Eric Holthaus notes that, as of last month, carbon dioxide concentrations in the Earth’s atmosphere exceeded 400 parts per million for the first time in human history:
Over the very long term, taking data from ice cores in Antarctica, paleoclimatologists have determined that there’s never been as quick a spike in carbon dioxide levels in at least the last 800,000 years … These data are painstakingly compiled by finding tiny air bubbles trapped in the ancient ice, and then analyzing their chemical composition. By this method, scientists have literally measured nearly a million years’ worth of the Earth’s atmosphere. Of course, looking at historical data, scientists could have made the same statement—we’re at levels not seen in human history!—in any year since about 1914 and would have been accurate. Problem is, the data didn’t exist then.
In fact, Brad Plumer notes, scientists believe CO2 levels haven’t been this high since long before humans even existed:
Indeed, some studies go further and estimate that carbon-dioxide levels may be at their highest point in 4.5 million years. During the Pliocene era, scientists have found, carbon-dioxide levels appeared to be around 415 ppm. (This rise was likely caused by wobbles in the Earth’s orbit — humans weren’t around then.)
The climate of the Pliocene was much warmer and wetter than it is today. Global average temperatures were 3°C or 4°C hotter (that’s 5.4°F to 7.2°C) and sea levels were between 5 and 40 meters higher.
That doesn’t mean we’ll get exactly those things today — the Pliocene isn’t perfectly comparable, since a variety of different factors were at play. But it’s the best guide we have to a fairly unprecedented situation.Other features of the Pliocene era: more frequent and intense El Niño events in the Pacific Ocean, intense flooding in the western United States, and severe coral reef extinctions as the oceans warmed.