A significant increase in carbon dioxide in the atmosphere, rather than changes to the Earth's orbit, was likely the main cause of global warming that ended the last ice age, a new study has found.
"The end of an ice age — you have a sense in your bones for what that means," said Jeremy Shakun, a visiting postdoctoral researcher at Harvard University, at a news conference organized by the journal Nature.
"It's obviously a significant change for the planet… I think this just provides a very palpable, tangible example of what rising CO2 can mean for the planet over the long term."
Shakun was the lead author of the study published Wednesday on Nature's website.
'It really leaves you thinking that CO2 was the big driver for global warming at the end of the last ice age.'—Jeremy Shakun, Harvard University
The analysis of historic temperature and atmospheric carbon dioxide data teased out of 80 ice and sediment cores from around the world also strongly discredits an argument used by some global warming skeptics to support the belief that rising carbon dioxide in the atmosphere does not cause the Earth to warm.
The last ice age ended about 20,000 to 10,000 years ago. Scientists had long agreed that the beginning of the process was first triggered by a gradual change in the Earth's orbit, which caused more sunlight to hit the northern hemisphere.
That was followed by an increase in carbon dioxide in the atmosphere and an increase in global temperatures. However, scientists were never sure how big a role the increase in carbon dioxide played in causing the warming.
Local, Antarctic data caused confusion
Researchers had a good record of global atmospheric carbon dioxide levels from bubbles trapped over tens of thousands of years in ice cores from Antarctica. But those ice cores only provided local information about the temperature, and they seemed to show that Antarctica started warming slightly before carbon dioxide levels rose.
"This is something that global warming skeptics have jumped on to say, 'Well, obviously CO2 doesn't cause warming because it came after the warming… so in turn, we have nothing to worry about today,'" Shakun said.
Many scientists did not think that was a good argument because the Antarctic records provided information about only local temperature changes, and they suspected average global temperatures didn't increase until later.
However, they weren't able to prove that until local temperatures records from around the world around the time of the last ice age were compiled into an average global temperature record for the purposes of the new study. The results clearly showed that average global temperatures didn't increase until after the rise in carbon dioxide.
"It really leaves you thinking that CO2 was the big driver for global warming at the end of the last ice age," Shakun said.
As the ice age came to a close, the amount of carbon dioxide in the atmosphere rose from 180 parts per million to 260 parts per million over around 7,000 years.
Shakun said current carbon dioxide levels in the atmosphere are around 390 parts per million, and they've risen about 100 parts per million over the last century, comparable to the amount they rose when the last ice age ended. However, he said, it will take centuries to see all the effects of the recent and the continuing increase in carbon dioxide levels, due to the "huge amount of inertia" within the Earth's climate systems.
While the current increase in atmospheric carbon dioxide has been caused by human activity, Shakun said it's not clear exactly what caused the rise at the end of the last ice age.
However, the theory is that the changes to the Earth's orbit caused more sunlight to hit the northern hemisphere, melting some of the ice sheets. The influx of freshwater into the North Atlantic disrupted ocean currents, causing the northern hemisphere to cool and the southern hemisphere to warm. The warming in the southern hemisphere may have allowed carbon dioxide to escape from the depths of the southern oceans by melting the sea ice that capped the oceans and kept the carbon dioxide trapped. Higher temperatures may also have strengthened winds that could help pull the carbon dioxide from the water into the atmosphere.