Despite vast changes in climate since the early Pleistocene, 2.1 million years ago, carbon dioxide levels have stayed pretty stable until lately, according to a study published online today in Science.

All of that consistency, in which peak CO2 levels averaged 280 parts per million, makes today's concentration—385 parts per millions, which is 38 percent higher—all the more stunning, the authors report. Previous work showed stable CO2 levels going back about 650,000 years. 

For the new estimate, the researchers analyzed ancient plankton shells beneath the Atlantic Ocean floor, rather than relying on data from polar ice cores, which can only give readings for the past 800,000 years. The tiny shells provided info on CO2 levels as well as temperature and ocean acidity.

The findings suggest that previous ice ages may have had more to do with other forces, such as changes in the Earth's orbit, than with plummeting levels of carbon dioxide in the atmosphere. But, spikes in CO2 levels did match up with periods of warming.

"Our data continues to suggest that greenhouse gases and global climate are intimately linked," lead study author Bärbel Hönisch, a geochemist at Columbia University's Lamont Doherty Earth Observatory, said in a statement.

Image of Bärbel Hönisch diving for plankton courtesy of Steve Doo