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 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."
On the heels of that last presentation stating that mankind has been effecting the climate at least as far back as the thousands of years ago because of the large scale rice production and clearing of Europe's forests, there's another new paper stating that the wipe-out of the native cultures in NorAm allowed the forests to reclaim vast regions prior to the European settlement of those areas. This sequestered a lot of carbon and exacerbated the Little Ice Age by allowing for more cooling.
I have to admit that this plays to my prejudices. I have long wondered if the Little Ice Age was really the kick-off of the next glacial cycle and we interrupted it with our contributions to the atmospheric changes over the last century or so. It seems that it was more than merely that. The arrival of the Spanish to the New World made the Little Ice Age worse and then, possibly, the Little Ice Age was shortened or possibly ended by the Industrial Revolution.
Now, the various solar minimums like the Maunder Minimum were possibly one of the overt drivers of the Little Ice Age, but, on the other hand, based on what others have said that our orbital position and whatnot, we ought to have ended or be ending the interglacial. Obviously, we're not and mankind is almost certainly the reason.
Now, a speculative comment: I have to wonder about the adaption of Homo erectus and fire as to whether or not there were couplings between the hypothesized use of fire to clear forest for our ancestors to hunt more and climate changes during the Pleistocene. Wouldn't that be a kicker if the interglacials were correlated to habitat changes brought on by hominids pre modern man.