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A new analysis by the World Weather Attribution (WWA) group has concluded that the extreme heat and humidity affecting large parts of the United States this week would have been “virtually impossible” without human-induced climate change.
The study comes as a strong “heat dome” system traps hot, moisture-laden air over the central and eastern United States and southern Canada, driving daytime temperatures above 100°F (38°C) in several regions. Combined with high humidity, the conditions are producing dangerous levels of heat stress across wide areas.
Researchers reported that the most relevant measure of risk, Wet Bulb Globe Temperature (WBGT), is expected to reach exceptionally high levels in parts of the affected region.
WBGT incorporates temperature, humidity, wind, and solar radiation and is widely used to assess the safety of outdoor physical activity.
According to the findings, values in some locations are approaching thresholds considered unsafe for prolonged exertion, including professional sporting events.
The WWA study compared present-day climate conditions, which have been warmed by approximately 1.4°C due to human greenhouse gas emissions, with a counterfactual scenario absent human-induced climate change.
In the simulated natural climate, the researchers found that the current level of heat stress would have been so rare as to be effectively impossible, occurring at most once in several thousand years.
In today’s climate, the event remains highly unusual and is estimated to be on the order of a one-in-200-year occurrence, although researchers noted significant uncertainty due to the extremity of the conditions.
The timing of the heat wave coincides with major public and international events, including FIFA World Cup matches scheduled in U.S. host cities such as Philadelphia, as well as Fourth of July celebrations marking the country’s 250th anniversary of independence.
Some forecast WBGT readings are reported to exceed thresholds at which player safety guidelines recommend delaying or rescheduling outdoor sporting events due to heat risk for athletes and spectators.
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