Rate of atmospheric carbon dioxide rise unprecedented

MONTREAL — Humans are dumping extra carbon into the atmosphere at a rate unprecedented since at least the time the dinosaurs went extinct about 66 million years ago, new research suggests.

Previously, a massive outpouring of carbon about 56 million years ago had been proposed as faster than the current rate of net increase in atmospheric carbon. But researchers comparing data collected from ocean sediment cores with climate simulations show that this event at most reached only about a tenth of today’s carbon increase rate. The work suggests that no direct historical analogs exist to help predict the planet’s response to rapidly amassing greenhouse gases, the researchers said May 6 at a meeting of the American Geophysical Union and other organizations.

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“Not a single event during the past 66 million years released carbon as fast as we’re releasing now,” said Richard Zeebe, a paleoclimatologist at the University of Hawaii at Manoa in Honolulu. “The current rate is remarkable.”

Climate scientists study past climate events to better predict future climate changes. If a climate simulation can’t accurately reproduce historical events, its predictions probably won’t be accurate either. With increasing atmospheric carbon dioxide levels shifting the global climate, scientists searched for a past event to use for comparison.

The likeliest contender appeared to be the Paleocene-Eocene Thermal Maximum, or PETM, around 56 million years ago. At this time, global CO₂ levels spiked from around 1,000 parts per million to roughly 1,700 to 2,000 ppm, raising global temperatures by several degrees Celsius. While the exact source of this extra carbon is unknown, proposed explanations range from volcanic eruptions to the release of ice-trapped methane, which breaks down in air to form CO₂. Understanding how long this carbon deluge lasted is the key to understanding how fast atmospheric carbon entered the atmosphere.

Scientists can track changes in the ancient air by looking at ocean sediments. Over time, sediments such as calcium carbonate form layers on the seafloor. As atmospheric carbon levels increase, the ocean becomes more acidic and dissolves more of the calcium carbonate. The amount of calcium carbonate in a sediment therefore serves as a proxy to the level of carbon released at the time the layer formed: A reduction in calcium carbonate signals an increase in atmospheric carbon levels.

Interpreting ocean sediment cores, researchers proposed in 2013 that the PETM event released its carbon in just over a decade. The cause, they suggested, was a comet impact that flung carbon into the atmosphere.

Zeebe and colleagues, however, doubted that the carbon release was that rapid. Using computer climate simulations, they calculated the effects of releasing the PETM carbon over various timescales and compared the results with the ocean sediment data. The simulations and sediment data agreed only when the carbon release took more than 4,000 years, Zeebe says. Over that timescale, the carbon release rate was at most 1.1 billion tons a year. That rate is far less than the approximately 10 billion tons of carbon humans released via fossil fuel burning in 2013 on top ofthe natural background carbon cycle.

“We’re a geological force to be reckoned with,” says climate scientist Gavin Schmidt of the NASA Goddard Institute for Space Studies in New York City, who was not involved in the work.

While the current rate of carbon entering the atmosphere is unprecedented, Schmidt says we can still learn from climate events such as the PETM. “When you look back in time, you don’t necessarily have to find a direct analog,” he says. “You’re looking for something that utilizes the same machinery, the same Earth processes, but it doesn’t have to be in exactly the same way.”