ACC News Brief
Extreme Heat Global
VERIFIED TREND - peer-reviewed reanalysis; the observational design does not directly attribute the change or isolate its causes
Extreme heat seasons are spreading beyond summer across about half of global land
What happened
An AGU Advances study compared MERRA-2 reanalysis data from 1980-1989 with 2015-2024 and found that extreme dry-heat and humid-heat seasons expanded significantly across about half of global land. The expansion was often asymmetric, shifting more into months before the customary heat season in some regions and extending more strongly afterward in others. ERA5 was used as a robustness check.
ACC context
The study defines extremes against each location's 1980-1989 95th-percentile threshold. Rare shoulder-season events and internal variability limit causal interpretation.
Why it matters
Heat alerts, cooling centers, work rules, health planning, agriculture, and wildfire preparation are often organized around a fixed summer calendar. A longer or lopsided extreme-heat season can leave people exposed before preparations begin or after protections wind down.
What to watch
- Climate-model experiments separating human-caused warming from natural variability and circulation changes.
- Local studies testing whether warning systems cover newly exposed shoulder months.
- Overlaps between late or early heat and other seasonal hazards such as wildfire or cyclones.
Sources & evidence
- Extreme Dry and Humid Heat Seasons Are Changing AsymmetricallyAGU Advances article first published September 10, 2026; MERRA-2 analysis spans 1980-2024.
Continue learning with ACC
See how long-term warming changes the timing and intensity of climate risks
