Columbus, smallpox and global cooling

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Claims that the rapid depopulation of the Americas around 1500 AD, leading to abandonment of cleared lands and reforestation, may have intensified 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.

By their figures, this “precipitous drop” and the consequent reforestation may account for around “10 to 50 percent” of the net carbon that left the atmosphere in that time frame. This doesn’t

  1. Using better historical data allows climate modelers to test their models more thoroughly. A model