Scientists can study Earth’s climate as far back as 800,000 years by drilling core samples from deep underneath the ice sheets of Greenland and Antarctica. Detailed information on air temperature and CO2 levels is trapped in these specimens. Current polar records show an intimate connection between atmospheric carbon dioxide and temperature in the natural world. In essence, when one goes up, the other one follows.
There is, however, still a degree of uncertainty about which came first—a spike in temperature or CO2. Until now, the most comprehensive records to date on a major change in Earth’s climate came from the EPICA Dome C ice core on the Antarctic Plateau. The data, covering the end of the last ice age, between 20,000 and 10,000 years ago, show that CO2 levels could have lagged behind rising global temperatures by as much as 1,400 years. “The idea that there was a lag of CO2 behind temperature is something climate change skeptics pick on,” says Edward Brook of Oregon State University’s College of Earth, Ocean and Atmospheric Sciences. “They say, ‘How could CO2 levels affect global temperature when you are telling me the temperature changed first?’”
Frédéric Parrenin of the Laboratory of Glaciology and Geophysical Environment in France and a team of researchers may have found an answer to the question. His team compiled an extensive record of Antarctic temperatures and CO2 data from existing data and five ice cores drilled in the Antarctic interior over the last 30 years. Their results, published February 28 in Science, show CO2 lagged temperature by less than 200 years, drastically decreasing the amount of uncertainty in previous estimates.
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The wide margin of error in the EPICA core data is due to the way air gets trapped in layers of ice. Snowpack becomes progressively denser from the surface down to around 100 meters, where it forms solid ice. Scientists use air trapped in the ice to determine the CO2 levels of past climates, whereas they use the ice itself to determine temperature. But because air diffuses rapidly through the ice pack, those air bubbles are younger than the ice surrounding them. This means that in places with little snowfall—like the Dome C ice core—the age difference between gas and ice can be thousands of years.
Parrenin’s team addresses these concerns with a new method that establishes the different ages of the gas and ice. They measured the concentration of an isotope, nitrogen 15, which is greater the deeper the snowpack is. Once they were able to determine snowpack depth from the nitrogen 15 data, a simple model can determine the offset in depth between gas and ice and the amount of time the difference represents. The researchers then compared results from multiple locations to reduce the margin of error.
“Our method takes into account more data and shows that the age difference in Antarctic temperature and CO2 levels is less than we previously thought,” Parrenin says. “I think this could help to change the tone of discussions about climate change.”