As North Carolinians head into the Fourth of July holiday, forecasters are urging people to prepare for dangerous heat.
Humidity can make hot weather significantly more dangerous, and a new Climate Central analysis finds those risky combinations of heat and humidity are becoming much more common because of human-caused climate change.
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The report found the number of dangerous humid heat days has more than doubled globally since the 1970s, increasing from an average of 10 days per year to 23 days per year over the past decade.
"When heat and humidity combine and reach a certain threshold, your body can no longer sweat efficiently to cool you down, and that's when things get dangerous for your health," said Shel Winkley, meteorologist at Climate Central.
According to the analysis, climate change is now the primary driver behind those dangerous days. In the 1970s, warming contributed about three dangerous humid heat days each year globally. During the past decade, that increased to 18 days annually. In 2025 alone, Climate Central found 83% of dangerous humid heat days worldwide were attributable to climate change.
Why humidity matters
Meteorologists often refer to the "heat index" or "feels like" temperature. It combines air temperature and humidity because humid air reduces the body's ability to cool itself.
"As the air gets more humid, it compromises our skin's ability to dissipate heat," said Dr. Chip Konrad, director of the Southeast Regional Climate Center at UNC Chapel Hill. "We perspire and we sweat, but the air doesn't have as much room to hold that water vapor, so we don't get as much evaporation, which means we don't get as much cooling."
Why climate change is increasing humid heat
Scientists have long expected humid heat to increase as the planet warms.
"Obviously the atmosphere is getting warmer, it's hotter, but also the oceans have gotten warmer, and so they're evaporating more water vapor into the atmosphere," Konrad said. "That's making it more humid."
The warmer atmosphere can also hold more moisture, creating a feedback loop that traps additional heat.
Konrad said natural weather patterns such as El Niño and La Niña still cause year-to-year swings in temperatures and humidity. Cooler summers or less humid stretches can still happen, but they don't erase the long-term warming trend.
"We have a really variable atmosphere," he said. "We have warm periods, cool periods. There's a lot of natural variability. But we've done a lot of good science on climate change... these models really show the climate warming up and getting more humid."
What it means for North Carolina
North Carolina's proximity to the Atlantic Ocean and Gulf of Mexico makes it especially susceptible to humid air during the summer.
Konrad said the state also faces another challenge in its cities. Pavement, buildings and other infrastructure absorb and retain heat, creating urban heat islands where temperatures can be substantially higher than nearby parks or wooded areas.
Research by the Southeast Regional Climate Center has identified neighborhoods where those local conditions can become dangerous even on days that might otherwise seem typical for central North Carolina.
Extreme heat is already one of the deadliest weather hazards in the United States, and Konrad said rising temperatures disproportionately affect people without reliable access to air conditioning.
"A lot of people in North Carolina are really vulnerable to the heat," he said. "When you get into rural areas, where there's a lot of poverty and what's called energy poverty, people may have air conditioners, but they're just too expensive to run."
With many people expected to spend time outdoors over the holiday weekend, experts recommend checking the heat index in addition to the air temperature, drinking plenty of water, taking frequent breaks in the shade or air conditioning and watching for signs of heat exhaustion or heat stroke.