Next Ice Age Delayed by Global Warming, Study Says

Christine Dell'Amore
National Geographic News
September 3, 2009
Humans are putting the brakes on the next ice age, according to the most extensive study to date on Arctic climate change.

The Arctic may be 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. (See photos of how climate change is transforming the Arctic.)

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 may have 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 26,000-year-long wobble, which causes the planet to spin on is axis like an unstable top, so that a line drawn from the axis would trace a cone in the sky.

The wobble causes Earth to make its closest pass by the sun in different months over the long term. For the past 7,000 years, Earth has passed closest to the sun in January.

This means less sunlight has been hitting the Arctic during its summertime, so the region should be cooling. (See an Arctic map.)

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 four of the five warmest decades in the past 2,000 years occurred after 1950, 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."

© 1996-2008 National Geographic Society. All rights reserved.