New maps of ancient warming show strong response to carbon dioxide

One A study published in PNASJessica Tierney, a professor at the University of Arizona, and colleagues have produced comprehensive global maps of the carbon-based warming that occurred during the Paleocene Eocene Thermal Maximum (PETM) 56 million years ago.

While the PETM has some parallels with current warming, the new work includes some unexpected results – the climate response to CO2.₂ then nearly twice as strong as the current best estimate by the Intergovernmental Panel on Climate Change (IPCC). But changes in rainfall patterns and increased polar warming were remarkably consistent with modern trends, even though it was a very different world back then.

A different world

The warming of the PETM was caused by a geologically rapid release Cho₂primarily a convulsion of magma In the Earth’s mantle where Iceland is now. Magma invaded oil-rich sediments in the North Atlantic and boiled off the CO₂ and methane. It already got hot, high CO₂ climate and made it warmer over tens of thousands of years, driving some deep sea creatures and some tropical plants don’t disappear mammals evolved smallerand there was a big migrations between continents; crocodiles, hippopotamus and palm trees all developed within only 500 miles of the North Pole and Antarctica it was ice free.

As our climate warms, so will scientists increasingly looking at past climates for concepts, but they temperature uncertainties, CO₂ levels and the precise timing of changes—for example, previous work on the PETM had temperature uncertainties of 8° to 10°C. Now, Tierney’s team has narrowed this range of uncertainty to just 2.4°C, showing that the PETM warmed by 5.6°C, an improvement of about 5°C from previous estimates.

“We were able to really narrow down that estimate compared to previous work,” Tierney said.

The researchers also calculated the CO₂ levels before and during the PETM derived from boron isotopes measured in fossil plankton shells. They found CO₂ It was about 1,120 ppm just before the PETM and rose to 2,020 ppm at its peak. In comparison, pre-industrial CO₂ was 280 ppmand we are currently approx 418 ppm. The team was able to use this new temperature and CO₂ values ​​to calculate how much the planet warms in response to a doubling of CO₂ values ​​or “Equilibrium Climate Sensitivity” for PETM.

High sensitivity

The IPCC’s best estimate for climate sensitivity in our time is 3°C, but this comes with a large amount of uncertainty – it could be anything in between. 2° to 5°C– to our imperfect knowledge reviews In the earth system. If the sensitivity is higher, we will get warmer for a given amount of emissions. Tierney’s study found PETM climate sensitivity to be 6.5°C—more than twice the IPCC’s best estimate.

The higher number “isn’t too surprising,” Tierney said, because previous research He showed the reaction of the Earth to CO₂ stronger at high CO₂ Past levels of the earth. Our climate sensitivity won’t be as high: “We don’t expect to experience 6.5°C climate sensitivity tomorrow,” Tierney said.

But their paper shows that if we continue to increase CO₂ levels, it will nudge the temperature response to CO₂ higher. “We can expect some level of climate sensitivity to increase in the near future, especially if we emit more greenhouse gases,” Tierney said.

Climate mapping with “Data Assimilation”.

A new, sharper picture emerges from how Tierney’s team tackles a perennial problem for geologists: We don’t have data for every part of the planet. Geologic data for PETM are limited to locations where sediments from that time are preserved and accessible—usually either through wells or outcrops. About the end of the year global climate must be augmented from these sparse data points.

“It’s actually a tough challenge,” Tierney said. “If you want to understand what’s going on in space, it’s really hard to do that based on geological data alone.” So Tierney and his colleagues borrowed a technique from weather forecasting. “What weather people do is they run a weather model and they measure wind and temperature as the day goes by, then they assimilate that into their model … and then they run the model again to improve the forecast,” Tierney said.

Instead of thermometers, his team used temperature measurements of microbes and plankton fossils preserved in 56-million-year-old sediments. Instead of a weather model, they used a climate model with Eocene geography and no ice sheets to simulate the climate at the peak of PETM warming. They ran the model repeatedly, changing CO₂ levels and the Earth’s orbital configuration because of the uncertainties in these. They then selected the simulation that best fit the data using the microbial and plankton data.

“The idea is to really take advantage of the spatial resolution of the model simulations. But they are models, so we don’t know if they are right or not. The data knows what’s going on, but it’s not spatially complete,” Tierney explained. “So by mixing them, we get the best of both worlds.”

To see how well the blend products matched reality, they compared it to independent data from pollen and leaves and from locations not involved in the blending process. “They’ve really adjusted really well, which is kind of comforting,” Tierney said.

“What is new about this study is the use of a climate model to rigorously examine which climate conditions best fit the data both before and during the PETM, providing a better estimate of global climate change patterns and global average temperature change.” said Dr. Tom Dunkley Jones of the University of Birmingham, who was not part of the study.