Roughly 20,000 years ago the great ice sheets that buried much of Asia, Europe and North America stopped their creeping advance. Within a few hundred years sea levels in some places had risen by as much as 10 meters—more than if the ice sheet that still covers Greenland were to melt today. This freshwater flood filled the North Atlantic and also shut down the ocean currents that conveyed warmer water from equatorial regions northward. The equatorial heat warmed the precincts of Antarctica in the Southern Hemisphere instead, shrinking the fringing sea ice and changing the circumpolar winds. As a result—and for reasons that remain unexplained—the waters of the Southern Ocean may have begun to release carbon dioxide, enough to raise concentrations in the atmosphere by more than 100 parts per million over millennia—roughly equivalent to the rise in the last 200 years. That CO2 then warmed the globe, melting back the continental ice sheets and ushering in the current climate that enabled humanity to thrive.
That, at least, is the story told by a new paper published in Nature on April 5 that reconstructs the end of the last ice age. Researchers examined sediment cores collected from deep beneath the sea and from lakes as well as the tiny bubbles of ancient air trapped inside ice cores taken from Antarctica, Greenland and elsewhere. (Scientific American is part of Nature Publishing Group.) The research suggests that—contrary to some prior findings—CO2 led the prior round of global warming rather than vice versa, just as it continues to do today thanks to rising emissions of CO2 and other greenhouse gases.
"We find that global temperature lags a bit behind the CO2 [levels]," explains paleoclimatologist Jeremy Shakun, a National Oceanic and Atmospheric Administration fellow at Harvard and Columbia universities, who led the research charting ancient CO2 concentrations and global temperatures. "CO2 was the big driver of global warming at the end of the Ice Age."
Shakun and his colleagues started by creating the first global set of temperature proxies—a set of 80 different records from around the world that recorded temperatures from roughly 20,000 years ago to 10,000 years ago. Ranging from the magnesium levels in microscopic seashells pulled from ocean sediment cores to pollen counts in layers of muck from lakebeds, the proxies delivered thousands of temperature readings over the period. "Ice cores only tell you about temperatures in Antarctica," Shakun notes of previous studies that relied exclusively on an ice core from Antarctica that records atmospheric conditions over the last 800,000 years. "You don't want to look at one spot on the map for global warming."
Comparing the global set of temperature records with the levels of CO2 in the ancient air bubbles trapped in ice cores reveals that global average temperatures started to rise at least a century after CO2 levels began to creep up. That's the reverse of what seems to have happened in Antarctica, where warming temperatures precede rising CO2 levels. But that local warming may be explained by this shutdown of ocean currents as a result of massive glacial melt in the Northern Hemisphere—a result further reinforced by computer modeling using the data gathered from the real-world record.
The reason for the retreat of the ice sheets remains elusive, however. Whereas there was a change in the relative strength of the sun roughly 20,000 years ago thanks to variations in the planet's orbit, it was smaller than changes that preceded it and failed to trigger a melt. In fact, ice cores from Greenland suggest there was an even larger warming event in the north roughly 60,000 years ago, notes climate scientist Eric Wolff of the British Antarctic Survey in a comment on the findings also published in Nature.
"We know that the only thing changing in the Northern Hemisphere [20,000 years ago] were these orbital changes" that affect the amount of sunlight striking the far north, explains geologist Peter Clark of Oregon State University, who guided Shakun's research. The melting in the north could have been triggered "because the ice sheets had reached such a size that they had become unstable and were ready to go." This may also help explain the cyclical rise and fall of ice ages over hundreds of thousands of years.
Just where the extra carbon dioxide came from remains unclear as well. "There is no convincing evidence that a sufficiently large reservoir of old metabolic carbon existed in some mysterious location in the glacial ocean only to be ventilated during deglaciation," argues paleoclimatologist Lowell Stott of the University of Southern California, who was not involved in the study. But a paper published online in Science on March 29 suggests that the extra CO2 did come from the Southern Ocean, based on analysis of the isotopes of carbon embedded in the molecule most responsible for global warming. Stott also argues that the timing of the warming versus that of increasing CO2 levels remain too close to be sure which came first.
Of course, modern global warming stems from a clear cause—rising levels of CO2 (and other greenhouse gases) from fossil fuel burning, cutting down forests and other human activities. And, in the past rising CO2 levels at the very least magnified global warming, ushering in the relatively balmy, stable climate sometimes called the "long summer" that has allowed human civilization to flourish. Humanity has now raised global CO2 levels by more than the rise from roughly 180 to 260 ppm at the end of the last ice age, albeit in a few hundred years rather than over more than a few thousand years. "The end of an ice age, you have a sense in your bones what that means: a big, significant change for the planet," Shakun says. "It's a tangible example of what rising CO2 can mean for the planet over the long-term."
In fact, the amount of global warming already guaranteed by existing concentrations of CO2 in the atmosphere—392 ppm and still rising—will also play out over centuries, if not millennia. "The rise at the end of the Ice Age and today is about the same [a rise of 100 ppm] and we're going to be well above and beyond," most likely increasing concentrations of greenhouse gases by hundreds of parts per million from preindustrial levels, Shakun notes. "We will only see some of that realized in this next century. It will be many centuries and beyond to feel the full effects."