New maps of ancient warming show strong response to carbon dioxide

In a study published in PNAS, University of Arizona professor Jessica Tierney and colleagues created complete global maps of the carbon-driven warming that occurred 56 million years ago in the Paleocene-Eocene Thermal Maximum (PETM).

While the PETM shows some parallels with current warming, the new work contains some unexpected findings – the climate’s response to CO₂ was about twice the current best estimate by the Intergovernmental Panel on Climate Change (IPCC). But changes in precipitation patterns and the increase in warming at the poles were remarkably consistent with modern trends, even though it was a very different world then.

Another world

The warming of the PETM was triggered by a geologically rapid release of CO₂, mainly from a magma convulsion in the Earth’s mantle where Iceland is today. The magma invaded oil-rich sediments in the North Atlantic and evaporated CO₂ and methane. It took an already warm, high CO₂ climate and made it hotter for tens of thousands of years, causing some deep-sea creatures and some tropical plants to become extinct. Mammals evolved smaller and there were great migrations across continents; Crocodiles, hippo-like creatures, and palm trees all thrived just 500 miles from the North Pole, and Antarctica was ice-free.

As our climate warms, scientists increasingly look to past climates for insight, but they are hampered by uncertainties related to temperature, CO₂ Levels and the exact timing of the changes — previous work on the PETM, for example, had temperature uncertainties on the order of 8° to 10°C. Now, Tierney’s team has narrowed that range of uncertainty down to just 2.4°C, showing that the PETM has changed by 5 .6 °C, a refinement of the previous estimate of about 5 °C.

“We’ve really been able to narrow this estimate down from previous work,” said Tierney.

The researchers also calculated the CO₂ Concentrations before and during PETM derived from isotopes of boron measured in fossil plankton shells. They found CO₂ was around 1,120 ppm just before the PETM and rose to 2,020 ppm at its peak. For comparison: pre-industrial CO₂ was 280ppm and we are currently at about 418ppm. The team was able to take advantage of this new temperature and CO₂ Values ​​to calculate how much the planet has warmed in response to a doubling of CO₂ Values ​​or the “Equilibrium Climate Sensitivity” for the PETM.

Highly sensitive

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° and 5°C – due to our imperfect knowledge of Earth system feedbacks. If the sensitivity turns out to be on the higher end, we’ll be heating up more for a given amount of emissions. Tierney’s study found that the PETM climate sensitivity was 6.5°C — more than double the IPCC’s best estimate.

A higher number is “not too surprising,” Tierney told me, because previous research had shown Earth’s response to CO₂ is stronger at higher CO₂ Planes of Earth’s past. Our climate sensitivity won’t be that high: “We don’t expect to see a climate sensitivity of 6.5°C tomorrow,” explained Tierney.

However, your paper suggests that if we continue to increase CO₂ levels, it will trigger the temperature response to that CO₂ higher. “We could expect some level of heightened climate sensitivity in the near future, particularly if we emit more greenhouse gases,” Tierney said.

Climate mapping through “data assimilation”

The new, sharper picture comes from the way Tierney’s team dealt with geologists’ perennial problem: We don’t have data for every place on Earth. Geological data for the PETM is limited to locations where sediments from this period are preserved and accessible – usually either via a borehole or outcrop on land. Any conclusions about global Climate needs to be scaled up from these sparse data points.

“It’s actually a tough problem,” Tierney remarked. “If you want to understand what’s happening spatially, it’s really difficult to do that from geological data alone.” So Tierney and colleagues borrowed a technique from weather forecasting. “Weather experts run a weather model and take measurements of wind and temperature throughout the day, which they then integrate into their model… and then re-run the model to improve the forecast. ‘ Tierney said.

Instead of thermometers, her team used temperature measurements of microbial and plankton debris 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 just before and at the peak of PETM warmth. They ran the model a few times and varied the CO₂ planes and the configuration of the Earth’s orbit because of the uncertainties in these. Then they used the microbial and plankton data to select the simulation that best matched the data.

“The idea is really to exploit the fact that model simulations are spatially complete. But they’re models, so we don’t know if they’re right. The data knows what happened, but it’s not spatially complete,” Tierney said. “So by mixing them, we get the best of both worlds.”

To see how well their blended product matched reality, they checked it against independent data obtained from pollen and leaves and from locations not involved in the blending process. “They were really, really a good match, which is a bit reassuring,” Tierney said.

“The novelty of this study is to use a climate model to rigorously work out which climate state best fits the data, both before and during the PETM, thereby providing patterns of climate change around the world and a better estimate of global mean temperature change.” be preserved.” said Dr. Tom Dunkley Jones from the University of Birmingham, who was not part of the study.