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Saturday, September 5, 2009

Joseph Romm: Human-caused Arctic warming overtakes 2,000 years of natural cooling, “seminal” study finds

Human-caused Arctic warming overtakes 2,000 years of natural cooling, “seminal” study finds

by Joseph Romm, Climate Progress blog, September 3, 2009

A Hockey Stick in Melting Ice


Arctic temperatures in the 1990s reached their warmest level of any decade in at least 2,000 years, new research indicates. The study, which incorporates geologic records and computer simulations, provides new evidence that the Arctic would be cooling if not for greenhouse gas emissions that are overpowering natural climate patterns.

So reports the National Center for Atmospheric Research (NCAR), which coauthored the study to be published in Science Friday. [I'll put the link up when it's posted.] The Washington Post story notes:

The analysis, based on more than a dozen lake sediment cores as well as glacier ice and tree ring records from the Arctic, provides one of the broadest pictures to date of how industrial emissions have shifted the Arctic’s long-standing natural climate patterns. Coupled with a separate report on the region issued Wednesday by the World Wildlife Fund, the studies suggest human-induced changes could transform not only the Arctic but climate conditions across the globe.

It’s basically saying the greenhouse gas emissions are overwhelming the system,” said David Schneider, a visiting scientist at the National Center for Atmospheric Research and one of the Science article’s co-authors.

The same could be said about the entire planetary ecosystem — on our current path, we’re going to overwhelm the whole system (see “Intro to global warming impacts: Hell and High Water “). Indeed, in some sense we already have, as a number of climate scientists have pointed out. The NYT’s Andy Revkin interviewed Thomas Crowley, a climate specialist at the University of Edinburgh:

“I would say that this is another piece of evidence that strengthens the argument that humans are now capable of preventing the onset of a future ice age,” he told me. Another scientist holding this view is James E. Hansen of NASA, whom I interviewed about the timing of the next ice age in 2003.

The NCAR graph appears to provide yet more support for the original, much-maligned “hockey stick,” which has been confirmed by recent analysis published in the Proceedings of the National Academy of Sciences (see “Sorry deniers, hockey stick gets longer, stronger: Earth hotter now than in past 2,000 years“):


The new study suggests (again) that the Medieval warm period was limited to only a part of the Northern Hemisphere, and that recent human-caused warming is quite outside the boundary of the last two millennia:

Darrell Kaufman of Northern Arizona University, the lead author and head of the synthesis project, says the results indicate that recent warming is more anomalous than previously documented.

“Scientists have known for a while that the current period of warming was preceded by a long-term cooling trend,” says Kaufman. “But our reconstruction quantifies the cooling with greater certainty than before.”

This new study made use of the “natural archives of Arctic climate”:

To reconstruct Arctic temperatures over the last 2,000 years, the study team incorporated three types of field-based data, each of which captured the response of a different component of the Arctic’s climate system to changes in temperature.

These data included temperature reconstructions published by the study team earlier this year. The reconstructions were based on evidence provided by sediments from Arctic lakes, which yielded two kinds of clues: changes in the abundance of silica remnants left behind by algae, which reflect the length of the growing season, and the thickness of annually deposited sediment layers, which increases during warmer summers as deposits from glacial meltwater increase.

The research also incorporated previously published data from glacial ice and tree rings that were calibrated against the instrumental temperature record.

The scientists compared the temperatures inferred from the field-based data with simulations run with the Community Climate System Model, a computer model of global climate based at NCAR. The model’s estimate of the reduction of seasonal sunlight in the Arctic and the resulting cooling was consistent with the analysis of the lake sediments and other natural archives. These results give scientists more confidence in computer projections of future Arctic temperatures.

Some of our leading climate scientists say this is especially important paper, as the WP piece notes:

Mark Serreze, director of the National Snow and Ice Data Center at the University of Colorado at Boulder, said the study was significant because it helps confirm scientists’ current understanding of how the earth’s climate has changed over millennia.

“It’s not that we don’t know how the climate works, it just we didn’t have anyone at that time measuring the climate forcing then,” referring to 2,000 years ago. “Climate doesn’t change all by itself for no good reason. Something has to force it.”


Robert Correll, who chairs the Arctic Climate Impact Assessment, said the paper in Science will likely “in the long haul become a seminal piece in the scientific literature” because it allows other climate researchers “to set their work in a long time scale.”

And Revkin’s print piece underscores the danger:

Jonathan T. Overpeck, a study author and climate specialist at the University of Arizona, said the rising concentration of long-lived greenhouse gases guaranteed warming at a pace that could stress ecosystems and cause rapid melting of Greenland’s great ice sheet.

“The fast rate of recent warming is the scary part,” Dr. Overpeck said. “It means that major impacts on Arctic ecosystems and global sea level might not be that far off unless we act fast to slow global warming.”

So now we know the answer to the question Robert Frost famously posed:

Some say the world will end in fire,
Some say in ice.

Fire it is — humanity’s burning of fossil fuels (and forests) trumps the natural ice age cycle.

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