In a study published in PNAS, University of Arizona Professor Jessica Tierney and colleagues produced complete global maps of carbon-induced warming that occurred in the Paleocene Thermal Maximum (PETM), 56 million years ago.
Although there are some similarities between the PETM period and current warming, the new work includes some unexpected consequences – the climate response to carbon dioxide.2 It was then twice stronger than the current best estimate by the Intergovernmental Panel on Climate Change (IPCC). But changes in precipitation patterns and amplification of warming at the poles were remarkably consistent with recent trends, despite it being an entirely different world at the time.
The warming of the Petam period was caused by a geologically rapid release of carbon dioxide2, primarily from the spasm of magma in the Earth’s mantle in the place where Iceland is now located. Magma invaded the oil-rich sediments in the North Atlantic, bringing carbon dioxide to a boil2 and methane. It required a really high carbon monoxide content to be warm2 It made the climate hotter for tens of thousands of years, driving some deep-sea creatures and some tropical plants to extinction. Mammals evolved smaller, and there were large transcontinental migrations. Crocodiles, hippos-like creatures, and palm trees thrived only 500 miles from the North Pole, and Antarctica was virtually ice-free.
As our climate warms, scientists increasingly look to past climates for insights, but are held back by temperature uncertainty, and carbon dioxide.2 Levels, and exact timing of changes – Previous work on PETM had an uncertain temperature in the range of 8° to 10°C, for example. Now Tierney’s team has narrowed this uncertainty down to just 2.4 °C, showing that PETM has warmed by 5.6 °C, an improvement over the previous estimate of ~5 °C.
“We were really able to narrow that estimate to previous work,” Tierney said.
The researchers also calculated carbon dioxide2 Levels before and during the PETM period derived from boron isotopes measured in fossil plankton shells. They found CO2 It was around 1,120ppm before the PETM period, and it rose to 2020ppm at its peak. For comparison, CO2 It was 280ppm, and we are currently about 418ppm. The team was able to use the new temperatures and carbon dioxide2 Values to calculate how much the planet is warming in response to the carbon dioxide doubling2 values, or “equilibrium climate sensitivity” for PETM.
The IPCC’s best estimate of the climate sensitivity of our time is 3°C, but this comes with a great deal of uncertainty – it could be anything between 2° to 5°C – due to our incomplete knowledge of the Earth system’s feedback. If sensitivity turns out to be on the higher end, we’ll heat up more for a given amount of emissions. Tierney’s study found that the climate sensitivity of the PETM period was 6.5 °C – more than double the IPCC’s best estimate.
Tierney told me the higher number “isn’t too surprising,” because previous research has indicated Earth’s response to carbon dioxide.2 It is stronger at higher CO2 level2 Earth’s past levels. Our climate sensitivity wouldn’t be that high: “We don’t expect we’ll have a 6.5°C climate sensitivity tomorrow,” Tierney explained.
However, their paper suggests that if we continue to raise CO2 levels, it will induce a temperature response to that CO2 higher. “We may expect some level of climate sensitivity increase in the near future, especially if we release more greenhouse gases,” Tierney said.
Climate Mapping Using Data Intake
The new, clearer picture emerges from the way Tierney’s team has tackled the perennial problem of geologists: we don’t have data for every place on the planet. PETM’s geological data is limited to sites where sediments from that time are preserved and accessible – usually either through a borehole or outcrop on the ground. Any conclusions about worldwide Climate must be scaled up from these scattered data points.
“It’s actually a tough problem,” Tierney said. “If you want to understand what’s going on spatially, it’s really hard to do that with geological data alone.” So Tierney and his colleagues borrowed a technique from weather forecasting. “What the weather experts do is they run a weather model, and as the day goes on, they take wind and temperature measurements, and then they integrate them into their model…and then they run the model again to improve the forecast,” Tierney said.
Instead of thermometers, her team used temperature measurements from microbial and plankton remains preserved in 56-million-year-old sediments. Instead of a weather model, they used a climate model with Eocene geography and no ice layers to simulate the climate prior to, and at, peak warmth PETM. They rotated the model several times, with varying degrees of CO2 levels and the Earth’s orbital configuration due to uncertainties in those. They then used the microbe and plankton data to choose 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 it up, we get The best of both worlds.”
To see how well their blended product matched reality, they checked it against independent data derived from pollen and leaves, and from places not included in the blending process. “They matched up actually, really well, which is rather comforting,” Tierney said.
“The novelty of this study is the use of a climate model to accurately work out the climate condition that best fits the data before and during the PETM period, giving patterns of climate change around the world and a better estimate of average global temperature change,” said Dr. Tom Dunkley-Jones from the University of Birmingham, which was not part of the study.
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