A new study reveals that extreme heat events are happening more often outside of traditional summer months, with different regions experiencing these events at varying times of the year. Published in AGU Advances, the research analyzed temperature data from six inhabited continents between 1980 and 1989 and compared it with the most recent decade, 2015 to 2024. Researchers defined "extreme" heat as days that fall in the hottest 5% of daily temperatures. They used both standard thermometer readings and wet-bulb globe temperature measurements, which account for humidity and radiant heat. The study found that the duration of extreme heat seasons has grown across more than half of the world’s land area for dry heat and nearly half for humid heat. Some regions are seeing more frequent heat events before their typical summer, while others are experiencing them after. In the western United States, eastern China, northern Africa, and eastern Europe, extreme heat events are becoming more common in the two months after their traditional summer. In contrast, western Europe, southern Africa, and northwestern India are seeing more frequent heat events in the two months before their usual hot season. Phoenix, Arizona, provides a striking example: it recorded 183 extreme heat days in the 1980s, but that number rose to 338 in the decade ending in 2024. The median date of dry heat extremes moved 10 days later in the year, while humid heat extremes shifted 5.5 days earlier. In 2024, Phoenix experienced 113 consecutive days above 100°F (38°C), followed by 21 days of record-breaking temperatures in late September and early October. That year, Maricopa County, where Phoenix is located, saw 608 heat-related deaths, with 46% occurring in July, the hottest month, compared to 64% in July the previous year. To confirm their findings, the researchers used two independent data sets—one from NASA and the other from the European Centre for Medium-Range Weather Forecasts. The study found that the expansion of extreme heat seasons is not uniform across the globe, with each region showing unique patterns. The researchers note that extreme heat is rare by definition, making it difficult to detect statistical changes. Moving forward, they plan to use climate simulations to determine how much of the shift in extreme heat timing is due to human-caused climate change versus natural variability. The study suggests that ordinary warming could explain changes in the core of the heat season, but not the uneven shifts at its beginning and end. The findings have significant implications for both human health and the environment. Dry heat can harm crops and ecosystems, while humid heat poses a greater risk to people, as it hinders the body’s ability to cool itself through sweating. Additionally, the shifting timing of extreme heat increases the chance of overlapping with other seasonal dangers, such as wildfire season in the western U.S. and hurricane season in the Southeast. As climate change continues to influence weather patterns, understanding these shifts is crucial for preparing communities and ecosystems for increasingly unpredictable heat events.