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.