"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."