CO2 Levels Highest in Two Million Years

Maggie Koerth-Baker
for National Geographic News
June 18, 2009
What happens when carbon dioxide levels skyrocket? Most climate scientists think they know the answer: global warming.

But to determine just how high temperatures may climb and how climate patterns may shift, researchers may need to pinpoint, for comparison, a time in our planet's past when a similar carbon dioxide jump happened.

Doing that may have just gotten a lot tougher—a new study says atmospheric carbon dioxide levels haven't been this high in more than two million years.

(Pictures: Five Global Warming "Tipping Points.")

Fuzzy But Far-Reaching New View

Carbon dioxide, or CO2, is a naturally occurring greenhouse gas that is also released into the atmosphere by the burning of fossil fuels, for example in cars and power plants (causes of global warming).

"We really don't know how high CO2 has been in the geologic past. Thus we don't know how sensitive the surface temperature of the Earth is to CO2," said Don DePaolo, head of the Earth Sciences Division at the Lawrence Berkley National Laboratory in California.

Most global warming predictions are based on fluctuations in CO2 levels and temperature that happened between a relatively recent series of ice ages, said DePaolo, who was not involved in the new study, which will appear in tomorrow's issue of the journal Science.

Chemicals in ice cores, for example, can show how CO2 levels have changed over time, down to five-year intervals.

But ice core records only go back about 800,000 years.

By studying chemicals in long-dead, single-celled plankton called foraminifera, though, the team behind the new study was able to extend the climate record back 2.1 million years (prehistoric time line).

The method doesn't provide as much detail, but it does give a pretty clear picture of what was going on at roughly thousand-year intervals.

Though he'd like to see the study results replicated for good measure, DePaolo is impressed by the report. For dates where the ice-core and plankton data overlap, the CO2 levels match, which suggests the new data for older time periods is accurate too.

Ice Age Theory Debunked?

The study team, led by geochemist Bärbel Hönisch, found evidence disproving the theory that the longer, stronger ice ages that kicked in about 850,000 years ago were caused by a steady, ongoing drop in CO2. Instead, CO2 levels seesawed over the 2.1 million years, dropping during ice ages then bouncing back.

What's more, the average CO2 level during warm periods was 38 percent lower than the average we see today.

That's significant, because it means that scientists will have to look back even further in time to find global warming answers.

Hönisch's next goal is to do just that.

"We know from the geologic record that, around 55 million years ago, the deep-sea temperature suddenly rose by 8 degrees C [14 degrees F]," said Hönisch, of the Lamont-Doherty Earth Observatory in Palisades, New York.

"It's a time that we would like to study, because it's probably the closest thing we'll find to what's happening today. And that's the best way to make estimates for our future."

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