Arctic temperatures are now higher than at any time in the last 2,000
years, research reveals. Changes to the Earth's orbit drove centuries
of cooling, but temperatures rose fast in the last 100 years as human
greenhouse gas emissions rose.
The study in the journal Science documents
2,000 years of geological history using tree rings, glacier ice and
lake sediment to reconstruct the Arctic summer temperature record.
Lead
author Darrell Kaufman, professor of environmental science and
environmental sustainability at Northern Arizona University, says the
warming trend over the last 50 years interrupted what was a natural
cooling cycle.
"We noted that the timing of the rapid increase
in temperature coincides with the timing of the buildup of greenhouse
gases, and there is no other mechanism or forcing that we can come up
with that would explain the rapid reversal of that natural cooling," he
said.
The earth's cool-down period started about 7,000 years
ago. Arctic temperatures bottomed out during the so-called "Little Ice
Age" that lasted from the 16th to the mid-19th centuries.
The root cause of the slow cooling was the orbital "wobble" that slowly varies over thousands of years.
Some
skeptics have argued that the fact that the Earth wobbles in its axis
of rotation has helped determine recent warming, rather than human
activity. But the new study shows that this wobble - which affects how
much sunlight Earth receives in the middle of the summer - actually
accounts for a long-term cooling trend in the Arctic, which has been
reversed only in the past half-century.
Studies show the earth
had been cooling at the rate of .2 degrees Celsius per
millennium. The last decade, however, was the warmest of the last
2,000 years, averaging 1.4 degrees Celsius higher than would have been
expected if the cooling trend had continued.
Kaufman says his research adds to the evidence that human-produced greenhouse emissions contribute to global warming.
"I
think that the important point there is that the very slow orbital
cycles would take thousands of years before the earth could enter an
ice age, before ice sheets miles thick would build up on the
continent," he said. "The rapid warming that we are experiencing
presently far out competes any kind of cooling trend that the natural
cycle would dictate."
Kaufman says another important finding
came from collaboration with the National Center for Atmospheric
Research, which simulated orbital variations over a 2,000 year time
period.
"And the output from that computer model showed the same
amount of temperature change for the arctic that we documented based on
the geologic evidence," he said. "So it is this match of the output of
the computer models and the natural data that gives us confidence in
the ability of the model to simulate the effects of factors that we
know causes climate to change."
Kaufman says the evidence is
conclusive that recent Arctic warming is unusual, that greenhouse gases
play a role, and that it's time to take decisive action to reduce
carbon emissions. The study was conducted by an international team of
scientists and primarily funded by the National Science Foundation.
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