If Columbus is Blamed for Little Ice Age, who Caused the Climate to Change?

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. This strongly suggests we humans have been tampering with climate for
centuries. 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.

Depopulation of Americas may have cooled
climate

By Devin Powell in Science News October 13th,
2011    T

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.

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.

Source: www.sciencenews.org