Arctic reverses trend, is warmest in two millennia

Randolph E. Schmid, Associated Press Yahoo News 3 Sep 09;

WASHINGTON – The Arctic is warmer than it's been in 2,000 years, even though it should be cooling because of changes in the Earth's orbit that cause the region to get less direct sunlight. Indeed, the Arctic had been cooling for nearly two millennia before reversing course in the last century and starting to warm as human activities added greenhouse gases to the atmosphere.

"If it hadn't been for the increase in human-produced greenhouse gases, summer temperatures in the Arctic should have cooled gradually over the last century," said Bette Otto-Bliesner, a National Center for Atmospheric Research scientist and co-author of a study of Arctic temperatures published in Friday's edition of the journal Science.

The most recent 10-year interval, 1999-2008, was the warmest of the last 2,000 years in the Arctic, according to the researchers led by Darrell S. Kaufman, a professor of geology and environmental science at Northern Arizona University.

Summer temperatures in the Arctic averaged 2.5 degrees Fahrenheit (1.4 degrees Celsius) warmer than would have been expected if the cooling had continued, the researchers said.

The finding adds fuel to the debate over a House-passed climate bill now pending in the Senate. The administration-backed measure would impose the first limits on greenhouse gases and eventually would lead to an 80 percent reduction by putting a price on each ton of climate-altering pollution.

It is the latest in a drumbeat of reports on warming conditions in the Arctic, including:

• A marine scientist reports that Alaskan waters are turning acidic from absorbing greenhouse gases faster than tropical waters, potentially endangering the state's $4.6 billion fishing industry.

• NASA satellite measurements show that sea ice in the Arctic is more than just shrinking in area, it is dramatically thinning. The volume of older crucial sea ice in the Arctic has shrunk by 57 percent from the winter of 2004 to 2008.

• Global warming effects in Alaska also include shrinking glaciers, coastal erosion and the march north of destructive forest beetles formerly held in check by cold winters.

And with the melting of land-based ice, such as the massive Greenland ice cap, sea levels could rise across the world, threatening millions who live in coastal cities.

The new report is based on a decade-by-decade reconstruction of temperatures over the past 2,000 years developed using information from ancient lake sediments, ice cores, tree rings and other samples. The findings were then compared with complex computer climate model simulations created at the National Center for Atmospheric Research in Boulder, Colo.

"This study provides us with a long-term record that reveals how greenhouse gases from human activities are overwhelming the Arctic's natural climate system," commented NCAR scientist David Schneider, a co-author on the study.

Added Jonathan T. Overpeck, a University of Arizona professor of geosciences: "The Arctic should be very sensitive to human-caused climate change, and our results suggest that indeed it is."

In addition, he pointed out, as the Arctic warms there is less snow and ice to reflect solar energy back into space and the newly exposed dark soil and dark ocean surfaces absorb solar energy and warm further, accelerating the warming process.

The Arctic cooling had been the result of a 21,000-year cycle in the Earth's movement that caused the far north to get progressively less summertime energy from the sun for the last 8,000 years. That process won't reverse for another several thousand years.

The research was funded by the National Science Foundation.
Science: http://www.sciencemag.org

Warming in Arctic reverses 8,000 year old cooling trend
Yahoo News 4 Sep 09;

CHICAGO (AFP) – An abrupt warming of the Arctic has reversed a cooling trend that began about 8,000 ago, according to a study published Thursday which sheds light on the threat of rising sea levels and climate change.

Increased greenhouse gas emissions appear to have overridden the natural cooling caused by a wobble in the Earth's axis which has been gradually pulling the planet away from the Sun.

This wobble has cooled summer temperatures by an average of about 0.2 degrees Celsius per thousand years, the study published in the journal Science found.

But Arctic temperatures began to rise at the beginning of the 20th century even though the orbital cycle that produced the cooling continued.

The result was summer temperatures that were about 1.4 degrees Celsius warmer than they should have been by the year 2000, according to the study which mapped Arctic temperatures for every decade of the past 2,000 years.

Temperatures for four of the last five decades were among the highest on record.

"This study provides us with a long-term record that reveals how greenhouse gases from human activities are overwhelming the Arctic's natural climate system," said co-author David Schneider of the Colorado-based National Center for Atmospheric Research.

"This result is particularly important because the Arctic, perhaps more than any other region on Earth, is facing dramatic impacts from climate change."

The Arctic tends to warm about three times faster than elsewhere in the Northern Hemisphere because of a phenomenon called Arctic amplification.

When highly reflective ice and snow melt, the exposed dark land and ocean absorb more sunlight. The warmer temperatures accelerate melting, which then accelerates warming.

"This has consequences globally because, as the Arctic warms, glacier ice will melt, contributing to (a) sea-level rise and impacting coastal communities around the globe," said lead author Darrell Kaufman of Northern Arizona University.

"Thawing permafrost will release methane adding to the global greenhouse effect," Kaufman added.

The researchers used glacial ice, tree rings and lake sediments to supplement a complex computer model of global climate and generate a reconstruction of temperatures for the past 2,000 years.

Previous studies had only pinpointed Arctic temperatures for the past 400 years.

"Scientists have known for a while that the current period of warming was preceded by a long-term cooling trend," Kaufman said.

"But our reconstruction quantifies the cooling with greater certainty than before."

Arctic now warmest in 2000 years, researchers say
Deborah Zabarenko, Reuters 3 Sep 09;

WASHINGTON (Reuters) - Climate-warming greenhouse gas emissions pushed Arctic temperatures in the last decade to the highest levels in at least 2,000 years, reversing a natural cooling trend that should have lasted four more millennia.

Carbon dioxide and other gases generated by human activities overwhelmed a 21,000-year cycle linked to gradual changes in Earth's orbit around the Sun, an international team of researchers reported on Thursday in the journal Science.

"I think it really underscores how sensitive the Arctic is to climate change ... and it's really the place where you can see first what's happening to the (climate) system and how the rest of the Earth will or might follow," David Schneider, a co-author and a scientist with the U.S. National Center for Atmospheric Research said in a telephone interview.

The big cool-down started about 7,000 years ago, and Arctic temperatures bottomed out during the so-called "Little Ice Age" that lasted from the 16th to the mid-19th centuries, dove-tailing with the start of the Industrial Revolution.

This cooling trend was caused by a characteristic wobble in Earth's orbit that very gradually pushed the Arctic away from the Sun during the northern summer. Earth is now about 620,000 miles (1 million km) farther from the Sun in the Arctic summer than it was 2000 years ago, said Darrell Kaufmann of Northern Arizona University.

This cooling should have continued through the 20th and 21st centuries and beyond as the 21,000-year cycle played out. This latest research confirms that it hasn't.

EARTH'S AIR CONDITIONER

"If it hadn't been for the increase in human-produced greenhouse gases, summer temperatures in the Arctic should have cooled gradually over the last century," Bette Otto-Bliesner, a co-author from the National Center for Atmospheric Research, said in a statement.

What happens in the Arctic doesn't stay there, since it is among the world's biggest weathermakers, sometimes called Earth's air-conditioner. As Arctic sea ice melts in summer, it exposes the darker-colored ocean water, which absorbs sunlight instead of reflecting it, accelerating the warming effect.

Arctic warming also affects land-based glaciers; if these melt, they would contribute to a global rise in sea levels.

Warming in this area could also thaw frozen ground called permafrost, sending methane, a powerful greenhouse gas, into the atmosphere.

Climate scientists have long known that Earth wobbles in its orbit, which affects how much sunlight reaches the Arctic in the summer. This is the first time a large-scale study has tracked decade-by-decade changes in Arctic summer temperatures this far back in time.

To figure this out, researchers looked at natural archives of temperature -- tree rings, ice cores and lake sediments -- along with computer models, which tallied closely with the natural record.

Average summer temperatures in the Arctic have increased by about 3 degrees F (1.66 degrees C) from what they would have been had the long-term cooling trend remained intact.

(Editing by Todd Eastham)

Arctic Temperatures Are Warmest in 2,000 Years
Andrea Thompson, livescience.com Yahoo News 3 Sep 09;

Arctic air temperatures in the 1990s were the warmest in the last 2,000 years and were a result of rising greenhouse gas levels, a new study concludes.

The findings, detailed in the Sept. 4 issue of the journal Science, also suggest that if it weren't for these manmade pollutants, temperatures around the North Pole would actually be cooling as a result of natural climate patterns.

"This result is particularly important because the Arctic, perhaps more than any other region on Earth, is facing dramatic impacts from climate change," said study team member David Schneider of the National Center for Atmospheric Research (NCAR). "This study provides us with a long-term record that reveals how greenhouse gases from human activities are overwhelming the Arctic's natural climate system."

Natural archives

The researchers uncovered this masked cooling trend by reconstructing Arctic temperatures over the past two millennia with data from Arctic lake sediments, glacial ice and tree rings, all of which provide records of the changes in temperatures up there.

These natural archives indicated a pervasive cooling across the Arctic on a decade-by-decade basis that is related to an approximately 21,000-year cyclical wobble in Earth's tilt relative to the sun.

Over the last 7,000 years, the timing of Earth's closest pass by the sun has shifted from September to January. This has gradually reduced the intensity of sunlight reaching the Arctic in the Northern Hemisphere's summertime, when Earth is farther from the sun (the main driver of summer temperatures is the fact that the hemisphere is tilted toward the sun during these months, while it is tilted away from the sun during winter).

The team's temperature analysis shows that summer temperatures in the Arctic, in step with the reduced energy from the sun, cooled at an average rate of about .35 degrees Fahrenheit (0.2 degrees Celsius) per thousand years. The temperatures eventually bottomed out during the "Little Ice Age," a period of widespread cooling that lasted roughly from the 16th to the mid-19th centuries.

The study is useful because it isolates the temperature changes of the Arctic region from the larger signal of the Northern Hemisphere - the orbital changes are known to have more of an effect on the high latitudes than the lower ones, and this is born out in the study's findings, said Michael Mann of Penn State University, who did not work on the new study.

Trend overwhelmed

Even though the orbital cycle that produced the cooling continued, it was overwhelmed in the 20th century by human-induced warming caused by the buildup of greenhouse gases, such as carbon dioxide, in the atmosphere.

"If it hadn't been for the increase in human-produced greenhouse gases, summer temperatures in the Arctic should have cooled gradually over the last century," said team member Bette Otto-Bliesner, also of NCAR.

The study found that the 10 years from 1999 to 2008 was the warmest in the Arctic in two millennia. Arctic temperatures are now 2.2 degrees Fahrenheit (1.2 C) warmer than in 1900.

"The amount of energy we're getting from the sun in the 20th century continued to go down, but the temperature went up higher than anything we've seen in the last 2,000 years," said team member Nicholas P. McKay of The University of Arizona in Tucson.

The scientists compared the temperatures inferred from the field-based data with computer model simulations. 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.

"This study provides a clear example of how increased greenhouse gases are now changing our climate, ending at least 2,000 years of Arctic cooling," says NCAR scientist and team member Caspar Ammann.

The new study follows previous work showing that temperatures over the last century warmed almost three times faster in the Arctic than elsewhere in the Northern Hemisphere. This phenomenon, called Arctic amplification, occurs as highly reflective Arctic ice and snow melt away, allowing dark land and exposed ocean to absorb more sunlight. This amplification could lead to potentially catastrophic melting of Arctic sea ice and land-based glaciers, which could impact Arctic wildlife, indigenous peoples and global sea levels.

"The warming of the past century, which has been shown to very likely be due in substantial part to human-caused increases in greenhouse gas concentrations, is once again seen to be without precedent in a very long-term (at least 2000 year in this case) context," Mann told LiveScience. "Yet one other line of evidence that the changes that are taking place today are indeed without precedent."

Arctic 'warmest in 2,000 years'
Richard Black, BBC News 3 Sep 09;

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.

Scientists took evidence from ice cores, tree rings and lake sediments.

Writing in the journal Science, they say this confirms that the Arctic is very sensitive both to changes in solar heating and to greenhouse warming.

The 23 sites sampled were good enough to provide a decade-by-decade picture of temperatures across the region.

The result is a "hockey stick"-like curve in which the last decade - 1998-2008 - stands out as the warmest in the entire series.

"The most pervasive signal in the reconstruction, the most prominent trend, is the overall cooling that took place for the first 1,900 years [of the record]," said study leader Darrell Kaufman from Northern Arizona University in Flagstaff, US.

"The 20th Century stands out in strong contrast to the cooling that should have continued. The last half-century was the warmest of the 2,000-year temperature record, and the last 10 years have been especially dramatic," he told BBC News.

On average, the region cooled at a rate of 0.2C per millennium until about 1900. Since then, it has warmed by about 1.2C.

Much debate on climate change has centred on the Mediaeval Warm Period, or Mediaeval Climate Anomaly - a period about 1,000 years ago when, historical records suggest, Vikings colonised Greenland and may have grown grapes in Newfoundland.

The new analysis shows that temperatures were indeed warmer in this region 1,000 years ago than they were 100 years ago - but not as warm as they are now, or 1,000 years previously.

"It shows that the Mediaeval Warm Period is real, and is... an exception from the general trend of cooling," commented Eystein Jansen from Bergen University in Norway, who was not involved in the research.

"It also shows there's lots of variability on the 100-year timescale, and that's probably more so in the Arctic than elsewhere."

Professor Jansen was a co-ordinating lead author on the palaeoclimate (ancient climate) chapter of the last Intergovernmental Panel on Climate Change (IPCC) assessment.

Arctic wobbles

The root cause of the slow cooling was the orbital "wobble" that slowly varies, over thousands of years, the month in which the Earth approaches closest to the Sun.

This wobble slowly decreased the total amount of solar energy arriving in the Arctic region in summertime, and the temperature responded - until greenhouse warming took over.

"The 20th Century is the first century for which how much energy we're getting from the Sun is no longer the most important thing governing the temperature of the Arctic," said another of the study team, Nicholas McKay from the University of Arizona.

The recent warming of the Arctic has manifested itself most clearly in the drastic shrinkage in summer sea-ice extent, with the smallest area in the satellite era documented in 2007.

As the Science study emerged, UN Secretary-General Ban Ki-moon was telling the World Climate Conference in Geneva that many of the "more distant scenarios" forecast by climate scientists were "happening now".

Earlier this week, Mr Ban visited the Arctic in an attempt to gain first-hand experience of how the region is changing.

"Scientists have been accused for years of scaremongering. But the real scaremongers are those who say we cannot afford climate action," he said in his Geneva speech, calling for world leaders to make bigger pledges of action in the run-up to December's UN climate summit in Copenhagen.

Next Ice Age Delayed by Global Warming, Study Says
Christine Dell'Amore, National Geographic News 3 Sep 09;

Humans are putting the brakes on the next ice age, according to the most extensive study to date on Arctic climate change.

The Arctic is now warmer than it's been in the past 2,000 years—a trend that is reversing a natural cooling cycle dictated by a wobble in Earth's axis.

Previously, researchers had looked at Arctic temperature data that went back just 400 years.

That research showed a temperature spike in the 20th century, but it was unclear whether human-caused greenhouse gas emissions or natural variability was the culprit, noted study co-author Gifford Miller of the Institute of Arctic and Alpine Research at the University of Colorado, Boulder.

By looking even farther back in time, Miller and colleagues' newest study reveals that the 20th century's abrupt warming in fact interrupted millennia of steady cooling.

It's "pretty clear that the most reasonable explanation for that reversal is due to increasing greenhouse gases," Miller said.

The researchers' computer climate models dovetails with field data such as sediment cores and tree rings, which "really … solidifies our understanding," he said.

Eventually Earth will slip again into the pattern of cyclical ice ages, Miller added, but it may be thousands of years before that happens.

Ice Age, Interrupted

Earth's angle toward the sun changes due to a natural wobble in the planet's tilt, which cycles every 26,000 years. Since the planet's tilt is what creates the seasons, the wobble means that Earth makes its closest pass by the sun in September for part of the cycle.

But the change in Earth's tilt has caused us to pass closest to the sun in January over the past 7,000 years.

This means less sunlight has been hitting the Arctic during its summertime, so the region should be cooling.

To estimate past temperatures, the research team looked at Arctic lake sediments and at previously published data of glacial ice cores and tree rings.

The team also examined a computer model of global climate based at the National Center for Atmospheric Research in Colorado.

Miller and colleagues found that the wobble in Earth's tilt causes Arctic temperatures to drop by about 0.36 degree Fahrenheit (0.2 degree Celsius) every thousand years during a cooling phase.

But human-caused global warming overwhelmed that gradual cooling in the mid-1990s, shooting temperatures up by about 2.5 degrees Fahrenheit (1.4 degrees Celsius) over the course of a few decades.

In fact, five of the warmest decades in the past 2,000 years have occurred between 1950 and 2000, according to the study, which will be published tomorrow in the journal Science.

Ecologist Syndonia Bret-Harte said she has seen the effects of climate change firsthand during her research on the changing Alaskan tundra.

The new study "doesn't seem that surprising, but it's good to confirm what researchers were already thinking," said Bret-Harte, of the Institute of Arctic Biology at the University of Alaska, Fairbanks.

Global Warming Amplifiers

The effects of climate change are powerfully amplified in the Arctic, which has been heating up faster than anywhere else on Earth.

That's because Arctic temperatures are greatly impacted by melting from both summer sea ice and permafrost, or frozen soil, experts say.

Summer Arctic sea ice, which hit a record low in 2007, will probably have dissolved completely by 2030.

Without swaths of white ice to reflect sunlight back into the atmosphere, those rays will instead be absorbed into the darker oceans—speeding up the region's warming.

And melting permafrost is already starting to release carbon dioxide and methane, two potent greenhouse gases that have long been trapped underground.

"It's pretty straightforward that amplification will continue in the future. There will be direct impacts in the Arctic," study co-author Miller said.

"The big issue is, when you melt ice, the sea level rises—that's a global issue, and that has major impacts."

Bret-Harte agreed that the amplifying effects are "going to continue until summer sea ice is gone—there's no way to reverse the trend in the short term."

But that doesn't mean people shouldn't act, she said.

"Realistically we're looking at a warmer world, and there are two questions: How do we adapt to that as humans but [also] slow down the pace of [climate] change? The faster it is, the harder it is to adapt."