European disease killed off so many in the new world that reforestation rates caused global cooling

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 Americas. 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.

Posted via email from Deane’s posterous