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Researchers at the University of Arizona helped create a state-of-the-art computer model that shows how a spike in carbon dioxide over 50 million years ago may have caused global temperatures to skyrocket—a "scary finding" that has major implications for Earth's future climate.

A team of researchers from the UA and the University of Michigan built a climate model to successfully simulate the extreme warming of the Early Eocene Period, considered an analog for Earth's future climate.

"We were surprised that the climate sensitivity increased as much as it did with increasing carbon dioxide levels," said Jiang Zhu, a postdoctoral researcher at the University of Michigan Department of Earth and Environmental Sciences, and the paper's first author.

"It is a scary finding because it indicates that the temperature response to an increase in carbon dioxide in the future might be larger than the response to same increase in CO2 now," Zhu told UA News. "This is not good news for us."

For the first time, researchers can examine how the rate of warming in the Early Eocene—which occurred between 48 to 56 million years ago—increased dramatically as carbon dioxide levels rose and temperatures were on average 14 degrees Celsius above what they are today — the warmest levels in 66 million years.

The research paper was published in the journal Science Advances.

In the simulations, the researchers found that there was a higher rate of climate sensitivity to increases in carbon dioxide levels because of "small-scale cloud processes" that helped determine "large-scale climate changes," and suggest "a potential increase in climate sensitivity with future warming."

By better understanding the Early Eocene, the researchers hope to better understand how large amounts of carbon dioxide pumped into the atmosphere by humans will drive future climate change. By better representing how clouds work in the climate, the researchers discovered that the climate could be more sensitive than previously observed, and their research helped improve a model known as the Community Earth System Model version 1.2, or CESM1.2.

Until now, climate models have struggled to simulate the warmth of the Early Eocene, including several short, intensely warm events, including one known as the Paleocene-Eocene Thermal Maximum, or PETM, which occurred over a few thousand years. And, the ecological consequences were dire, with widespread extinctions for sea-life and land animals.

Global warming is expected to change the distribution and types of clouds in the Earth’s atmosphere, and clouds can have both warming and cooling effects on the climate, the researchers said. In their simulations of the Early Eocene, Zhu and his colleagues found a reduction in cloud coverage and opacity that actually drove CO2-induced warming.

And, these same cloud process are active today, according to the authors of the Science Advances paper.

"Our findings highlight the role of small-scale cloud processes in determining large-scale climate changes and suggest a potential increase in climate sensitivity with future warming," said University of Michigan paleoclimate researcher Christopher Poulsen, a co-author of the Science Advances paper.

"The sensitivity we're inferring for the Eocene is indeed very high, though it's unlikely that climate sensitivity will reach Eocene levels in our lifetimes," said Jessica Tierney, an associate professor in the UA Department of Geosciences and the paper's third author.

The Early Eocene was a time of elevated atmospheric carbon dioxide concentrations, with temperatures that were on average at least 14 degrees Celsius—25 degrees Fahrenheit—warmer than they are today. During this period, plants and animals were found at much higher latitudes, and the poles lost their ice.

The reasons for the warming during the PETM are not known, but researchers have estimated that the event was linked to the sudden release of methane hydrates from the ocean's floor that were triggered by a massive volcanic eruption.

The temperature is linked to the increase of carbon dioxide levels that reached 1,000 parts per million in the Early Eocene, more than twice the present-day level of 412 ppm, according to geological evidence. In May, the Mauna Loa Atmospheric Baseline Observatory at the National Oceanic and Atmospheric Administration recorded the highest CO2 concentration in 61 years, at 414 ppm.

And, climate scientists have estimated that CO2 levels could once again hit 1,000 parts per million by 2100.

"For decades, the models have underestimated these temperatures, and the community has long assumed that the problem was with the geological data, or that there was a warming mechanism that hadn't been recognized," Poulsen said.

Until now, climate models have struggled to simulate the warmth of the Early Eocene, and researchers would adjust the model to make it work.

Unsubstantiated changes to the models were required to make the numbers work, Poulsen told UA News.

But the CESM1.2 model was able to simulate both the warm conditions and the low equator-to-pole temperature gradient seen in the geological records.

CESM1.2 was one of the climate models used by Intergovernmental Panel on Climate Change for its Fifth Assessment Report released in 2014.

And, the updated climate model's ability to simulate Early Eocene warming shows that the model's prediction of future warming may be right, including a key climate measurement called "equilibrium climate sensitivity." The measurement means that the planet may be more likely to continue to warm, even with deep cuts to CO2 emissions.

With sustained doubling of CO2 from pre-industrial levels of 285 ppm, climate scientists have estimated that the temperature will be 2.7 to 8.1 degrees Fahrenheit higher. But, the ECS shown in CESM1.2 is near the upper-end of the range, and the study's Early Eocene simulations showed that the era was more sensitive to warming.

New models will help underpin the IPCC's next report due in 2021.

"For the first time, a climate model matches the geological evidence out of the box—that is, without deliberate tweaks made to the model. It's a breakthrough for our understanding of past warm climates," Tierney said.

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