Carbon Dioxide Rise Came First

A new study shows that the warming of Earth at the end of its last ice age was preceded by a rise in atmospheric CO₂, supporting a causal relationship between increasing atmospheric CO₂ and global warming (Nature, DOI: 10.1038/nature10915). These phenomena have long been known to be linked, but ice-core studies had generated conflicting results about which event happened first. Now, Jeremy D. Shakun, at Harvard University, and colleagues have assembled data from 80 temperature records taken around the world, dating back 10,000 to 20,000 years, when the last ice age ended. Previous studies linked global atmospheric CO₂ levels with temperatures in the Antarctic inferred from Antarctic ice cores. But armed with the worldwide temperature data, the group generated a picture of global temperatures in relation to atmospheric CO₂, showing clearly that atmospheric CO₂ levels rose before global temperature changed. “Most scientists are not in doubt about the human-enhanced greenhouse effect,” Shakun says. “There are nearly a dozen strong pieces of evidence that it is affecting global climate. This is just one more log on the fire that confirms it.”