Data collected from Antarctic and Greenland ice cores seems to show CO2 levels rising centuries after temperature increases. However, new research suggests that this may be a misinterpretation of the evidence.

Scientists have been using bubbles of air trapped in the ice when it was formed to detemine the CO2 level in the air at the time, the isotopes of elements like hydrogen, carbon and oxygen in the ice to calculate the temperature at which it formed and the depth of the ice sample in the core to estimate its age. This work led to the conclusion that CO2 levels rose centuries after the temperature increased.

In 2012, Australian and Danish scientists published research suggesting that the air bubbles were first trapped in snow which did not solidify into ice until new snow falls had buried it to a depth of 50 to 100 metres. This means that the air bubbles could be hundreds of years old when the ice in which they were found was formed. On this basis, their studies of Antarctic ice cores indicated that CO2 and temperatures rose at the same time and that the CO2 rise may have even been slightly ealier than the temperature rise.

Now, Frédéric Parrenin and colleagues at the French National Centre for Scientific Research in Grenoble, has delevoped a new approach using concentrations of nitrogen-15 in the air bubbles to estimate the depth at which the bubbles were trapped in the ice. Their research, published in the journal Science, confirmed that CO2 levels and temperature rises happened together, at least during the last deglaciation, when ice sheets retreated during an abrupt warming period 20,000-10,000 years ago.