According to a new study, when it comes to measuring global warming, humidity, not just heat, matters in generating dangerous climate extremes.
The energy generated in extreme weather, such as floods, storms and rainfall is related to the amount of water in the air. So a team of scientists in the U.S. and China decided to use an obscure weather measurement called equivalent potential temperature-or theta-e-that reflects “the moisture energy in the atmosphere,”said study co-author V. Ramanathan, a climate scientist at the University of California. Like temperature, it is expressed in degrees.
“There are two drivers of climate change: temperature and humidity,” said Ramanathan. “And so far we measured global warming just in terms of temperature.”
But by adding the energy from humidity, “the extremes-heat waves, rainfall and othe measures of extremes-correlate much better,” he said.
That’s because as the Earth warms, the air holds more moisture, nearly 7% for every degree Celsius. When moisture condenses, it releases heat or energy, “that’s why when it rains, now it pours,” said Ramanathan.
Also, water vapor is a powerful heat-trapping gas in the atmosphere that increases climate change, he said.
When judging by temperature alone, it looks like warming is more pronounced in North America, mid-latitudes and especially the poles-and less so in the tropics, said Ramanathan.
However that is not the case, he said, because the high humidity in the tropics juices up storm activity, from regular storms to tropical cyclones and monsoons.
“This increase in latent energy is released in the air which leads to weather extremes: floods, storms and droughts,” said Ramanathan.