We are living through the most humid summer on record

The United States and the entire planet are poised to clinch their most humid summer on record, scientists say. The sweltering conditions, which have pushed this year’s heat close to the limits of survivability in some areas and fueled flooding downpours, are part of a long-term increase in humid heat driven by human-caused climate change.

Climate models have long predicted that a warming world would lead to higher humidity, because warmer air evaporates more water from Earth’s surface and can hold more moisture. The consequences of more humid heat include greater stress on the human body, increased odds of more extreme rainfall, warmer nights and higher cooling demand.

With only a few days left in meteorological summer, defined as June to August, this summer is on track to be the most humid in the United States in 85 years of recordkeeping based on observations of dew point - a measure of humidity - compiled by Hudson Valley meteorologist Ben Noll. It’s also likely to end up being the most humid summer globally, Alaska-based climate scientist Brian Brettschneider said in an email to The Washington Post.

If both trends hold, then five of the most humid summers in both the United States and worldwide will have occurred since 1998.

“I have been tracking increasing surface moisture at the monthly time scale. June 2024 and July 2024 both set records for highest dew point for their respective months,” Brettschneider said. “I expect August 2024 to be a record too. Summer 2024 should break the record set in summer 2023.”

This summer’s surge in humidity continues a trend that goes back several decades, with extreme humid heat having more than doubled in frequency since 1979, according to a 2020 study led by UCLA climate scientist Colin Raymond.

Increasing humidity “makes summer heat feel more relentless, with a particular effect on nighttime temperatures,” Raymond said in an email. “That means more demand for cooling, and worse health consequences when cooling is unavailable or unaffordable.”

Another study, published in 2022 by climate scientists in the United States and China, found that increasing humidity has “led to more frequent and stronger extreme events such as heat waves, hurricanes, convection storms, and flash floods.”

To cool itself down, the human body reacts to heat by sweating. The evaporation of sweat into the surrounding air is what cools the body. But evaporation occurs more slowly when the air is humid, making it harder to cool off and increasing the risk of heat-related illness or death. The heat index measures how hot the air feels when factoring in the humidity.

In one of the most extreme episodes of heat, portions of the Persian Gulf saw the heat index exceed 140 degrees in both mid-July and this week as dew points soared to around 90 degrees. In the United States, any dew point over 70 degrees is usually considered uncomfortable. The extreme humidity levels were tied to bathtub-like sea-surface temperatures as high as 95 degrees in the Persian Gulf.

A Post analysis found that heat and humidity levels in northern India in May, part of a broader heat wave that spanned much of South and Southeast Asia, surpassed a threshold that researchers have identified as posing a risk to human survival if such heat in prolonged.

“In places like India and areas near the Red Sea and Persian Gulf, the increase in moisture and higher temperatures drives [conditions] to near the point where heat illness is almost certain without air conditioning,” Brettschneider said.

Record heat and humidity also have scorched large portions of the United States. Historically hot conditions arrived in Florida in May. That’s when Key West, Fla., one of the nation’s southernmost points, tied its highest heat index on record at 115 degrees. Northern locales have seen humid heat, too, as the heat index in Preston, Minn., surged to 120 earlier this week as the dew point climbed to 86 degrees, just two degrees short of the state record. On Tuesday, the heat index in Chicago spiked to 115.

Globally, July was the world’s warmest July on record and the 14th consecutive month of record global heat, according to the National Oceanic and Atmospheric Administration. It was a month in which the planet saw its four hottest days ever observed and followed the warmest January-to-July period in 175 years of recordkeeping. “There is a 77% chance that 2024 will rank as the warmest year on record,” NOAA said.

Even before summer started, more than 1.5 billion people worldwide experienced at least one day between January and May in which the combination of heat and humidity reached a threshold defined as “dangerous” by the National Weather Service.

While it’s challenging to quantify the contribution of global warming to any one flood event, scientists say that warm air is clearly prone to holding more moisture and producing more intense rainfall.

High humidity near the ground, as well as record amounts of moisture from the ground to the top of the atmosphere, has helped fuel at least 10 significant flood events in the United States this year, including severe flooding in northeast Vermont after 8 inches of rain fell in just six hours. Other notable flood events occurred in Minnesota and in western Connecticut and Long Island, while Tropical Storm Debby flooded portions of the eastern United States from Florida to New York earlier this month.

In several cases, the National Weather Service had to issue a flash flood emergency, its most dire flood alert, reserved “for the exceedingly rare situations when extremely heavy rain is leading to a severe threat to human life and catastrophic damage from a flash flood.”

The nonprofit Climate Central says the atmosphere holds 4 percent more moisture for every one degree Fahrenheit of climate warming, which “supercharges” the water cycle and accounted for more than one-third of the $230 billion in damage from inland flooding in the country from 1988 to 2021.