Today's world: Fastest rate of carbon dioxide rise over the last 50,000 years

The findings, just published in the Proceedings of the National Academy of Sciences, provide important new understanding of abrupt climate change periods in Earth's past and offer new insight into the potential impacts of climate change today.

"Studying the past teaches us how today is different. The rate of CO₂ change today really is unprecedented," said Kathleen Wendt, an assistant professor in Oregon State University's College of Earth, Ocean, and Atmospheric Sciences and the study's lead author.

"Our research identified the fastest rates of past natural CO₂ rise ever observed, and the rate occurring today, largely driven by human emissions, is 10 times higher."

Carbon dioxide, or CO₂, is a greenhouse gas that occurs naturally in the atmosphere. When carbon dioxideenters the atmosphere, it contributes to warming of the climate due to the greenhouse effect. In the past, the levels have fluctuated due to ice age cycles and other natural causes, but today they are rising because of human emissions.

Ice that built up in Antarctic over hundreds of thousands of years includes ancient atmospheric gasses trapped in air bubbles. Scientists use samples of that ice, collected by drilling cores up to 2 miles (3.2 kilometers) deep, to analyze the trace chemicals and build records of past climate. The U.S. National Science Foundation supported the ice core drilling and the chemical analysis used in the study.

Previous research showed that during the last ice age, which ended about 10,000 years ago, there were several periods where carbon dioxide levels appeared to jump much higher than the average. But those measurements were not detailed enough to reveal the full nature of the rapid changes, limiting scientists' ability to understand what was occurring, Wendt said.

"You probably wouldn't expect to see that in the dead of the last ice age," she said. "But our interest was piqued, and we wanted to go back to those periods and conduct measurements at greater detail to find out what was happening."

Using samples from the West Antarctic Ice Sheet Divide ice core, Wendt and colleagues investigated what was occurring during those periods. They identified a pattern that showed that these jumps in carbon dioxide occurred alongside North Atlantic cold intervals known as Heinrich Events that are associated with abrupt climate shifts around the world.

"These Heinrich Events are truly remarkable," said Christo Buizert, an associate professor in the College of Earth, Ocean, and Atmospheric Sciences and co-author of the study. "We think they are caused by a dramatic collapse of the North American ice sheet. This sets into motion a chain reaction that involves changes to the tropical monsoons, the Southern hemisphere westerly winds and these large burps of CO₂ coming out of the oceans."

During the largest of the natural rises, carbon dioxide increased by about 14 parts per million in 55 years. And the jumps occurred about once every 7,000 years or so. At today's rates, that magnitude of increase takes only 5 to 6 years.

Evidence suggests that during past periods of natural carbon dioxide rise, the westerly winds that play an important role in the circulation of the deep ocean were also strengthening, leading to a rapid release of CO₂ from the Southern Ocean.

Other research has suggested that these westerlies will strengthen over the next century due to climate change. The new findings suggest that if that occurs, it will reduce the Southern Ocean's capacity to absorb human-generated carbon dioxide, the researchers noted.

"We rely on the Southern Ocean to take up part of the carbon dioxide we emit, but rapidly increasing southerly winds weaken its ability to do so," Wendt said.

Additional coauthors include Ed Brook, Kyle Niezgoda and Michael Kalk of Oregon State; Christoph Nehrbass-Ahlesof the University of Bern in Switzerland and the National Physical Laboratory in the United Kingdom; Thomas Stocker, Jochen Schmitt and Hubertus Fischer of the University of Bern; Laurie Menviel of the University of New South Wales in Australia; James Rae of the University of St. Andrews in the United Kingdom; Juan Muglia of Argentina; David Ferreira of the University of Reading in the United Kingdom and Shaun Marcott of University of Wisconsin-Madison.