By Ker Than
March 23, 2006


About 130,000 years ago, an ice age ended and there was a period of few centuries before the next one began. During this lull, Earth's temperature warmed, glaciers retreated and ice sheets melted. Sea levels rose by up to 20 feet.

Scientists warn that this could happen again -- and soon. But while the last great thaw was the result of a natural tilt in the Earth's axis towards the Sun, the next one will be caused by humans, some scientists argue.

If global warming continues at its current pace, by 2100 Earth could be up to 8 degrees Fahrenheit warmer than it is today. If steps are not taken soon to reduce greenhouse emissions, the Arctic will be as warm as it was 130,000 years ago and similar rises in sea level will occur, according to two new studies released today.

"Although the focus of our work is polar, the implications are global," said Bette Otto-Bliesner from the National Center for Atmospheric Research (NCAR) in Colorado, who was involved in both studies. "These ice sheets melted before and sea levels rose. The warmth needed isn't that much above present conditions."

The findings are detailed in the March 24 issue of the journal Science.

Antarctica's role

The studies are the first to definitively link Antarctica to the sea level rises that occurred between the last two ice ages, the researchers say.

Called the Last Interglaciation, this period lasted from about 129,000 to
116,000 years ago. Scientists had previously known that meltwater from Greenland and other Arctic ice sheets were important factors in sea level rises during this period, but it was unclear what Antarctica's contribution was.

The new results, which draw upon a combination of computer simulations and paleoclimate records, suggests that Arctic melting caused sea levels to rise by up to 11 feet during the Last Interglaciation.

This in turn triggered melting in Antarctica, causing sea levels to rise further.

Rising seas

The researchers combined a computer climate prediction model, the NCAR-based Community Climate System Model (CCSM), with ice sheet simulations to estimate what the Earth's climate was like 130,000 years ago.

They crosschecked the computer's estimates with data from natural records of ancient climate change such as sediments, fossils and ice cores.

All the methods indicated similar warming. However, the computer model showed meltwater from Greenland and other Arctic sources raising sea levels by only about 11 feet, while coral records indicate that the sea level actually rose up to 20 feet.

The researchers think this discrepancy can be explained by meltwater from Antarctica, which could have caused sea levels to rise by another 6 to 10 feet.

Rising seas from Arctic meltwater would have destabilized ice shelves in Antarctica, causing them to melt or break apart and fall into the ocean.

"It's just like throwing a bunch of ice cubes into a full glass of water and watching the water spill over the top," said Jonathon Overpeck of the University of Arizona, who was also involved in both studies.

This hypothesis is consistent with earlier studies based on fossilized microscopic marine organisms, which showed that parts of the West Antarctic Ice Sheet disappeared at some point over the last several hundred thousand years.

3 feet per century

Once the researchers were confident that their computer model could accurately simulate past climate conditions, they used it to predict future climate change.

"Getting the past climate change correct in these models gives us more confidence in their ability to predict future climate change," Otto-Bliesner said.

The researchers concluded that if greenhouse gas emissions are not curbed and we continue with "business as usual," Arctic temperatures will become at least as warm as it was during the Last Interglacial.

If this happens, humanity will be committing the planet to a sea level rise as drastic as, or worse than, the 20-foot increase that occurred 130,000 years ago, Overpeck said.

"Paleoclimatic data shows that we could get 3 feet of sea level rise per century," he told LiveScience. "That's what we would be triggering later in the century. We'd be committing to a sea level rise of that magnitude."

Currently, global sea level rises at a rate of about an inch per decade.

Not too late

Scientists warn that if the warming seen 130,000 years ago occurred today, it would be accelerated by global warming and other human activities.

"The ocean is the vehicle by which this heat is getting to the edges of the ice sheets, so if you increase the rate at which you're putting heat into the ocean, then it would further accelerate the melting," said Robert Bindschadler, a glaciologist at NASA's Goddard Space Flight Center who was not involved in the study by Otto-Bliesner and Overpeck.

Bindschadler is the author of another study, also published in this week's issue of Science, which shows how glaciers can be melted from below by pockets of warm water.

The pace of Arctic melting would also quicken because of pollution-darkened snow, scientists say, which absorbs more sunlight and melts faster than regular snow.

The process will become irreversible sometime in the second half of the 21st century unless steps are taken in the next few decades to curb greenhouse gas emissions, Overpeck said.

"We need to start serious measures to reduce greenhouse gases within the next decade. If we don't do something soon, we're committed to four-to-six meters (13 to 20 feet) of sea level rise in the future."


By John Roach
National Geographic News
March 30, 2006


The air over Antarctica has warmed dramatically over the past 30 years, according to a new study of archived data collected by weather balloons floated over the icy continent.

The greatest warming -- nearly 1.4ºF (0.75ºC) per decade in the winter -- has occurred about 3 miles (5 kilometers) above the surface.

Scientists are hard pressed to explain the temperature spike, which is three times larger than the global average. The rise cannot be explained by the climate models scientists use to predict the effects of global warming from increased greenhouse gases.

"That could point to some mechanism of climate change we don't understand, a failing in these models, or just a result of natural climate variability," said John Turner, a climate scientist with the British Antarctic Survey in Cambridge, England.

Meanwhile, surface temperatures have increased 4.5ºF (2.5ºC) in the last 50 years on the Antarctic Peninsula, the mountainous arm that trails toward the southern tip of South America.

"But the rest of Antarctica has done virtually nothing [at the surface]", Turner said.

Turner is the lead author of the study, which appears in tomorrow's issue of the journal Science.

David Bromwich, a meteorologist with the Byrd Polar Research Center at Ohio State University in Columbus, said there's "no doubt this [warming] is real."

But, he added, the finding only "deepens the mystery of what's going on over Antarctica."

Potential Implications

According to Turner, the unexpected warming could affect snowfall across the continent, which might have implications for global sea-level rise.

Snowfall records of the past three decades show no change, Turner said. "But measuring snowfall is hard. Measuring temperature is obviously easier," he added.

Scientists expect the warming to create a small increase in snowfall over Antarctica, as the warmer, moister air blows over the continent and is cooled to form snow.

This in turn could mitigate, to a small extent, sea-level rise by "locking up" meltwater in the form of snow.

Since the atmospheric warming is greatest three miles (five kilometers) up in the atmosphere, Turner said it is unlikely to result in extensive melting of ice on the surface. The continent's tallest mountains are 2.5 miles (4 kilometers) high.

Complex Signals

Turner and his colleagues are now trying to understand why the atmosphere warming is disconnected from surface temperatures.

One possibility, he said, is that the region is showing a greater than expected sensitivity to greenhouse gases in the winter.

Antarctica is dark during the winter months, which means there is no sunlight to heat the surface.

However, the heat that is on the surface continues to radiate into the atmosphere, where it is trapped by the blanket of greenhouse gases, Turner explained.

Alternatively, the warming may reflect a change in air circulation patterns, though data collected at Antarctic weather stations suggest this has not happened, he said.

Bromwich, of the Byrd Center, said the findings fit the emerging picture of Earth experiencing the effects of global warming, such as the widely reported melting in the Arctic.

"To understand what is happening to our world, we also need to understand what is happening in Antarctica," Bromwich said.

"This [research] deepens the mystery rather than solves it, but it shows us the direction we should be looking."

Informant: NHNE


Antarctic Air is Warming Faster Than Rest of World


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