The “Little Ice Age:” When Forests Cooled the World

This is major…


“They concluded that reforestation of agricultural lands-abandoned as the
population collapsed-pulled so much carbon out of the atmosphere that it
helped trigger a period of global cooling …”
Stanford University News Service

News Release
December 17, 2008

Post-pandemic reforestation in New World helped
trigger Little Ice Age, Stanford researchers say

The power of viruses is well documented in human
history. Swarms of little viral Davids have
repeatedly laid low the great Goliaths of human
civilization, most famously in the devastating
pandemics that swept the New World during
European conquest and settlement.

In recent years, there has been growing evidence
for the hypothesis that the effect of the
pandemics in the Americas wasn’t confined to
killing indigenous peoples. Global climate
appears to have been altered as well.

Stanford University researchers have conducted a
comprehensive analysis of data detailing the
amount of charcoal contained in soils and lake
sediments at the sites of both pre-Columbian
population centers in the Americas and in
sparsely populated surrounding regions. They
concluded that reforestation of agricultural
lands-abandoned as the population
collapsed-pulled so much carbon out of the
atmosphere that it helped trigger a period of
global cooling, at its most intense from
approximately 1500 to 1750, known as the Little
Ice Age.

“We estimate that the amount of carbon
sequestered in the growing forests was about 10
to 50 percent of the total carbon that would have
needed to come out of the atmosphere and oceans
at that time to account for the observed changes
in carbon dioxide concentrations,” said Richard
Nevle, visiting scholar in the Department of
Geological and Environmental Sciences at
Stanford. Nevle and Dennis Bird, professor in
geological and environmental sciences, presented
their study at the annual meeting of the American
Geophysical Union on Dec. 17, 2008.

Nevle and Bird synthesized published data from
charcoal records from 15 sediment cores extracted
from lakes, soil samples from 17 population
centers and 18 sites from the surrounding areas
in Central and South America. They examined
samples dating back 5,000 years.

What they found was a record of slowly increasing
charcoal deposits, indicating increasing burning
of forestland to convert it to cropland, as
agricultural practices spread among the human
population-until around 500 years ago: At that
point, there was a precipitous drop in the amount
of charcoal in the samples, coinciding with the
precipitous drop in the human population in the

To verify their results, they checked their fire
histories based on the charcoal data against
records of carbon dioxide concentrations and
carbon isotope ratios that were available.

“We looked at ice cores and tropical sponge
records, which give us reliable proxies for the
carbon isotope composition of atmospheric carbon
dioxide. And it jumped out at us right away,”
Nevle said. “We saw a conspicuous increase in the
isotope ratio of heavy carbon to light carbon.
That gave us a sense that maybe we were looking
at the right thing, because that is exactly what
you would expect from reforestation.”

During photosynthesis, plants prefer carbon
dioxide containing the lighter isotope of carbon.
Thus a massive reforestation event would not only
decrease the amount of carbon dioxide in the
atmosphere, but would also leave carbon dioxide
in the atmosphere that was enriched in the heavy
carbon isotope.

Other theories have been proposed to account for
the cooling at the time of the Little Ice Age, as
well as the anomalies in the concentration and
carbon isotope ratios of atmospheric carbon
dioxide associated with that period.

Variations in the amount of sunlight striking the
Earth, caused by a drop in sunspot activity,
could also be a factor in cooling down the globe,
as could a flurry of volcanic activity in the
late 16th century.

But the timing of these events doesn’t fit with
the observed onset of the carbon dioxide drop.
These events don’t begin until at least a century
after carbon dioxide in the atmosphere began to
decline and the ratio of heavy to light carbon
isotopes in atmospheric carbon dioxide begins to

Nevle and Bird don’t attribute all of the cooling
during the Little Ice Age to reforestation in the

“There are other causes at play,” Nevle said.
“But reforestation is certainly a first-order

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