How humans cooled the earth -- 500 years ago

After pandemics caused a mass die-off in the New World, farmland turned to forest and temperatures dropped

Published January 8, 2009 12:05AM (EST)

One of the tell-tale signs of a really thought-provoking book is that soon after reading it, you start seeing its thesis replicated everywhere you look. So it has been with one of the tomes I referred to in yesterday's post "Polynesian Chickens in Peru and other Mysteries," Charles Mann's "1491: New Revelations of the Americas Before Columbus."

The massive depopulation of the Americas via smallpox, hepatitis and other diseases introduced by Westerners (perhaps as much as 95 percent of the existing population died in vast pandemics) and the large landscape-altering scale of agriculture practiced across the "New World" by pre-Columbian cultures are two of the big themes of "1491." Both popped up in a presentation made by two scientists at the annual meeting of the American Geophysical Union last December. (Thanks to MongaBay for the tip.)

The scientists contend that after the die-off, massive reforestation on abandoned agricultural land occurred on a large enough scale to contribute significantly to the period of global cooling between 1500 and 1750 known as the "Little Ice Age."

After examining soil samples and sediment cores from numerous locations in Central and South America, Richard Nevle, a visiting scholar at Stanford's Department of Geological and Environmental Sciences at Stanford, and Dennis Bird, also from Stanford, concluded that the reforestation sequestered as much as 10 to 50 percent of the carbon necessary to cool the earth. Up until 1500, the soil samples showed a steady increase in charcoal content, likely generated from human-caused fire used to clear forest. After 1500, the scientists discovered a drastic drop in charcoal content.  No more burning.

The scientists acknowledge that reforestation was just one factor in contributing to global cooling. It may not even have been the most critical factor. But the research is sobering nonetheless, in its hint as to humanity's power to alter the fundamental characteristics of life on this planet, long before we were burning fossil fuels as if there were no tomorrow. We did it back then, we're doing it now, and maybe, just maybe, if we exert our collective will in the proper direction, we can fix our mistakes.

Let's just hope it doesn't require another vast die-off to set things to rights.

By Andrew Leonard

Andrew Leonard is a staff writer at Salon. On Twitter, @koxinga21.

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