(Inside Science) -- Climate change is alarming enough if you only consider temperature changes. But the warming trend stands out even more sharply when you use a metric that reflects how much the heat stresses human bodies, according to a new study.
A team of researchers looked at average summertime conditions across a wide swath of the Northern Hemisphere using a measure of heat stress that combines temperature and humidity. Their analysis indicated that within two to three decades, summers in each region studied will routinely feel hotter than the muggiest summer on record from 1973 to 2012.
"Every second year you'd have a summer that is warmer than that record warm level," said Francis Zwiers, chief of the Canadian Centre for Climate Modelling and Analysis in Victoria and senior author of a paper published last month in the journal Earth's Future.
If you want to know whether conditions are dangerously hot, humidity is a crucial part of the equation. That's because humans cool themselves by sweating, and sweat doesn't evaporate as well when the air is already full of moisture.
Climate models have long predicted that global warming will make Earth more humid on average, since warm air can hold more moisture than cold air. But until recently, it was hard to demonstrate rising humidity trends in the real world because of a lack of reviewable historical data.
That changed in 2014, when the U.K.'s Met Office published a humidity dataset compiled from measurements taken at weather stations since 1973. This dataset allowed Zwiers and his colleagues to analyze historical conditions for land-covered regions between 10 degrees north and 60 degrees north latitude.
To estimate how hot conditions felt, the researchers used a metric called wet bulb globe temperature. This metric traditionally combines temperature, humidity, solar radiation and wind, but the researchers used a simplified version with only humidity and temperature. Wet bulb globe temperature is the heat stress metric used by the International Organization for Standardization, and it has well-validated thresholds for when humans can safely work, giving it advantages over alternative metrics such as heat index, said study co-author Chao Li, a hydroclimatology scientist at the University of Victoria in Canada.
The researchers divided the land in their study into patches 5 degrees latitude by 5 degrees longitude. For each patch, they calculated average wet bulb globe temperature across the three months of summer, producing average summer values for each year from 1973 to 2012.
Once they had summer averages for each patch, they combined these values into summer averages for 10 larger regions such as northern Europe, the Mediterranean, and eastern North America. Next, they compared the real-world values with thousands of simulations using climate models. Only simulations that included fossil fuel emissions matched the real-world wet bulb globe temperature trends, once more confirming humanity's role in climate change.
The researchers also looked at the changing likelihood of "extremely warm summers," defined as the real-world summer in each region with the highest average wet bulb globe temperature between 1973 and 2012. By running thousands of simulations, they could compare how often extremely warm summers cropped up in each of those 40 years. They found that the likelihood of such extreme summers rose sharply in every region, cropping up at least 70 times more often in 2012 than in 1973 -- a tenfold increase over the equivalent change looking at temperature alone.
Next, the researchers projected wet bulb globe temperatures into the future. They found that by the 2030s, extremely warm summers would occur about every other year. By the 2050s, they happened almost every year.
Real-world data from 2012 to 2016 is in line with their predictions. The upward trend has continued over the last few years, with two of the 10 regions breaking their previous 40-year record, according to Chao.
The researchers were initially surprised by the dramatic findings, said Zwiers. But when they looked closer, they saw that the wet bulb metric didn't actually rise any more sharply than temperature. The difference was that temperature jumped around from year to year, while wet bulb was much less variable. Because the range of modeled wet bulb globe temperatures in a given time and place was narrow, even small increases quickly moved a region out of its historical range, making it easy to see the steady rise in humid warmth.
"For this particular indicator, it's a really rapid separation of what the current range of variability is, and what we're projecting the future range of variability will be," said Zwiers. "That signal just stands out even more."
This isn't the first time climate researchers have looked at temperature and humidity together. Last year, Thomas Knutson and Jeffrey Ploshay at the U.S. National Oceanic and Atmospheric Administration's geophysical fluid dynamics laboratory at Princeton University in New Jersey used wet bulb globe temperature to detect human impacts on the climate. But according to Knutson, Zwiers and his colleagues used even more sophisticated methods, and they took things a step further by predicting future conditions.
"It considers both the past and the future projections angle, and sort of ties them together nicely," said Knutson. "I think it's a commendable job that they've done."
But Matthew Huber, a climate scientist at Purdue University in West Lafayette, Indiana, had several criticisms of the researchers' methods -- issues that he said could have led them to overestimate the risk of heat stress on humans. Huber said his lab is in the middle of its own project to predict future heat stress using wet bulb globe temperatures.
First, Huber raised concerns about the formula Zwiers and his colleagues used to calculate wet bulb globe temperature, both on technical grounds and because it ignored solar radiation. Huber said that while it's reasonable to ignore wind, the sun's brightness is crucial if you want to know how people will feel outside, when they are at the greatest risk of heat stress.
In addition, Huber criticized the researchers' decision to use monthly averages of temperature and rainfall to calculate the average wet bulb globe temperature for the whole summer, rather than deriving wet bulb values for shorter time intervals and combining them to get a summer average. Zwiers said that they tried calculating wet bulb values for each day before deriving seasonal averages, and it made no difference to the final results.
While the researchers disagreed about methods, they were unanimous about one thing: the need to look beyond the thermometer to assess how hot our changing climate will feel.
"It has important implications for human health," said Knutson. "It's something that we should not be overlooking."