Scientists are looking back to a time when "greenhouse gases" were not the problem they are today, and it is giving them a clearer picture of how people are making it worse.
A team of European researchers analyzed tiny air bubbles preserved in Antarctic ice for millennia and determined there is more carbon dioxide in the atmosphere now than at any point during the last 650,000 years.
The study by the European Project for Ice Coring in Antarctica, published Friday in the journal Science, promises to spur "dramatically improved understanding" of climate change, said geosciences specialist Edward Brook of Oregon State University.
Today, scientists directly measure levels of carbon dioxide and other greenhouse gases, which accumulate in the atmosphere as a result of fuel-burning and other processes. Those gases help trap solar heat, like the greenhouses for which they are named, resulting in a gradual warming of the planet.
Those measurements are disturbing: Levels of carbon dioxide have climbed from 280 parts per million two centuries ago to 380 ppm today.
Earth's average temperature, meanwhile, increased about 1 degree Fahrenheit in recent decades, a relatively rapid rise. Many climate specialists warn that continued warming could have severe impacts, such as rising sea levels and changing rainfall patterns.
Skeptics sometimes dismiss the rise in greenhouse gases as part of a naturally fluctuating cycle. The new study provides ever-more definitive evidence countering that view, however.
Deep Antarctic ice encases tiny air bubbles formed when snowflakes fell over hundreds of thousands of years. Extracting the air allows a direct measurement of the atmosphere at past points in time, to determine the naturally fluctuating range.
A previous ice-core sample had traced greenhouse gases back about 440,000 years. This new sample, from East Antarctica, goes 210,000 years further back in time.
Today's still rising level of carbon dioxide already is 27 percent higher than its peak during all those millennia, said lead researcher Thomas Stocker of the University of Bern, Switzerland.
"We are out of that natural range today," he said.
Moreover, that rise is occurring at a speed that "is over a factor of a hundred faster than anything we are seeing in the natural cycles," Stocker added. "It puts the present changes in context."
The team, which included scientists from France and Germany, found similar results for methane, another greenhouse gas.
Researchers also compared the gas levels to the Antarctic temperature over that time period, covering eight cycles of alternating glacial or ice ages and warm periods. They found a stable pattern: Lower levels of gases during cold periods and higher levels during warm periods.
The bottom line: "There's no natural condition that we know about in a really long time where the greenhouse gas levels were anywhere near what they are now. And these studies tell us that there's a strong relationship between temperature and greenhouse gases," said Oregon State's Brook. "Which logically leads you to the conclusion that maybe we should worry about temperature change in the future."
A lengthening history of greenhouse gas concentrations should help climate specialists build better models about what the future might bring, Stocker said. It also may help answer additional questions such as how long ago humans started influencing greenhouse gas accumulations, and what impact other factors such as ocean currents play in the complexities of climate change.
Just a decade ago, scientists weren't sure it was possible to trace greenhouse gas concentrations back so far in ice. Now, Brook is part of another international research team preparing to hunt an ice-core sample dating back a million years or more, hoping to reach eras when Earth's temperature was significantly warmer.