New World post-pandemic reforestation helped start Little Ice Age, say Stanford scientists


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."

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 increase.

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

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

Source: Stanford University

Explore further

This is what you need to know about carbon capture and storage

Citation: New World post-pandemic reforestation helped start Little Ice Age, say Stanford scientists (2008, December 18) retrieved 15 October 2019 from
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.

Feedback to editors

User comments

Dec 18, 2008

Dec 18, 2008
Very well said.

Dec 19, 2008
I too think it is a bit of a stretch. :P

Dec 19, 2008
this is a scientific result, presented at the American Geophysical Union. If you guys can't say anything scientific in response, don't say anything at all. This result is more evidence linking CO2 levels to temperature and frankly I find it really interesting that "primitive" civilisations could potentially impact the global climate.

Dec 19, 2008
Velanarris, it never fails to surprise me how you smear mainstream science which supports global warming as fraud while promoting isolated and unsubstantiated research as a viable alternative. In retrospect, I understand that this is all you can do, so go ahead my misguided friend.

Dec 19, 2008
That is a very strange article. It never once mentioned the very low level of solar activity that occurred at that time of the Little Ice Age (~1500 until ~1750). Had they noted that correlation, their research would have also suggested that the reforestation was influencing not just the global climate, but the sun as well.

Of course, another possibility is that the Little Ice Age was a tough time for humans which resulted in a dramatic decrease in human population. And that it was the cooling that caused the decrease in population which allowed reforestation of previously cultivated land.

The article fails to mention Polar Bears which surprises me.

Dec 19, 2008
I wonder if not the global warming scientists or as I like to call thermodynamic fundamentalists (those who believe that trace gases are driving Earth%u2019s climate), if they never blush when they present such garbage.
But hey, their tabloid article may even be presented on the BBC and CBS as real science:

Dec 19, 2008
And how did the Incas create a big enough rise in CO2 to create the Medieval Warm spell to which reforestation moved them into the little ice age?

Dec 19, 2008
You know how clever those Incas are, Nar...

Dec 21, 2008
The scientists do not consider the reverse of their theory. As the oceans cool they can absorb more carbon dioxide, an undisputed fact. Cause and effect arse backwards.

Dec 22, 2008
More trees anyone? Seems like a good idea to beat the heat

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more