CO2 Levels Are as High as They Were Three Million Years Ago

Climate change researchers are peering into the past in hopes of anticipating the future, but as a group of British scientists reported at a recent Royal Meteorological Society meeting, the historical record is far from an encouraging sign of the planet’s fate.

In fact, Jonathan Amos writes for BBC News, the last time Earth’s atmosphere contained the amount of carbon dioxide present today, Antarctica was a plant-covered oasis, sea levels were an estimated 10 to 20 meters higher, and global temperatures were an average of 2 to 3 degrees Celsius warmer. In the Arctic, summer temperatures were a full 14 degrees higher than they are now.

These conditions, seen some 2.6 to 5.3 million years ago during a period known as the Pliocene Epoch, are a far cry from our current climate. But if decisive steps to combat global warming are not taken soon, researchers warn, history may be poised to repeat itself.

The ratio of carbon dioxide molecules found in the atmosphere reached Pliocene levels of 400 parts per million, or ppm, in 2015. Comparatively, Damian Carrington notes for the Guardian, carbon levels during the Industrial Revolution—when temperatures were an average of one degree Celsius cooler—hovered around 218 ppm.

As Metro’s Jeff Parsons observes, meeting chair Martin Siegert, a geophysicist and climate change scientist at Imperial College London, says there will be a “lag” before the most devastating effects of crossing this threshold come to light.

Although the rate at which carbon levels have and still are rising suggests Earth will experience an unavoidable temperature uptick of 3 to 4 degrees Celsius, this warming is expected to happen gradually over the next several centuries. Large-scale melting capable of elevating sea levels and turning Antarctica’s icy landscape green will, in turn, take closer to a few millennia.

“If you put your oven on at home and set it to [400 degrees Fahrenheit], the temperature does not get to that immediately,” Siegert explained at the April 3 session, titled “The Pliocene: The Last Time Earth had >400 ppm of Atmospheric CO2.”

He continued, “It takes a bit of time, and it is the same with climate.”

As the Guardian’s Carrington reports, fossilized plants and tree remains attest to Antarctica’s once-verdant environment. At Oliver Bluffs, located some 300 miles away from the South Pole, beech and conifer trees flourished alongside cushion plants and mosses. In other spots across the continent, according to a presentation by British Antarctic Survey Director Jane Francis, flora including ferns, Monkey Puzzle trees and ancestral predecessors of plants currently found in South America, Australia and New Zealand thrived.

“What we're looking at is a time when the glaciers retreated and we were left with areas in front of those glaciers where tiny little shrubs could grow,” Francis tells BBC News’ Amos. “It wouldn't have been a nice place to live; it would have been a tundra kind of environment, but with enough sunshine in summer and temperatures [able to rise] up to about 5 [degrees Celsius].”

Expanding on this line of thought at the meeting, Francis added, “The really important significance of this is that we've got 400 ppm now, and if we had 400 ppm in the past, this is maybe where we are going back to, which is the ice sheets are going to shrink at times ... which may allow plants to colonize in Antarctic land again.”

If carbon emissions continue at their current rate, Siegert warns Metro’s Parsons, Earth could reach a staggering 1,000 ppm by 2100. When carbon levels rose to this level during the Cretaceous Period some 100 million years ago, the planet was a very different place: Dinosaurs ruled, for one, but more germanely, Antarctica was far warmer and covered in vast swaths of forest.

To ward off this unwelcome scenario, Siegert says humans must work to restrict this century’s temperature rise to 1.5 degrees.

"We've got to bring CO2 levels down to 40 percent of what they are today by 2030,” he explains to BBC News’ Amos. “And then to zero by 2050, and then negative after that. That's a massive undertaking but it's possible."

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's senior associate digital editor, history.