Maps of the past can shed light on our climate

image: Reconstructed surface air temperature (left) and amount of precipitation (right) during the Paleocene-Eocene thermal peak warming event, 56 million years ago. The maps were created by mixing geological data with climate model simulations using a technique called paleoclimate data assimilation.
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Credit: Jessica Tierney

Around 56 million years ago, volcanoes rapidly dumped massive amounts of carbon dioxide into the atmosphere, rapidly warming the Earth.

This period – called the Paleocene-Eocene Thermal Maximum, or PETM – is often used as a historical parallel for our own future under climate change, since humans have also been rapidly dumping carbon dioxide into the atmosphere over the past 250 years. .

A team of researchers led by the University of Arizona published a study in Proceedings of the National Academy of Sciences that includes temperature and precipitation maps of Earth during the PETM to help better understand conditions at that time. time and climate sensitivity to high levels of carbon dioxide.

The team, led by UArizona geosciences professor Jessica Tierneycombined temperature data and previously published climate models to confirm that the PETM is, in fact, a good indicator of what could happen to the climate based on future projections of carbon dioxide levels.

“The PETM is not a perfect analogue for our future, but we were somewhat surprised to find that yes, the climate changes we reconstructed share many similarities with future predictions, as shown in the latest AR6 report from the IPCC (Intergovernmental Panel on Climate Change),” Tierney said.

Both the distant period and our future are characterized by faster warming at the poles than the rest of the globe – a phenomenon called Arctic amplification – as well as stronger monsoons, more intense winter storms and less precipitation at the far reaches of the Arctic. tropics. . The researchers also found that as more carbon dioxide is pumped into the air, the climate becomes more sensitive than previously predicted.

“Overall, our work helps us better understand our future in the face of climate change,” Tierney said. “This provides some confirmation that the underpinnings of climate change – such as polar amplification, more intense monsoons and winter storms – are features of past and future greenhouse gas-rich climates.”

Tierney and his team constructed their PETM maps by combining so-called proxy temperature data with climate models. Paleoclimatologists like Tierney can infer past temperatures by chemically analyzing certain types of fossils from a given time period. This proxy temperature data, combined with modern climate modeling technology, allowed Tierney and his collaborators to create global temperature maps of the PETM.

The climate models used by researchers to create the maps of the past are generally used to make future climate predictions – including those in the IPCC Assessment Reports. Instead, Tierney and his team used them to generate simulations of what Earth looked like 56 million years ago.

“We moved the continents to match the PETM, then we ran simulations at different levels of carbon dioxide, ranging from three to 11 times current levels – or from 850 parts per million to a very high 3,000 parts. by millions – because these are all possible levels of carbon dioxide that could have occurred in the PETM,” Tierney said. “For context, the carbon dioxide in our atmosphere today is about 420 parts per million and it was around 280 parts per million before the industrial revolution. By adding the geological evidence, we narrowed down the simulations to those that best matched that evidence.”

Tierney and his team have used this method in previous studies to reconstruct climate during more recent times.

The new study also more accurately estimates how much the globe warmed during the PETM. Previous studies have suggested that the PETM was 4 to 5 degrees Celsius warmer than the period just before. Tierney’s research, however, found that number to be 5.6 degrees Celsius, suggesting the climate is more sensitive to increases in carbon dioxide than previously thought.

Climate sensitivity is the amount of global warming per double of carbon dioxide.

“It’s really important to fix that number, because if the climate sensitivity is high, we’ll see more warming by the end of the century than if it’s lower,” Tierney said. “IPCC AR6 predictions range from 2 to 5 degrees Celsius per doubling of carbon dioxide. In this study, we quantify this sensitivity during PETM and found that the sensitivity is between 5.7 and 7.4 degrees Celsius per doubling, which is much higher.

Ultimately, this means that with higher levels of carbon dioxide than we have today, the planet will become more sensitive to carbon dioxide, which Tierney says “is something significant. to think about longer-term climate change, beyond the end of the century.”

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