What is the estimated frequency of such extreme heat events today?

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Extreme Heat Events Now Occur Roughly Once Every 200 Years

The extreme heat and humidity currently affecting parts of the United States, including areas hosting World Cup matches, are estimated to occur approximately once every 200 years in today's climate. This assessment is based on a new study by the World Weather Attribution group (WWA), which compared climate models of the world with and without human-induced greenhouse gas emissions.
Extreme Heat Events Estimated at Once Every 200 Years

Researchers from the World Weather Attribution group (WWA) have determined that the current heatwave, exacerbated by a high-pressure "heat dome," has a frequency of about one in 200 years. This is a significant increase in rarity compared to a hypothetical world without climate change, where such events would be virtually impossible, occurring perhaps once every 5,000 years. The study's findings highlight a fundamental shift in the climate since the United States' founding.
Uncertainty Due to Event Extremity

Despite the 200-year estimate, the researchers acknowledge a high degree of uncertainty. This uncertainty stems from the extreme nature of the event itself, making precise statistical calculations more challenging. The WWA group, composed of climate researchers from several prominent institutions, utilized climate models to analyze the impact of human-caused emissions, which have raised global temperatures by 1.4C (2.5F) since pre-industrial times.
Humidity's Significant Role in Perceived Heat

The study also emphasizes the role of humidity in exacerbating the perceived heat. While daytime temperatures are frequently exceeding 38 degrees Celsius (100.4 degrees Fahrenheit), the combined effect of high humidity, measured by Wet Bulb Globe Temperatures (WBGT), makes the conditions feel even more intense. WBGT values are projected to reach record highs across the affected regions, indicating significant heat stress for individuals.
Minimal Impact of El Nino Conditions

To isolate the impact of climate change, the researchers also investigated the influence of El Nino conditions in the Pacific. Their findings indicated that El Nino's effect on northeastern North America during this period was a minor cooling influence, suggesting it did not significantly alter the extreme heat driven by other factors. The study's conclusions are particularly relevant as World Cup matches, such as the July 4 clash between France and Paraguay in Philadelphia, are forecast to occur under conditions with WBGT levels exceeding 82F, prompting safety concerns from FIFPRO, the players' union.

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What is the estimated frequency of such extreme heat events today?

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