“It's actually a hard problem,” remarked Tierney. Any conclusions about global climate must be scaled up from those sparse data points. Geological data for the PETM is limited to locations where sediments from that time are preserved and accessible-typically either via a borehole or outcropping on land. The new, sharper picture emerges from the way Tierney’s team dealt with geologists’ perennial problem: We don’t have data for every place on the planet. “We might expect some level of increased climate sensitivity in the near future, especially if we emit more greenhouse gases,” Tierney said. Their paper does, however, suggest that if we continue to raise CO 2 levels, it will nudge the temperature response to that CO 2 higher.
Our climate sensitivity won’t be that high: “We don't expect that we're going to experience a climate sensitivity of 6.5° C tomorrow,” Tierney explained.
Tierney’s study found the PETM climate sensitivity was 6.5° C-more than double the IPCC best estimate.Ī higher number is “not too surprising,” Tierney told me, because earlier research had indicated Earth’s response to CO 2 is stronger at the higher CO 2 levels of Earth’s past. If the sensitivity turns out to be on the higher end, then we’ll warm more for a given amount of emissions. The IPCC’s best estimate for climate sensitivity in our time is 3° C, but that comes with a large uncertainty-it could be anything between 2° to 5° C-due to our imperfect knowledge of feedbacks in the Earth system. The team was able to use these new temperature and CO 2 values to calculate how much the planet warmed in response to a doubling of CO 2 values, or the “Equilibrium Climate Sensitivity” for the PETM. For comparison, preindustrial CO 2 was 280 ppm, and we’re currently at about 418 ppm. They found CO 2 was about 1,120 ppm just before the PETM, rising to 2,020 ppm at its peak. The researchers also calculated the CO 2 levels before and during the PETM derived from isotopes of boron measured in fossil plankton shells. “We were really able to narrow that estimate down over previous work,” said Tierney. Now Tierney’s team has narrowed that uncertainty range to just 2.4° C, showing that the PETM warmed by 5.6° C, a refinement on the previous estimate of approximately 5° C. Mammals evolved smaller, and there were big migrations across continents crocodiles, hippo-like creatures, and palm trees all thrived just 500 miles from the North Pole, and Antarctica was ice-free.Īs our climate warms, scientists are increasingly looking at past climates for insights, but they are hampered by uncertainties in temperature, CO 2 levels, and the exact timing of changes-prior work on the PETM had temperature uncertainties on the order of 8° to 10° C, for example. It took an already warm, high-CO 2 climate and made it hotter for tens of thousands of years, driving some deep-sea creatures and some tropical plants to extinction. The magma invaded oil-rich sediments in the North Atlantic, boiling off CO 2 and methane. The warming of the PETM was triggered by a geologically rapid release of CO 2, primarily from a convulsion of magma in Earth’s mantle at the place where Iceland is now situated. But changes in rainfall patterns and the amplification of warming at the poles were remarkably consistent with modern trends, despite being a very different world back then. While the PETM has some parallels to present warming, the new work includes some unexpected results-the climate response to CO 2 then was about twice as strong as the current best estimate by the Intergovernmental Panel on Climate Change (IPCC). In a study published in PNAS, professor Jessica Tierney of the University of Arizona and colleagues have produced globally complete maps of the carbon-driven warming that occurred in the Paleocene Eocene Thermal Maximum (PETM), 56 million years ago.
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