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There Hasn’t Been This Much Carbon in the Atmosphere for Millions of Years

So what did the climate look like back then?

March 2015 marked the first time that carbon dioxide levels hit 400 parts per million in Earth's atmosphere—and stayed there all month. We'd seen 400 ppm registered, albeit briefly, in previous periods. But now, it really is a 400 ppm world. And to begin to grasp what that means for life in the near-future, we might look to the past.

"CO2 concentrations haven't been this high in millions of years," NASA's Erika Podest, a carbon and water research scientist, said—a common refrain among the scientists who researched or observed the event.

"This event is a milestone on a road to unprecedented climate change for the human race, Dr. Ed Hawkins, a climatologist at the University of Reading, told the Guardian. "The last time the Earth had this much carbon dioxide in the atmosphere was more than a million years ago, when modern humans hadn't even evolved yet." The environmentalist Bill McKibben echoed the sentiment, too: "We're in new territory for human beings—it's been millions of years since there's been this much carbon in the atmosphere."

Yep: it's been millions of years since there's been this much carbon in the atmosphere. That, inevitably, brings on the question: What was the climate like back then, a few million years ago?

According to the world's leading authority on climate science, the last time carbon levels reached 400 ppm, and "mean global temperatures were substantially warmer for a sustained period," was probably 2-3 million years ago, in the Mid-Pliocene era. The Intergovernmental Panel on Climate Change (IPCC), whose job is to synthesize the latest and most accurate climate science in order to make recommendations to policymakers, devotes a section of its annual report to the era, because "Understanding the climate distribution and forcing for the Pliocene period may help improve predictions of the likely response to increased CO2 in the future." During that time, carbon levels registered from 360 to 400 ppm.

It's not a perfect analog, of course, and much of the information we have about the climate comes from so-called proxy data derived from samples taken from ice cores and ancient tree rings. But still! It's worth taking a look at what the climate was like then to get an idea of what a more carbon-saturated world might look like now.

Here are a few of the characteristics of the Pliocene's climate, which I also noted back when atmospheric carbon levels first crept past 400 ppm:

-Sea levels were, on average, between 50 and 82 feet higher.
-Temperatures were 2-3 ˚C higher—about 4-6 ˚F—above pre-industrial levels.
-Arctic temperatures were between 10-20 ˚C hotter.
-Many species of both plants and animals existed several hundred kilometers north of where their nearest relatives exist today.

Mid-Pliocene sea surface temperature anomaly. USGS

It's also worth noting that the tropics weren't much hotter, and were perhaps even a bit cooler—it's the regions near the poles that were cooked. Arctic ice, for instance, was "ephemeral", as in, not permanent, and melted in the warm season. North Atlantic regions warmed considerably.

Again, this period in the Pliocene is surely not a perfect corollary, but the best science indicates we're heading towards a climate not unlike that warmer one, if not even hotter. We've already seen global temperatures increase 1˚ F since preindustrial times, which has changed the face of the planet. Glaciologists predict an ice-free Arctic in coming decades—and temperatures around the poles are climbing fast.

But passing the 400 ppm mark may be more symbolic than anything, as a number of researchers have noted. In fact, we're on track to blow right past it—towards a world that might make those marginally higher temps and raised sea levels of the Pliocene look quaint in comparison.