The edge of the Greenland ice sheet as it recedes.

An image shows the edge of the Greenland ice sheet, where recent melting has left bare ground.

(Photo: Kevin Krajick/Columbia University)

'Call to Action': CO2 Now at Levels Not Seen in 14 Million Years

"It really brings it home to us that what we are doing is very, very unusual in Earth's history," the lead author of a new study said.

The last time that levels of atmospheric carbon dioxide were as high as they are today, Greenland was free of ice and the savanna and grassland ecosystems where humans evolved didn't exist yet.

That's the conclusion of a study published in Science Friday, which researchers say compiles "the most reliable data available to date" on atmospheric carbon dioxide levels over the last 66 million years.

"It really brings it home to us that what we are doing is very, very unusual in Earth's history," lead author Baerbel Hoenisch of the Columbia Climate School's Lamont-Doherty Earth Observatory toldAgence France-Presse.

"We've already pushed the atmosphere way beyond anything we've seen as a species."

By burning fossil fuels and clearing natural carbon sinks like forests, industrial capitalism has raised global carbon dioxide levels to 419 parts per million (ppm) today from around 280 ppm at the beginning of the industrial revolution.

"Rising atmospheric CO2 is the most obvious and startling expressions of our impact on the global environment," study corresponding author and University of Utah geologist Gabe Bowen wrote on social media. "The concentration has risen by ~50% in the past 100 years. Every year is now marked by the highest CO2 levels *ever observed* by humans!"

To understand how such a spike in carbon dioxide might impact Earth's climate and ecosystems, it's helpful to look at the past. This presents challenges, however, because the most reliable record of past carbon dioxide concentrations—gas bubbles preserved in ice cores—only goes back to around 800,000 years ago, when atmospheric concentrations of carbon dioxide were still at around preindustrial levels.

"Once you lose the ice cores, you lose direct evidence. You no longer have samples of atmospheric gas that you can analyze," Bowen said in a University of Utah press release. "So you have to rely on indirect evidence, what we call proxies. And those proxies are tough to work with because they are indirect."

Proxies are evidence in the geologic record that can stand in for carbon dioxide levels, such as mineral isotopes or the shape of fossilized leaves. Scientists have looked at these proxies before, but the current study represents the most comprehensive effort to date. A team of around 90 researchers from 16 countries spent seven years synthesizing and reviewing previous work under the banner of the Cenozoic CO2 Proxy Integration Project, according to the University of Utah and AFP.

The new study represents the scientific consensus on the carbon dioxide record, and it concludes that the last time carbon dioxide levels were around 419 ppm was 14 million years ago. That's much earlier than previous estimates of 3 to 5 million years ago.

However, the record goes back further than that to the Cenozoic Era, when the dinosaurs died and mammals began to emerge.

That record revealed a very clear pattern, Bowen tweeted: "CO2 goes up, the world warms. CO2 down, and things get icy."

The record enabled the scientists to predict the consequences of current and projected carbon dioxide levels.

"This is an incredibly important synthesis and has implications for future climate change as well, particularly the key processes and components of the Earth system that we need to understand to project the speed and magnitude of climate change," University of Utah biology professor William Anderegg said in the press release.

One of the report's messages, Bowen tweeted is that "the future is now."

"We've already pushed the atmosphere way beyond anything we've seen as a species," Bowen continued, "and if it stays this way we're in for big changes in the environment we live in."

If policy-makers don't restrict the burning of fossil fuels, atmospheric carbon dioxide could reach 600 to 800 ppm by 2100, AFP reported. According to the record, the last time levels were this high was 30 to 40 million years ago, when Antarctica was also ice-free and the Earth was home to giant insects.

Even today's concentrations are bound to have lasting consequences. For example, when carbon dioxide levels rapidly increased around 56 million years ago, it significantly altered ecosystems and took around 150,000 years to decrease again.

"We are in this for a very long time," Hoenisch told AFP, "unless we sequester carbon dioxide, take it out of the atmosphere, and we stop our emissions sometime soon."

However, the that doesn't mean the most extreme changes are locked-in. Instead, Bowen tweeted that the report was a "call to action."

"The geological changes we studied lasted for thousands and millions of years," Bowen said, "and if human-induced CO2 change is short-lived it won't have as big an impact on the climate."

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