A new study shows that the increase of atmospheric carbon dioxide that contributed to the end of the last ice age more than 10,000 years ago did not occur gradually but rather was characterized by three abrupt pulses.
Scientists are not sure what caused these abrupt increases, during which carbon dioxide levels rose about 10 to 15 parts per million – or about 5 percent per episode – during a span of one to two centuries. It likely was a combination of factors, they say, including ocean circulation, changing wind patterns and terrestrial processes.
The finding, published Oct. 30 in the journal Nature, casts new light on the mechanisms that take the Earth in and out of ice ages.
“We used to think that naturally occurring changes in carbon dioxide took place relatively slowly over the 10,000 years it took to move out of the last ice age,” said lead author Shaun Marcott, who did the work as a postdoctoral researcher at Oregon State University and is now at the University of Wisconsin-Madison. “This abrupt, centennial-scale variability of CO2 appears to be a fundamental part of the global carbon cycle.”
Previous research has hinted at the possibility that spikes in atmospheric carbon dioxide may have accelerated the last deglaciation, but that hypothesis had not been resolved, the researchers say. The key to the new finding is the analysis of an ice core from the West Antarctic that provided the scientists with an unprecedented glimpse into the past.
“An important element in ice core research is being able to calculate the difference in age between the gas, in this case carbon dioxide, and the ice that surrounds it. To do that, we need good measurements of both the snow accumulation rate and of temperature,” said Eric Steig, a UW professor of Earth and space sciences.
Steig and T.J. Fudge, a postdoctoral researcher in Earth and space sciences, are co-authors of the Nature paper, and provided analysis from an ice core from the surface to more than 2 miles deep and covering some 68,000 years of climate history, taken from the West Antarctic Ice Sheet.
Fudge used electrical measurements on the core to detect seasonal variations in acidity, an indicator of thickness of the snow layer from one year to the next. Steig’s lab analyzed the ratio of heavy oxygen to light oxygen in the ice to provide a measure of temperature differences through time. Details of that work were published in Nature last year.
Past climate studies have been hampered by the limitations of previous ice cores. Cores from Greenland, for example, provide unique records of rapid climate events going back 120,000 years, but high concentrations of impurities don’t allow researchers to accurately determine atmospheric carbon dioxide records. Antarctic ice cores have fewer impurities, but generally have had lower resolution and so provided less-detailed information about atmospheric carbon dioxide.
However, the West Antarctica core has “extraordinary detail,” said co-author Edward Brook, an Oregon State paleoclimatologist. Because the area where the core was taken gets high annual snowfall, the new ice core provides one of the most-detailed records of atmospheric carbon dioxide.
“It is a remarkable ice core and it clearly shows distinct pulses of carbon dioxide increase that can be very reliably dated,” Brook said. “These are some of the fastest natural changes in CO2 we have observed, and were probably big enough on their own to impact the Earth’s climate.
“The abrupt events did not end the ice age by themselves,” he said. “That might be jumping the gun a bit. But it is fair to say that the natural carbon cycle can change a lot faster than was previously thought – and we don’t know all of the mechanisms that caused that rapid change.”
The researchers say the increase in atmospheric carbon dioxide from the peak of the last ice age to complete deglaciation was about 80 parts per million, taking place over 10,000 years. Thus, the finding that 30 to 45 ppm of the increase happened in just a few centuries was significant.
The overall rise of atmospheric carbon dioxide during the last deglaciation was thought to have been triggered by the release of CO2 from the deep ocean – especially the Southern Ocean. However, the researchers say that no obvious ocean mechanism is known that would trigger rises of 10 to 15 ppm over a timespan as short as one to two centuries.
“The oceans are simply not thought to respond that fast,” Brook said. “Either the cause of these pulses is at least part terrestrial, or there is some mechanism in the ocean system we don’t yet know about.”
One reason the researchers are reluctant to pin the end of the last ice age solely on carbon dioxide increases is that other processes were taking place, Marcott said. As carbon dioxide levels were increasing, atmospheric methane was increasing at the same or a slightly higher rate. He added that ocean circulation changed during at least two of the pulses, and such changes would have affected carbon dioxide and, indirectly, methane by affecting global rainfall patterns.
The research was supported by the National Science Foundation.