Global warming started with human deforestation about 5,000 years ago

Using powerful supercomputers and advanced climate models, the
researchers concluded that methane and carbon dioxide – the building
blocks of global warming – began rising with the introduction of rice
cultivation and large-scale tree removal. “I think that the take-home
message is that this hypothesis shows that climates are extremely
sensitive to small variations in greenhouse gases,” said Steve Vavrus,
a climatologist at UW’s Center for Climatic Research. Vavrus and his
colleagues John Kutzbach and Gwenäelle Philippon were to discuss their
research today at a meeting of the American Geophysical Union in San
Francisco.

The work of the UW team also shows that the build-up of
greenhouse gases over thousands of years has prevented the start of a
new glacial age. Ice ages have occurred at regular 100,000-year
intervals over the last 1 million years, they said, mirroring
predictable changes in the orbit of the Earth, known as Milankovitch
cycles. Had the man-made gases not been emitted into the atmosphere,
portions of the arctic and some mountainous regions would contain more
permanent snow and ice than they do today, Vavrus said. The
researchers used data from other scientists who have been measuring
oxygen and other gases trapped in ice core samples from Antarctica
that are 850,000 years old. The air samples from 5,000 to 8,000 years
ago contained unmistakable levels of methane and carbon dioxide gases.
Methane levels rose from decomposing vegetation in terraced rice
paddies, Vavrus said. The rising levels of carbon dioxide came from
deforestation known to have taken place in Europe.
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