Reframing Future Risks of Extreme Heat in the United States

Area of Science:

• Climate Science
• Environmental Science
• Atmospheric Science

Background:

• Extreme heat poses significant risks to public health and infrastructure.
• Accurate projections of future extreme heat are crucial for effective adaptation strategies.
• Current communication of extreme heat risks may not fully capture the severity of future events.

Purpose of the Study:

• To reframe the analysis and discussion of extreme heat projections.
• To improve the communication of future extreme heat risks in the United States.
• To examine future extreme heat exposure under various global temperature rise scenarios.

Main Methods:

• Utilized data from 31 Coupled Model Intercomparison Project Phase 5 (CMIP5) models.
• Analyzed future extreme heat exposure for global average temperature increases of 1.5, 2, 3, and 4°C above preindustrial levels.
• Compared projections based on absolute and relative metrics for defining extreme heat.

Main Results:

• Historically rare extreme heat events are projected to become increasingly common across the US.
• The definition of extreme heat (absolute vs. relative metrics) significantly influences the depiction of exposure.
• Under a 4°C rise, virtually all of the US may experience over 4 weeks of extreme heat per summer, even if absolute temperature thresholds are not always exceeded.
• All explored extreme temperature metrics worsen with rising global average temperatures.
• A moderate climate scenario delays impacts by nearly a generation compared to a higher scenario and avoids the most severe outcomes of a 4°C rise.

Conclusions:

• Future extreme heat events in the US will intensify and become more frequent with global warming.
• Clearer communication of extreme heat risks requires careful consideration of both absolute and relative metrics.
• Mitigation efforts, even those leading to moderate warming scenarios, can substantially reduce the severity of future extreme heat impacts.