Earth’s ice ages are typically thought of as seemingly unending periods of bitter cold. But a new study suggests bursts of carbon dioxide (CO2) often entered the atmosphere during these times, providing decades or even centuries of relative warmth amid 10,000-year stretches of chill. Such pulses may have caused glaciers and ice sheets to retreat somewhat, thus opening up new areas for plants and animals.
Researchers can track fluctuations in atmospheric CO2 by looking at ice cores drilled in Antarctica, Greenland, and some tall mountains in lower latitudes. The cores contain trapped air bubbles as fluffy snow was compressed into ice by the weight of newer, fresher overlying layers.
Some of these climate chronicles go back hundreds of thousands of years. But high-resolution data, tracking centennial-scale or shorter variations, have been lacking, in part because of contamination from techniques used to extract CO2 from the ice.
In the new study, Christoph Nehrbass-Ahles, a climate physicist at the University of Cambridge, and his colleagues came up with a way to extract CO2 from ancient ice that’s about three times more precise than previous methods. The ice-crushing technique they developed doesn’t involve friction between metal parts, which can be a source of fresh CO2. The new approach allowed them to measure CO2 concentrations to within one part per million.
Nehrbass-Ahles’s team then analyzed portions of a 3.5-kilometer-long ice core drilled at one of the highest points in eastern Antarctica. Their samples capture times between 330,000 and 450,000 years ago—an interval that includes one complete ice age as well as the warm spells on either side. On average, each data point was separated from its neighbors by about 300 years, a four- to sixfold improvement in time resolution over previous studies.
Across the 120,000-year interval, the researchers identified eight episodes when CO2 levels rose quickly. Seven of those eight pulses lasted more than 100 years, and during six of them, CO2 levels rose more than nine parts per million, the researchers report today in Science. (For comparison, before human activity started to pump large amounts of CO2 into the air at the beginning of the Industrial Revolution, the global average of atmospheric CO2 was about 280 parts per million.)
Previously, researchers have discerned century-scale jumps in CO2 only from ice samples representing the late stages of the most recent ice age. So, to find them in ice that instead accumulated during a warm, interglacial period was a surprise, Nehrbass-Ahles says.
The team’s new analysis shows Earth’s climate “can change a lot faster than we’ve previously thought,” says Shaun Marcott, a paleoclimatologist at the University of Wisconsin, Madison, who wasn’t involved in the new study. The resulting shifts in ecosystems, although short-lived, could have been profound.
Nehrbass-Ahles and his colleagues suggest the jumps in atmospheric CO2 result from changes in a conveyor belt of ocean currents in the Atlantic Ocean. When the Gulf Stream weakens, that warm current brings less heat to North Atlantic waters. Those changes in sea-surface temperature, in turn, cause weather patterns in the tropics to shift, triggering a shrinkage of wetlands, Nehrbass-Ahles says. The carbon-rich material stored in those formerly swampy zones then decomposes, sending a pulse of CO2 into the air to warm the climate.
In modern times, these ancient pulses wouldn’t be impressive: A 10-part-per-million jump in CO2, which may have unfolded over 100 years or more in preindustrial times, could these days take only 4 or 5 years to transpire.
Nevertheless, Marcott says, “Finding these rapid jumps [in CO2] is quite exciting from my perspective.” The team’s results show sudden pulses of CO2 occur not only as ice ages are waning, but can be triggered at any time during the ice age cycle.