Depopulation of Americas may have cooled climate
By Devin Powell Science News
MINNEAPOLIS — By sailing to the New World, Christopher Columbus and the other explorers who followed may have set off a chain of events that cooled Europe’s climate for centuries.
The European conquest of the Americas decimated the people living there, leaving large areas of cleared land untended. Trees that filled in this territory pulled billions of tons of carbon dioxide from the atmosphere, diminishing the heat-trapping capacity of the atmosphere and cooling climate, says Richard Nevle, a geochemist at Stanford University.
“We have a massive reforestation event that’s sequestering carbon … coincident with the European arrival,” says Nevle, who described the consequences of this change October 11 at the Geological Society of America annual meeting.
Tying together many different lines of evidence, Nevle estimated how much carbon all those new trees would have consumed. He says it was enough to account for most or all of the sudden drop in atmospheric carbon dioxide recorded in Antarctic ice during the 16th and 17th centuries. This depletion of a key greenhouse gas, in turn, may have kicked off Europe’s so-called Little Ice Age, centuries of cooler temperatures that followed the Middle Ages.
By the end of the 15th century, between 40 million and 80 million people are thought to have been living in the Americas. Many of them burned trees to make room for crops, leaving behind charcoal deposits that have been found in the soils of Mexico, Nicaragua and other countries.
About 500 years ago, this charcoal accumulation plummeted as the people themselves disappeared. Smallpox, diphtheria and other diseases from Europe ultimately wiped out as much as 90 percent of the indigenous population.
Trees returned, reforesting an area at least the size of California, Nevle estimated. This new growth could have soaked up between 2 billion and 17 billion tons of carbon dioxide from the air.
Ice cores from Antarctica contain air bubbles that show a drop in carbon dioxide around this time. These bubbles suggest that levels of the greenhouse gas decreased by 6 to 10 parts per million between 1525 and the early 1600s.
“There’s nothing else happening in the rest of the world at this time, in terms of human land use, that could explain this rapid carbon uptake,” says Jed Kaplan, an earth systems scientist at the Federal Polytechnic School of Lausanne in Switzerland.
Natural processes may have also played a role in cooling off Europe: a decrease in solar activity, an increase in volcanic activity or colder oceans capable of absorbing more carbon dioxide. These phenomena better explain regional climate patterns during the Little Ice Age, says Michael Mann, a climate researcher at Pennsylvania State University in State College. [Mann still refuses to admit the Little Ice Age and Medieval Warming Period were global, not local, phenomena]
But reforestation fits with another clue hidden in Antarctic ice, says Nevle. As the population declined in the Americas, carbon dioxide in the atmosphere got heavier. Increasingly, molecules of the gas tended to be made of carbon-13, a naturally occurring isotope with an extra neutron. That could be because tree leaves prefer to take in gas made of carbon-12, leaving the heavier version in the air.
Kaplan points out that there’s a lot of uncertainty in such isotope measurements, so this evidence isn’t conclusive. But he agrees that the New World pandemics were a major event that can’t be ignored — a tragedy that highlighted mankind’s ability to influence the climate long before the industrial revolution.
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