Chronic Heat in a Warming World: A Global Assessment of Climatic Trends

Chronic heat, exacerbated by climate change, presents a growing global concern, particularly as cities increasingly experience periods of dangerous heat. This study identifies and analyzes cities worldwide that experience chronic humid heat. Here chronic heat is defined as a heat index exceeding 90°F—a threshold at which the National Weather Service advises 'extreme caution'—and occurs more than 90 days per year. We use the HadISD global sub-daily dataset, comprising temperature and dew point records from 10,501 stations across all seven continents. Heat metrics, including maximum temperature, heat index, and wet bulb temperature, are calculated using data from stations with at least 30 years of records, covering the period from January 1, 1993, to January 1, 2023, to align with climatological norms. Findings reveal global patterns of chronic heat exposure, with a particular focus on regions such as North America, Europe, and Australia, where data coverage is more comprehensive, though gaps remain in areas like Central Africa and Northern South America. In addition to the baseline climate, this work identifies trends in chronic humid heat metrics where data is available. Geographic shifts of the number of dangerous days has drastic effects on existing ecosystems and human health as the planet continues to warm.