There is some bad news from the last big heat wave of the Earth

50 million years ago, in the early Eocene era, the world was significantly warmer than today. This period is considered the best guide to the conditions that humanity can expect if we take no action to reduce greenhouse gas emissions. The most advanced model of early Eocene atmospheric systems provides a warning that extreme warming, although unlikely, is a greater risk than previously thought.

Although the Earth has experienced many climates over its history, the temperatures over the last million years have been similar to those of today, even cool enough, but never warmer . To see what a much hotter world would look like, we have to go back to the early Eocene (48 to 56 million years ago), as did Dr. Jiang Zhu of China. University of Michigan. Disturbingly, Zhu found that global conditions at the time were 50% more sensitive to mild carbon dioxide changes than today.

"We were surprised by the increase in climate sensitivity relative to the level of carbon dioxide," Zhu said in a statement. "This is a frightening finding because it indicates that the reaction of temperature to a future increase in carbon dioxide may be more important than the response to the same increase in CO₂ now. This is not good news for us. "

Zhu and his co-authors of an article in Science Advances are not the first to attempt to model the conditions of the Eocene, and previous efforts have come to different conclusions. The authors attribute the differences in their results to more detailed modeling of cloud behavior changes as carbon dioxide levels increase, with low and mid-level clouds turning to rain much faster than normal. aujourd & # 39; hui.

Depending on their thickness and altitude, clouds can warm or cool the planet. Therefore, switching from one type of cloud to another can have a major impact. A recently published article identified a threshold of carbon dioxide above which the main cooling clouds disappear almost entirely, causing a sudden and violent jump in temperature.

It is thought that the beginning of the Eocene was so hot because of the release of huge amounts of greenhouse gases. Although geologically rapid, the increase in temperature probably occurred on much longer time scales than the Anthropocene.

Like all scientific articles, the peer review process for this one will not stop at publication and errors can be found. Equally important differences between current conditions and those of the Eocene can be identified. However, if this does not happen, we run an even greater risk than is suggested by existing climate models.

The authors doubt that the threat is immediate. "It is unlikely that climate sensitivity will reach Eocene levels in our lifetime," said co-author Dr. Jessica Tierney of the University of Arizona. CO at the beginning of the Eocene₂ was 2-6 times the current levels, something that can not be equaled until at least 80 years old. However, this work shows that if we do not control our emissions quickly, the 22nd century could be even more hostile to human survival than predicted by climate models.