A team of climate experts say they've solved the chicken-or-egg question—Which came first? Carbon dioxide or temperature increases?—surrounding the global warming debate for years: Around 10,000 years ago, increased carbon dioxide levels in the atmosphere caused the end of the last Ice Age.
Scientists have known for decades that an increased level of carbon dioxide gas in the atmosphere accompanied higher global temperatures, but no one was able to prove that the gas increases came before higher temperatures.
"We've known that CO2 was a big driver of the end of the ice age for a while now, but this is just another nail in that coffin," says Jeremy Shakun, who completed much of the research for his Ph.D. at Oregon State University and is now a post-doctoral fellow for NOAA at Harvard and Columbia Universities. His article was published Wednesday in the science journal Nature.
Global warming skeptics argue that carbon dioxide levels—measured in icicle records in Antarctica—seemed to have increased slightly after warming temperatures. That argument has been used by many, including Texas Rep. Joe Barton, in an attempt to discredit Al Gore's movie An Inconvenient Truth.
"Historically, a rise in CO2 concentrations did not precede a rise in temperatures, but actually lagged temperature by 200 to 1,000 years," Barton argued. "The temperature appears to drive CO2, not vice versa. On this point, Mr. Vice President, you're not just off a little. You're totally wrong."
Shakun says his research vindicates Gore. In the past, scientists measured temperatures near where the icicles were sampled—in Antarctica. But when averaging the global temperature change, he found on average, temperatures changed after the CO2 levels did.
"We knew CO2 was connected to global warming, but it got a little chicken-and-egg like. The icicle records tell you what global carbon dioxide levels were, but they only tell you what the local temperature over Antarctica was," he says. "We said 'Let's look at the bigger picture,' and found that there's a strong argument that global temperature changes after the CO2 does."
His team compiled temperature records from 80 sites around the world and found that the temperature rises closely matched the global warming curve scientists predicted. The temperature change in Antarctica lags behind the rest of the world by a couple centuries, according to those records.
Leighton Steward, who runs Plants Need CO2, an organization that advocates for putting more carbon dioxide into the atmosphere in order to fertilize plants, says there is more than one climate change-driver, and that temperature records dating back several thousand years could be inaccurate.
Scientists aren't positive why CO2 levels rose during the end of the Ice Age, but they have a few hypotheses. Their best guess is that pent up carbon dioxide at the bottom of the Southern Ocean near Antarctica somehow escaped, but Shakun says that's a "big outstanding mystery" in climate science.
Shakun says his study should open the eyes of those who say human carbon emissions don't contribute to modern climate change.
"Back then, it was natural, but the key thing is—we found that carbon dioxide was driving global warming back then. The Earth doesn't care where the carbon dioxide came from—if it caused it back then, it's going to cause it today," he says. "Over the last 7,000 years of the Ice Age, the CO2 levels rose by 100 parts per million. Over the last century, we've run up the same amount. You look at that, and it's sobering."