Scripps scientists answer classic "chicken or egg" question of climate change

provided by Scripps Institution of Oceanography


t is a "chicken or egg" question that scientists studying the dramatic swings in climate that occur at the end of ice ages have pondered for years: which comes first -- a rise in global temperature followed by an increase in atmospheric carbon dioxide or vice versa?

New analysis of carbon dioxide trapped inside Antarctic ice core samples spanning 250,000 years indicates the former is true.

Scientists at the Scripps Institution of Oceanography at the University of California, San Diego, reported in the March 12 issue of Science that CO2 levels did not rise until hundreds of years after the warming periods that triggered the end of the last three ice ages.

"The atmospheric CO2 level in glacial times is about 180 to 200 ppm and then when the temperature rises it goes up to about 280 ppm," said Martin Wahlen, a professor in the Scripps Geosciences Research Division who coauthored the paper. "What we have found is that at these periods when the climate is transitioning from a glacial to an interglacial period, the atmospheric CO2 concentrations lag behind the rise in temperature by about 600 years."

The scientists further discovered that the elevated CO2 levels sometimes persisted for thousands of years after the onset of the next ice age. How long the CO2 levels remained high appears to be determined by the duration of the preceding warm period, Wahlen said.

"The way we interpret this is that if the climate stays warm for only a short period of time, then the amount of terrestrial biosphere which can be built up is relatively small. Thus, there is less organic material to decay and put CO2 back into the atmosphere," Wahlen said. "But when it stays warm for long periods, then the amount of biosphere is larger and the following CO2 flux to the atmosphere from decaying organic material lingers for quite some time."


The ice cometh, man


The ice core records were derived from samples taken from the Vostok ice core and the Taylor Dome ice core, both in Antarctica. The scientists analyzed CO2 trapped inside air bubbles in ice samples from these cores representing glacial-to-interglacial transition periods that started 18,000, 135,000 and 240,000 years ago, each lasting about 10,000 years. The scientists then compared the CO2 records with temperature records gleaned from deuterium isotope concentrations in ice core samples over the same periods measured by French scientists.

Wahlen notes that the burning of fossil fuels since the beginning of the Industrial Revolution already has caused about the same change in CO2 levels that the planet experienced from the depth of the last ice age to the beginning of the current warm interglacial period called the Holocene that began about 11,000 years ago. Carbon dioxide levels have jumped from about 280 ppm at the beginning of the Industrial Revolution to about 360 ppm today, a change of about 80 ppm. During the transition from the last glacial maximum to the beginning of the Holocene, CO2 increased from 200 ppm to 280 ppm.

In order to trace the change in CO2 levels over time, Wahlen's team analyzes tiny air bubbles that are trapped within the layers of snow-turned-to-ice that have been deposited in polar locations such as Antarctica and Greenland during the past hundreds of thousands of years. Similar to the way in which sediments are laid down at the bottom of the ocean, the ice cores are made up of ice-crystal layers that become older with depth. Using physical, isotopic, and chemical markers, scientists are able to count the layers at near-yearly resolution back through time, much as researchers analyze annual tree rings.

In order to analyze the ice samples, the scientists don hooded parkas and enter a freezer maintained at -16.6 degrees Fahrenheit where they cut the ice into small cubes. Each sample is then placed in a vacuum chamber and crushed by a bed of steel needles to release the CO2 trapped inside. The researchers then use a tunable infrared laser spectrometer to analyze the CO2 in the released gas. Because each sample contains only a minuscule amount of carbon dioxide, the measurements must be very accurate to measure concentrations down to a precision of a few parts per million.

Other coauthors of the paper include Hubertus Fischer, a former Scripps post-doctoral student now at the Alfred Wegener Institute in Bremerhaven, Germany, Jesse Smith, Derek Mastroianni, and Bruce Deck, all of Scripps. The work was funded by the National Science Foundation.