A spell of severe global warming 55 million years ago was triggered by not one but two injections of greenhouse gases into the atmosphere—and the rate of increase suggests that the ancient warming event may hold important lessons for what to expect with today’s climate change.
One of the best ways to explore how human-induced global warming will affect Earth in future is to study the way our planet responded to climate change in the past. Many researchers think the Paleocene–Eocene Thermal Maximum (PETM) 55 million years ago is particularly relevant to our current situation, because it also involved a massive injection of carbon into the atmosphere that led to spiraling global temperatures.
Not all researchers have been convinced we have much to learn from the PETM, though. Some studies suggested the climate back then changed far more slowly than it is changing now, with atmospheric carbon building up gradually over about 20,000 years, perhaps because of the slow release of volcanic gases. Still other studies concluded that the PETM changes occurred far too rapidly for comparisons with our present situation. A study last year suggested that atmospheric carbon soared within just a few years, maybe due to a massive influx of carbon from a comet impact.
But the latest geological evidence, published today in Nature Geoscience, makes these “too-slow” and “too-fast” warming scenarios begin to look unlikely. Instead, the rate of change was probably “just right” for making modern comparisons.
Researchers led by Gabe Bowen at the University of Utah drilled an 820-foot core out of the ground near Powell, Wyoming. Their sample cuts through ancient soils—now turned to stone—that formed before, during and after the PETM. The soil layers contain thousands of carbonate nodules, each of which holds varieties of carbon that reflect the composition of the atmosphere at the time the nodule formed.
By measuring these varieties, or isotopes, within each nodule, the researchers could build a picture of how carbon was added to the ancient atmosphere. It isn’t the first time researchers have built such a carbon record for the PETM—similar profiles have been made using information from marine sediment cores. But marine sediments get churned up by burrowing animals, blurring the detail of the carbon picture in a way that doesn’t happen so readily on land, say the researchers.
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