Fifty million years ago, at the start of the Eocene Era, the world was substantially warmer than today. This period is regarded as the best guide we have to the conditions humanity can expect if we don't take action to stem greenhouse emissions. The most advanced model yet of early Eocene atmospherics provides a warning, suggesting really extreme warming, while still unlikely, is a greater risk than has previously been acknowledged.

Although the Earth has had many different climates in its history, temperatures over the last few million years have been similar to today or quite a bit cooler, but never warmer. To see what a substantially warmer world would look like, we need to go back to the early Eocene (48-56 million years ago) as Dr Jiang Zhu of the University of Michigan has done. Disturbingly, Zhu found global conditions at the time were 50 percent more sensitive to small changes in carbon dioxide than today.

"We were surprised that the climate sensitivity increased as much as it did with increasing carbon dioxide levels," Zhu said in a statement. "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 the same increase in CO₂ now. This is not good news for us."

Zhu and his co-authors of a paper in Science Advances are not the first to try to model Eocene conditions, and previous efforts have reached different conclusions. The authors attribute the differences in their findings to a more detailed modeling of changes in cloud behavior as carbon dioxide levels rise, with low- and mid-altitude clouds turning to rain much more quickly than they do today.

Depending on their thickness and altitude, clouds can either warm or cool the planet, so a shift from one type of cloud to another can have a major impact. One recently published paper identified a carbon dioxide threshold above which the main cooling clouds almost entirely vanish, causing a sudden, drastic temperature jump.

The early Eocene is thought to have been so warm because of the release of enormous quantities of greenhouse gasses. Although rapid by geological standards, the temperature increase still probably took place on time-scales far longer than those of the Anthropocene.

Like all scientific papers, the peer review process for this one will not stop at publication and errors may be found. Equally important differences between current conditions and those in the Eocene may be identified. If neither happens, however, we are in even greater danger than existing climate models suggest.

The authors doubt the threat is immediate. “It's unlikely that climate sensitivity will reach Eocene levels in our lifetimes," co-author Dr Jessica Tierney of the University of Arizona said. Early Eocene atmospheric CO₂ was 2-6 times today's levels, something unlikely to be matched for at least 80 years. However, this work makes a case that, unless we quickly get our emissions under control, the 22nd century could be even more hostile to human survival than climate models have predicted.