This animation depicts climatic conditions out to 2280 assuming emissions of greenhouse gases are not restrained. The color coding shows how future conditions would compare to past climates, with lighter orange corresponding to the Pliocene Epoch about 3 million years ago, and darker orange to the Eocene about 50 million years ago. Dark red indicates future climatic conditions with no past analog. (Source: "Pliocene and Eocene provide best analogs for near-future climates," K. D. Burke et al, Proceedings of the National Academy of Sciences Dec 2018, 2018) Absent serious action on climate change, we'll continue careening toward a climatic cliff. And modern civilization will be hard-pressed to survive the plunge. This is the essential take-away from new research probing Earth's climatic past to yield insights into our future. The research finds that if our emissions of carbon dioxide and other greenhouse gases continue unabated, Earth's climate will warm by the year 2150 to levels not seen since the largely ice-free Eocene Epoch about 50 million years ago. That may seem like a long time on a human timescale. But consider that the research shows we're currently on course to reversing 50 million years of cooling in just a couple of centuries. That may be so rapid that it will outpace our ability to adapt our agricultural and other modern life support systems. Thanks to our emissions of greenhouse gases, "the Earth system is well along on a trajectory to a climate state different from any experienced in our history of agricultural civilizations," write Kevin Burke and his co-authors in a paper published in the Proceedings of the National Academy of Sciences. Here's why that's troubling: Modern human civilizations, made possible by agriculture, have thrived in what scientists term "a safe operating space," meaning a stable, relatively benign climate. But now, staying within that safe space seems to be "increasingly unlikely," the researchers write. "If we think about the future in terms of the past, where we are going is uncharted territory for human society," says the study's lead author, Kevin Burke, in a press release. Burke led the research while he was a graduate student in the lab of paleoecologist John "Jack" Williams of the University of Wisconsin-Madison. "We are moving toward very dramatic changes over an extremely rapid time frame, reversing a planetary cooling trend in a matter of centuries," Burke says.
Temperature trends for the past 65 million years and potential past analogs for future climates. Red arrows show "geohistorical" states of the climate system. From the left, these are the early Eocene Epoch; the mid-Pliocene; the last interglacial, or LIG; the mid-Holocene; the pre-industrial period; the historical period; and projections for the future (at extreme right). The time series shows how global mean annual temperatures departed from the 1961 to 1990 mean. Major patterns include a 50-million-year cooling trend beginning in the Eocene, periodic fluctuations driven by changes in the Earth’s orbit, the recent warming caused by human activities, and projected warming. (Source: "Pliocene and Eocene provide best analogs for near-future climates," K. D. Burke et al, Proceedings of the National Academy of Sciences Dec 2018, 2018) To conduct the research, Burke and his colleagues compared climatic conditions of several periods in geologic history with computer model projections of the future. These projections assumed different greenhouse gas emission scenarios. One sobering finding was that we are already headed toward a climatic state like that of the mid-Pliocene Epoch about 3 million years ago even if we do manage to reduce our emissions of greenhouse gases. In fact, thanks to burning of fossil fuels and other human activities, carbon dioxide concentrations in the atmosphere are already at Pliocene levels: more than 400 parts per million. We just don't have a Pliocene-like climate yet because most of the planet's surface is covered in oceans — and all that water takes time to warm up. The research shows that if we undertake moderate emissions reductions, we'll stabilize at Pliocene-like temperatures by 2040. During the Pliocene, temperatures were between 3.2 and 6.5 degrees Fahrenheit (1.8 to 3.6 degrees Celsius) warmer than they are today. At their peak, sea level during the Pliocene stood 72 feet (22 meters) higher than today. Luckily, inertia in the climate system means that it would take a few centuries before sea level would come up to that level again. (Looking at past climates, scientists have documented maximum rates of sea level rise of three to five meters per century.) Even so, Pliocene-like temperatures should bring a host of climate impacts that will prove challenging for modern societies to adapt to. And over time, we'll have to deal with that Pliocene-like sea level rise. "There is a very good relationship between sea level and temperature on the planet," says James White, a paleoclimatologist at the University of Colorado who was not involved in the current research. "If you warm up air, and you warm up water, you melt ice. And when land ice melts and goes into the ocean, you raise sea level." The 20 meters of sea level rise that correspond to a Pliocene-like climate "is enormous," White says. It would submerge coastal cities around the globe which are currently home to many tens of millions of human beings. The silver lining is that by reducing carbon emissions, we'd at least stabilize climatically without things getting even more challenging. But if we don't reduce our emissions, the planet will continue to warm — quite dramatically. By the year 2150, Earth's average temperature would rise by a staggering 23 degrees Fahrenheit (13 degrees Celsius) above current conditions, taking them to Eocene-like levels. During that geologic epoch, there was no permanent ice on the planet, and sea level is estimated to have been at least 100 meters higher than today. If we follow this path, the rapid change we'd trigger would be without precedent during the entire Cenozoic Era, the age of mammals, which began about 65 million years ago when the dinosaurs went extinct. "What the planet’s going to do is very predictable," White observes. Basic physics plus research on past climatic states provides confidence that "temperatures will rise, sea level will go up, and storms will get worse. It’s all very predictable. What’s not is how we will respond." I don't know about you, but I'd prefer to respond in a way that would leave us in the Pliocene, not the Eocene.
