科学美国人60秒:Climate 420 Million Years Ago Poised for Comeback

“We’re hearing a lot of the arguments against action on reducing CO2 being based on, ‘well CO2 was higher in the past, so we don’t have to worry about it.’

Gavin Foster, a geochemist at the University of Southampton. But Foster says that’s a flawed argument. For starters, just how far back in time do you have to look to find CO2 concentrations like what we expect to see in the future, and does it even make sense to compare the levels now and then?

To answer these questions, Foster and his colleagues reconstructed the history of atmospheric carbon dioxide for the last 420 million years. They compiled roughly 1,500 estimates of CO2 concentrations from 112 previous studies. When the researchers combined these data, they found that atmospheric carbon dioxide went up and down over time, but that, in general, it gradually declined from almost 3,000 parts per million down to less than 300 parts per million before humans started burning fossil fuels.

However, we have already started to reverse that trend. If we continue on a business-as-usual scenario, by the middle of this century, CO2 could reach levels not seen in 50 million years, according to Foster’s reconstruction. That’s long before humans evolved, back when the climate was much warmer and there were no large ice sheets at the poles. If we continue on that trajectory, by the year 2250, concentrations could approach what they were in the Triassic, 200 million years ago, when dinosaurs roamed the Earth.

But greenhouse gases aren’t the only factor impacting Earth’s climate. The sun also plays a major role. It’s grown brighter over time, offsetting most of the cooling related to dropping CO2 levels, Foster’s team found. And that fact has important implications for modern climate change. Because while we’re headed toward a world with CO2 levels similar to what they were in the distant geologic past, it won’t just be like rewinding the clock.

“So, because the sun is now brighter than it was 200 million years ago, or 400 million years ago, that radiative forcing from CO2 in the future is going to be that much more potent. And that, we thought, was quite a strong message that hadn’t been noted before.”

The findings are published in the journal Nature Communications. [Gavin L. Foster, Dana L. Royer and Daniel J. Lunt, Future climate forcing potentially without precedent in the last 420 million years]

Foster stresses that this isn’t a vision of what will be, but what could be. “It’s more of cautionary note that, in the absence of any action, we will be entering a world quite rapidly—in the next 150 years—where the climate is receiving a magnitude of forcing that, as far as we know, it hasn’t received for 420 million years…it’s outside the bounds at which the Earth is normally functioning. Doesn’t sound like a good place to be to me.”

—Julia Rosen