When Loss of Forest Cover Made the World Hotter

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” … physical evidence, backed by powerful
simulations on the world’s most advanced computer
climate models, is reshaping that view and
lending strong support to the radical idea that
human-induced climate change began not 200 years
ago, but thousands of years ago with the onset of
large-scale agriculture in Asia and extensive
deforestation in Europe.”

“No one disputes the large rate of increase in
greenhouse gases with the Industrial Revolution,”
Kutzbach notes. “The large-scale burning of coal
for industry has swamped everything else” in the
record.

“But looking farther back in time, using climatic
archives such as 850,000-year-old ice core
records from Antarctica, scientists are teasing
out evidence of past greenhouse gases in the form
of fossil air trapped in the ice. That ancient
air, say Vavrus and Kutzbach, contains the
unmistakable signature of increased levels of
atmospheric methane and carbon dioxide beginning
thousands of years before the industrial age.”
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Public release date: 17-Dec-2008
University of Wisconsin-Madison

Contact: Steve Vavrus
sjvavrus@wisc.edu
608-265-5279

Study: Did early climate impact divert a new glacial age?

SAN FRANCISCO – The common wisdom is that the
invention of the steam engine and the advent of
the coal-fueled industrial age marked the
beginning of human influence on global climate.

But gathering physical evidence, backed by
powerful simulations on the world’s most advanced
computer climate models, is reshaping that view
and lending strong support to the radical idea
that human-induced climate change began not 200
years ago, but thousands of years ago with the
onset of large-scale agriculture in Asia and
extensive deforestation in Europe.

What’s more, according to the same computer
simulations, the cumulative effect of thousands
of years of human influence on climate is
preventing the world from entering a new glacial
age, altering a clockwork rhythm of periodic
cooling of the planet that extends back more than
a million years.

“This challenges the paradigm that things began
changing with the Industrial Revolution,” says
Stephen Vavrus, a climatologist at the University
of Wisconsin-Madison’s Center for Climatic
Research and the Nelson Institute for
Environmental Studies. “If you think about even a
small rate of increase over a long period of
time, it becomes important.”

Addressing scientists here today (Dec. 17) at a
meeting of the American Geophysical Union, Vavrus
and colleagues John Kutzbach and Gwenaëlle
Philippon provided detailed evidence in support
of a controversial idea first put forward by
climatologist William F. Ruddiman of the
University of Virginia. That idea, debated for
the past several years by climate scientists,
holds that the introduction of large-scale rice
agriculture in Asia, coupled with extensive
deforestation in Europe began to alter world
climate by pumping significant amounts of
greenhouse gases – methane from terraced rice
paddies and carbon dioxide from burning forests –
into the atmosphere. In turn, a warmer atmosphere
heated the oceans making them much less efficient
storehouses of carbon dioxide and reinforcing
global warming.

That one-two punch, say Kutzbach and Vavrus, was
enough to set human-induced climate change in
motion.

“No one disputes the large rate of increase in
greenhouse gases with the Industrial Revolution,”
Kutzbach notes. “The large-scale burning of coal
for industry has swamped everything else” in the
record.

But looking farther back in time, using climatic
archives such as 850,000-year-old ice core
records from Antarctica, scientists are teasing
out evidence of past greenhouse gases in the form
of fossil air trapped in the ice. That ancient
air, say Vavrus and Kutzbach, contains the
unmistakable signature of increased levels of
atmospheric methane and carbon dioxide beginning
thousands of years before the industrial age.

“Between 5,000 and 8,000 years ago, both methane
and carbon dioxide started an upward trend,
unlike during previous interglacial periods,”
explains Kutzbach. Indeed, Ruddiman has shown
that during the latter stages of six previous
interglacials, greenhouse gases trended downward,
not upward. Thus, the accumulation of greenhouse
gases over the past few thousands of years, the
Wisconsin-Virginia team argue, is very likely
forestalling the onset of a new glacial cycle,
such as have occurred at regular 100,000-year
intervals during the last million years. Each
glacial period has been paced by regular and
predictable changes in the orbit of the Earth
known as Milankovitch cycles, a mechanism thought
to kick start glacial cycles.

“We’re at a very favorable state right now for
increased glaciation,” says Kutzbach. “Nature is
favoring it at this time in orbital cycles, and
if humans weren’t in the picture it would
probably be happening today.”

Importantly, the new research underscores the key
role of greenhouse gases in influencing Earth’s
climate. Whereas decreasing greenhouse gases in
the past helped initiate glaciations, the early
agricultural and recent industrial increases in
greenhouse gases may be forestalling them, say
Kutzbach and Vavrus.

Using three different climate models and removing
the amount of greenhouse gases humans have
injected into the atmosphere during the past
5,000 to 8,000 years, Vavrus and Kutzbach
observed more permanent snow and ice cover in
regions of Canada, Siberia, Greenland and the
Rocky Mountains, all known to be seed regions for
glaciers from previous ice ages. Vavrus notes:
“With every feedback we’ve included, it seems to
support the hypothesis (of a forestalled ice age)
even more. We keep getting the same answer.”

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