Global view answers ice age CO2 puzzle

Paleoclimate researcher Jeremy Shakun. Credit: Harvard University

Paleoclimate researcher Jeremy Shakun. Credit: Harvard University

Previous data suggesting that the world started warming out of the last ice age before CO2 levels in the atmosphere started rising don’t show the full picture. That’s according to US, French and Chinese scientists who have added to those Antarctic measurements with more taken from 80 locations across the globe. Harvard University’s Jeremy Shakun and colleagues show the greenhouse gas rises before temperature, supporting the case that CO2 drove climate change then, as it is now. “This provides a very tangible example of what rising CO2 can mean for the climate over the long term,” Jeremy said.

In the 1980s, researchers began building the history of CO2 in the atmosphere from cylinders of ice drilled from the Antarctic. Bubbles in the ice contain air from the time they formed, which researchers can measure. They can also figure out how old the ice holding the bubbles is from how deep it is in the core. And finally they can also work out temperature from the amount of the different forms, known as isotopes, of elements like hydrogen, carbon and oxygen in the ice. That’s because the temperature at which the snow that eventually became the ice formed affects how much of each it contains. And because some isotopes are radioactive and decay to a more stable isotope with time, studying them gives scientists another way to check the ice’s age.

The 800,000 year record of atmospheric CO2 from Antarctic ice cores, and a reconstruction of temperature based on hydrogen isotopes in the ice. The current CO2 concentration of 392 parts per million (ppm) is shown by the blue star. Credit: Jeremy Shakun/Harvard University

The 800,000 year record of atmospheric CO2 from Antarctic ice cores, and a reconstruction of temperature based on hydrogen isotopes in the ice. The current CO2 concentration of 392 parts per million (ppm) is shown by the blue star. Credit: Jeremy Shakun/Harvard University

Such methods show temperature and CO2 levels rising and falling together for 800,000 years, Jeremy told journalists over the phone on Tuesday. “The question is: Which is the cause and which is the effect?” he asked. “If you look up close you see temperature changed before CO2 did. This is something the global warming skeptics have jumped on to say, ‘Obviously CO2 doesn’t cause warming because it came after the warming in these records’. But these ice cores only tell you about temperatures in Antarctica. For the same reason that you don’t look at just one thermometer from London or New York to prove or disprove global warming, you don’t want to look at just one spot in the map to reconstruct the past either.” Read the rest of this entry »