A team of scientists has established a clear cause-and-effect relationship between rising levels of carbon dioxide and global warming that ended the last Ice Age.
The Earth, as seen from Apollo 17 (NASA / JPL-Caltech)
The new study, published in the journal Nature, identifies this relationship and provides compelling evidence that rising CO2 caused much of the global warming.
“The key to understanding the role of CO2 is to reconstruct globally averaged temperature changes during the end of the last Ice Age, which contrasts with previous efforts that only compared local temperatures in Antarctica to carbon dioxide levels,” said lead author Jeremy Shakun, a paleoclimatologist and a National Oceanic and Atmospheric Administration post-doctoral fellow at Harvard University and Columbia University. “Carbon dioxide has been suspected as an important factor in ending the last Ice Age, but its exact role has always been unclear because rising temperatures reflected in Antarctic ice cores came before rising levels of CO2.”
“But if you reconstruct temperatures on a global scale – and not just examine Antarctic temperatures – it becomes apparent that the CO2 change slightly preceded much of the global warming, and this means the global greenhouse effect had an important role in driving up global temperatures and bringing the planet out of the last Ice Age,” the paleoclimatologist added.
The team suggests that small changes in the Earth’s orbit affected the amount of sunlight striking the northern hemisphere, melting ice sheets that covered Canada and Europe. Fresh water flowed off of the continent into the Atlantic Ocean, where it formed a lid over the sinking end of the Atlantic Meridional Overturning Circulation – a part of a global network of currents that brings warm water up from the tropics and today keeps Europe temperate despite its high latitudes.
The ocean circulation warms the northern hemisphere at the expense of the south, but when the fresh water draining off the continent at the end of the last Ice Age entered the North Atlantic, it essentially put the brakes on the current and disrupted the delivery of heat to the northern latitudes.
“When the heat transport stops, it cools the north and heat builds up in the Southern Hemisphere,” Shakun explained. “The Antarctic would have warmed rapidly, much faster than the time itakes to get CO2 out of the deep sea, where it was likely stored. The warming of the Southern Ocean may have shifted the winds as well as melted sea ice, and eventually drawn the CO2 out of the deep water, and released it into the atmosphere. That, in turn, would have amplified warming on a global scale.”
The researchers constructed a record of global surface temperature from 80 temperature reconstructions spanning the end of the Ice Age and found that average temperature around the Earth correlated with – and generally lagged behind – rising levels of carbon dioxide.
“Changes in solar radiation were the likely trigger for the series of effects that followed,” said Prof. Peter Clark of Oregon State University, a co-author on the paper. “It has long been known that Earth’s slow wobble is caused primarily by the gravitational influences of the larger planets, such as Jupiter and Saturn, which pull and tug on the Earth in slightly different ways over periods of thousands of years.”
“CO2 was a big part of bringing the world out of the last Ice Age,” Shakun said. “And it took about 10,000 years to do it. Now CO2 levels are rising again, but this time an equivalent increase in CO2 has occurred in only about 200 years, and there are clear signs that the planet is already beginning to respond.”
