in a study published in PNASProfessor Jessica Tierney of the University of Arizona and colleagues have produced globally comprehensive maps of the carbon-driven warming that occurred in the Paleocene Eocene Thermal Maximum (PETM), 56 million years ago.
While the PETM has some parallels to current warming, the new work includes some unexpected results: The climate response to COtwo then it was about twice as strong as the current best estimate from the Intergovernmental Panel on Climate Change (IPCC). But the changes in rainfall patterns and the amplification of warming at the poles were remarkably consistent with modern trends, despite it being a very different world back then.
a different world
The warming of the PETM was caused by a geologically rapid release of COtwofirst of a convulsion of magma in the Earth’s mantle at the place where Iceland is now. The magma invaded oil-rich sediments in the North Atlantic, boiling COtwo and methane. He took a ya warm, high COtwo climate and made it hotter for tens of thousands of years, leading to some deep sea creatures Y some tropical plants to extinction Mammals evolved minorand there were big migrations across the continents; crocodiles, hippopotamus-like creatures and palms all prospered just 500 miles from the North Pole, and Antarctica it was ice free.
As our climate warms, scientists are looking more and more at past climates for information, but are hampered by uncertainties in temperature, COtwo levels and exact timing of changes: previous work at the PETM had temperature uncertainties on the order of 8° to 10°C, for example. Now, Tierney’s team has narrowed that uncertainty range down to just 2.4°C, showing that the PETM warmed by 5.6°C, a refinement of the previous estimate of about 5°C.
“We were actually able to lower that estimate from previous work,” Tierney said.
The researchers also calculated the COtwo levels before and during the PETM derived from boron isotopes measured in fossil plankton shells. they found COtwo it was about 1120 ppm just before the PETM, rising to 2020 ppm at its peak. For comparison, pre-industrial COtwo I was 280ppmand we are currently at approx. 418ppm. The team was able to use these new temperatures and COtwo values to calculate how much the planet warmed in response to a doubling of COtwo values, or the “Equilibrium Climate Sensitivity” for the PETM.
The IPCC’s best estimate for climate sensitivity in our time is 3°C, but that comes with a lot of uncertainty: it could be anything between 2° to 5°C—due to our imperfect knowledge of feedbacks in the Earth system. If the sensitivity turns out to be on the higher end, then we will heat more for a given amount of emissions. Tierney’s study found the PETM’s climate sensitivity to be 6.5°C, more than twice the IPCC’s best estimate.
A higher number “isn’t too surprising,” Tierney told me, because previous investigations had indicated the Earth’s response to COtwo is stronger at higher COtwo levels of Earth’s past. Our climate sensitivity won’t be as high: “We don’t expect that we’ll experience 6.5°C climate sensitivity tomorrow,” Tierney explained.
However, his paper suggests that if we continue to raise COtwo levels, it will push the temperature response to that COtwo higher. “We might expect some level of increased climate sensitivity in the near future, especially if we emit more greenhouse gases,” Tierney said.
Climate mapping by “data assimilation”
The new, sharper picture emerges from how Tierney’s team tackled the perennial problem for geologists: We don’t have data for every place on the planet. Geologic data for the PETM is limited to locations where sediments from that time are preserved and accessible, usually through a well or outcrop on land. any conclusion about global the weather needs to be zoomed in from those sparse data points.
“It’s actually a tough problem,” Tierney said. “If you want to understand what’s happening spatially, it’s really hard to do that with just the geological data.” So Tierney and his colleagues borrowed a technique from weather forecasting. “What the weather people are doing is running a weather model, and as the day goes on, they take measurements of wind and temperature, and then assimilate those into their model … and then they run the model again to improve the prognosis. Tierney said.
Instead of thermometers, his team used temperature measurements of the remains of microbes and plankton preserved in 56-million-year-old sediments. Instead of a weather model, they used a climate model that had Eocene geography and no ice sheets to simulate the climate just before and at the peak of the PETM heat. They ran the model a bunch of times, varying COtwo levels and the Earth’s orbital configuration due to uncertainties therein. They then used the microbe and plankton data to select the simulation that best fit the data.
“The idea is really to take advantage of the fact that the model simulations are spatially complete. But they are models, so we don’t know if they are right. The data knows what happened, but it’s not spatially complete,” Tierney explained. “So by mixing them together, we get the best of both worlds.”
To see how well their blended product matched reality, they compared it to independent data derived from pollen and leaves, and from locations not included in the blending process. “They actually matched very, very well, which is kind of comforting,” Tierney said.
“The novelty of this study is to use a climate model to rigorously determine which climate state best fits the data before and during the PETM, providing patterns of climate change around the world and a better estimate of global mean temperature change. ”. said Dr. Tom Dunkley Jones of the University of Birmingham, who was not part of the study.
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