Even the most extreme natural global warming episode of the last 65 million years can't hold a candle to what we're doing to the atmosphere now. If a new study of layered rock deposits from the Norwegian Arctic is correct, then we're releasing carbon dioxide 10 times faster than at any other time since the dinosaurs died out.
The Paleocene-Eocene Thermal Maximum (PETM), is one of the best geo-historical examples of what a future super-greenhouse could be like. It happened about 56 million years ago, and although nobody knows exactly what triggered it, numerous studies show that carbon emissions were the main culprits--perhaps arising from various coal seams, limy organic ocean muds, and frozen methane ices. This, of course, was long before people appeared on Earth, so we had nothing to do with it. Instead, undersea volcanism might instead have started it by immolating carbon-rich marine deposits.
What did the PETM do to the planet and its inhabitants? Carbon dioxide concentrations jumped as much as five times higher than those in our present atmosphere, adding at least 10°F to climates that were already much warmer than today and probably finishing off what little ice and snow may have whitened the polar regions in winter. The greenhouse gas surplus lingered for 150,000 to 200,000 years, and turned the deepest parts of the oceans into stagnant, acidified dead zones.
As unpleasant as being suffocated to death by the carbonic acid bath must have been for many deep-sea denizens, the PETM wasn't the great mass extinction or ecological "disaster" that some media accounts describe it as.Most of the extinctions occurred among a relatively narrow range of organisms--including tiny ocean-dwelling amoebae--but in shallower waters and on land, most species pulled through.
No polar bears survived the PETM, but that's not because of warming; it's because the hot spell happened so far back in time that bears hadn't even evolved yet. In fact, none of our modern mammals existed back then, and a tour of the Earth of 56 million years ago would reveal a fantastical menagerie that included hoofed, carnivorous "wolf-sheep" and our earliest, lemur-like primate ancestors scampering freely among universally warm latitudes and continents. Dawn redwood forests ringed the shorelines of the ice-free Arctic Ocean, and lush beech woods covered Antarctica. Deep ocean life suffered the most because the huge slug of carbon dioxide acidified the bottom waters (much as we see beginning to happen today), and greenhouse warming may also have slowed or stopped the sinking of well-oxygenated polar waters to the sea floor.
These facts are well known to geo-historians, but this latest study adds a new wrinkle to the PETM story by comparing it explicitly to what's going on today. So what, if anything, is the value of such a finding to us here and now? The article posits one lesson: Although many species survived the rapid onset of the PETM greenhouse, it doesn't mean that modern biodiversity isn't threatened by today's gas buildup--because this time it's happening much faster than before.
Still, some relatively recent natural climatic changes have actually been just as abrupt as our present warming; perhaps that's why the paper focuses on CO2 buildups rather than temperature. Furthermore, we simply don't know how adaptable most species on Earth are--we haven't even cataloged all of them yet. And regardless of how the pace of that earlier event compares with our own, today's species didn't live through it anyway.
The most compelling lesson is that yet another independent, peer-reviewed exploration of the past has shown that a super-greenhouse is not just the delusional fantasy of enviro-doomsayers. Scott Wing, a top PETM expert with the Smithsonian Institution, thinks that this kind of study is especially important because it clearly shows that adding lots of CO2 to the atmosphere does warm the planet and make the oceans more acidic. "People can be as skeptical as they want about global warming resulting from human carbon emissions," he told me recently, "but the geological fact is that this kind of thing really has happened before."
This latest addition to the PETM story offers us a warning. For the first time, billions of people and swarms of powerful machines are unearthing and igniting fossil carbon deposits on a planetary scale. This means that even the best paleo studies can't fully inform us about what lies ahead, and we're essentially driving ourselves and our planet into the future with our eyes half closed.
But I suppose we knew that already, didn't we?
[Image by Flickr user echobase_2000]
Curt Stager is an ecologist, paleoclimatologist, and science journalist with a Ph.D. in biology and geology from Duke University. His new book is DEEP FUTURE: The Next 100,000 Years of Life on Earth (St. Martin's Press, March 2011).