New metric shows how severe global warming is getting

By taking into account how increasing surface temperatures will alter both humidity and a measure of the energy contained in the atmosphere, a new study finds the world is at a growing risk of extreme weather events.

Driving the news: The study, published Monday in Proceedings of the National Academy of Sciences, focuses on an integrated temperature and humidity metric.

• The metric, researchers in China and the U.S. find, shows that as global temperatures climb, humidity and atmospheric energy do so even faster.
• The boost in humidity and atmospheric energy, the study shows, are strongly correlated with trends in extreme heat and precipitation.

What they found: Surface warming is causing a faster increase in humidity, since warm air can hold more water vapor, and warming seas and land surfaces are giving up more water into the atmosphere through evaporation.

• While unchecked emissions might bring up to 4.8°C (8.64°F) of surface warming by 2100, the study finds it could cause the integrated measure to climb by up to 12°C (21.6°F) by 2100, relative to the preindustrial era.
• This could yield an increase of up to 60% in extreme precipitation, with a 40% increase in the energy to power tropical thunderstorms.

Threat level: At the same time, heat extremes could become 14 to 30 times more frequent, due to the combination of high heat and humidity.

• The most lethal combinations of ultra-high heat and humidity, which are being seen now in parts of India, the Persian Gulf, North America and Europe, would get hotter and even more deadly, the study finds.
• It calls this increase "debilitating," especially for vulnerable populations that lack access to air conditioning.

What they're saying: "It is the humidity increase accompanied by warming which makes climate changes into a climate crisis worldwide," study co-author V. Ramanathan of Scripps Institution of Oceanography said in an email.

• "The humidity amplification of the warming becomes more pronounced as the climate becomes warmer in the future because it increases exponentially with temperature," study lead author Ghuang Zhang of the Ocean University of China told Axios in an email.
• Andrew Dessler, a climate scientist at Texas A&M University, who was not involved in the new research, told Axios the findings make sense, but aren't completely surprising given what is already known about the tie between increasing temperatures and humidity.
• "How much more evidence do we need to see that it's going to be bad if we don't bend the emissions curve [downward]?" he said. "If you're not convinced now, this is probably not going to change your mind."