actually farmed at any given time. Only as populations grew much larger, and less land was available
for farming or for laying fallow, did societies adopt more intensive
farming techniques and slowly gain more food yield from less land.
Ruddiman
noted that with the highly efficient and intensive farming of today,
growing populations are using less land per capita for agriculture.
Forests are returning in many parts of the world, including the
northeastern United States, Europe, Canada, Russia and even parts of
China.
However, the positive environmental effects of this
reforestation, are being canceled out by the large-scale
burning of fossil fuels, present since the advent of the Industrial Revolution about 150 years ago. Humans continue to add excessive
levels of carbon dioxide to the atmosphere, contributing to a global
warming trend, Ruddiman said.
Five years ago, Ruddiman made
headlines with a hypothesis that humans began altering global climate
thousands of years ago, not just since the Industrial Revolution. That
theory has since been criticized by some climate scientists who believe
that early populations were too small to create enough carbon dioxide
to alter climate.
According to projections from some models of
past land use, large-scale land clearing and resulting carbon emissions
have only occurred during the industrial era, as a result of huge
increases in population. But Ruddiman, and his co-author Erle
Ellis, an ecologist at UMBC who specializes in land-use change, say
these models are not accounting for the possibly large effects on
climate likely caused by early farming methods.
"Many climate
models assume that land use in the past was similar to land use today;
and that the great population explosion of the past 150 years has
increased land use proportionally," Ellis said. "We are proposing that
much smaller earlier populations used much more land per person, and
may have more greatly affected climate than current models reflect."
Ruddiman
and Ellis based their finding on several studies by anthropologists,
archaeologists and paleoecologists indicating that early civilizations
used a great amount of land to grow relatively small amounts of food.
The researchers compared what they found with the way most land-use
models are designed, and found a disconnect between modeling and
field-based studies.
Only as populations grew
larger over thousands of years and needed more food, did humans improve
farming technologies enough to begin using less land for more yield, according to Ruddiman. He stated, "We suggest in this paper that climate modelers might
consider how land use has changed over time, and how this may have
affected the climate."
Source: University of Virginia August 17, 2009, press release.
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