New maps of ancient warming reveal strong response to carbon dioxide

In a study published in PNAS, University of Arizona Professor Jessica Tierney and her colleagues produced comprehensive, global-scale maps of the carbon-induced 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 findings – the climate response to CO₂ was then about twice as high as the current best estimate of the Intergovernmental Panel on Climate Change (IPCC). But the changes in precipitation patterns and the amplification of warming at the poles were remarkably consistent with modern trends, despite being a very different world at the time.

A different world

The warming of PETM was triggered by a geologically rapid release of CO₂, primarily from a convulsion of magma in the Earth’s mantle where Iceland now stands. Magma invaded the oil-rich sediments of the North Atlantic, boiling CO₂ and methane. It took an already hot, high-CO₂ climate and made it warmer for tens of thousands of years, leading to the extinction of some deep-sea creatures and tropical plants. 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 are increasingly looking to past climates for information, but they are hampered by uncertainties in temperature, CO₂ levels and exact timing of changes – previous work on PETM had temperature uncertainties in the range of 8° to 10°C, for example. Now Tierney’s team has reduced that uncertainty range to just 2.4°C, showing that PETM has warmed by 5.6°C, a refinement from the previous estimate of around 5 °C.

“We were really able to lower this estimate compared to previous work,” Tierney said.

The researchers also calculated the CO₂ levels before and during PETM derived from boron isotopes measured in fossil plankton shells. They found CO₂ was around 1120 ppm just before the PETM, dropping to 2020 ppm at its peak. For comparison, pre-industrial CO₂ was 280 ppm, and we’re currently at about 418 ppm. The team was able to use this new temperature and CO₂ values ​​to calculate how much the planet has warmed in response to a doubling of CO₂ or the “equilibrium climate sensitivity” for the PETM.

Very sensitive

The IPCC’s best estimate for climate sensitivity in our time is 3°C, but this comes with great uncertainty – it could be between 2° and 5°C – due to our imperfect knowledge of feedbacks in the earth system. If the sensitivity turns out to be at the higher end, we will warm up more for a given amount of emissions. Tierney’s study found the climate sensitivity of the PETM to be 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 indicated Earth’s response to CO₂ is stronger at higher CO₂ levels of the Earth’s past. Our climate sensitivity won’t be that high: “We don’t expect to experience a climate sensitivity of 6.5°C tomorrow,” Tierney explained.

Their paper, however, suggests that if we continue to increase CO₂ levels, it will push the temperature response to that CO₂ upper. “We might expect some level of increased climate sensitivity in the near future, especially if we emit more greenhouse gases,” Tierney said.

Mapping the climate by “Data Assimilation”

The new, sharper picture emerges from how Tierney’s team dealt with the perennial problem for geologists: we don’t have data for every place on the planet. PETM geologic data is limited to locations where sediments from this era are preserved and accessible, usually via a borehole or land outcrop. Any conclusion on overall the climate must be extrapolated from these scattered data points.

“It’s actually a tough problem,” Tierney remarked. “If you want to understand what’s going on in space, it’s really hard to do that from geological data alone.” So Tierney and his colleagues borrowed a technique from weather forecasting. “What meteorologists do is they run a weather model, and as the day progresses, they take measurements of wind and temperature, and then they assimilate that into their model…and then run again the model to improve predictions,” said Tierney.

Instead of thermometers, his team used temperature measurements from remnants 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 caps to simulate the climate just before and at the peak of the PETM heat. They ran the model several times, varying the CO₂ levels and orbital configuration of the Earth due to uncertainties in these. Then they 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’re models, so we don’t know if they’re right. The data knows what happened, but it’s not spatially complete,” Tierney explained. “So by mixing them, 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 heartwarming,” Tierney said.

“The novelty of this study is to use a climate model to rigorously determine which climate state best matches data before and during the PETM, yielding worldwide climate change patterns and a better estimate of global average temperature change. “said Dr Tom Dunkley Jones of the University of Birmingham, who was not part of the study.