Next icy era may be on hold

The next big chill may be overdue. If humans hadn’t boosted levels of atmospheric carbon dioxide, Earth’s next frosty bout of glacial growth probably would have already started, new research suggests.

For the last 11,700 years, Earth has been on a break between periods of ice expansion called glaciations. A similar interglacial period occurred around 790,000 years ago. A new climate reconstruction based on sediments from an ancient Italian lake shows that this historical interglacial lasted only about 10,800 years. The result suggests that the current interglacial should be wrapping up — but it’s not.

The difference between now and then, the researchers found, is the extra CO₂ pushed into the atmosphere by human activities such as deforestation. Humans, therefore, may have postponed the next glaciation, the researchers propose online May 19 in Geology.

“Most probably the next glaciation should have already started,” says coauthor Paul Renne, a geoscientist at the Berkeley Geochronology Center in California. “We are experiencing a climate that has been modified by human activity.”

The global climate naturally warms and cools in long cycles due to periodic changes in the planet’s orientation to the sun, which determines how much solar energy reaches the upper atmosphere. If the amount of solar warming is low enough, more ice builds up during winter than melts away during summer, triggering a glaciation.

Earth’s current orbit and tilt closely match its orientation during the gap between glaciations that took place about 790,000 years ago. This similarity makes the historical interglacial useful for predicting when the current interglacial might end and Earth’s next glaciation may start. Determining the warm period’s duration is tricky, though.

Led by geologist Biagio Giaccio of the Institute of Environmental Geology and Geoengineering in Rome, Renne and colleagues studied an ancient lake bed in Sulmona, Italy. Hundreds of thousands of years ago, nearby volcanoes spat ash containing radioactive potassium that left distinct layers in the lake sediments. By measuring the fraction of potassium that decayed into argon, the researchers precisely pinpointed the relative ages of the sediment layers.

Oxygen atoms in each layer revealed the local climate at the time the layer formed. As the hemisphere cooled, the concentration of heavier oxygen atoms in the lake sediments increased. Using this trend, the researchers determined the probable start and end of the ancient warm period. They calculated that it lasted around 10,800 years, around 1,700 years shorter than previously thought and nearly 1,000 years shorter than the current interglacial.

Extra CO₂ in the atmosphere may have prolonged the current interglacial, the researchers propose. At the time the historical interglacial ended, atmospheric CO₂ levels were around 250 parts per million. Carbon dioxide levels, which have not dropped below 260 ppm over the last few thousand years, recently rose above 400 ppm. A 2007 paper estimated that without the extra CO₂ from human activities such deforestation and fossil fuel burning, present-day CO₂ levels would have fallen to 245 ppm, within the range amiable to a new glaciation.

“We basically short-circuited the beginning of a glaciation,” says climatologist Stephen Vavrus of the University of Wisconsin–Madison. “The future is murky now that humans are exerting such a big influence on the climate system.”