September 6, 2023 — As climate change intensifies, Southern California will face increasingly frequent and severe heat waves, warns a new study from NASA’s Jet Propulsion Laboratory (JPL). According to the scientists, focusing solely on air temperature does not capture the complete risk profile. Instead, they recommend that humidity should also be considered when issuing extreme heat warnings. The study points out that heat stress due to high wet-bulb temperatures—linked to human body cooling capabilities—can be worse in muggy conditions, even if air temperatures are cooler.

Heat Islands and Variable Impact Zones.

The study places particular emphasis on the Greater Los Angeles area as an “urban heat island,” where heat-trapping concrete and asphalt elevate temperatures compared to rural regions. The area is unique for its complex geography and population density, making it an important case study for heat wave impacts. The scientists used advanced modeling to simulate heat waves and found that different regions within the Greater L.A. area experience heat waves differently due to local climate and vegetation.

From Dry to Muggy Heat Waves.

Climate change has transformed the nature of heat waves in the region. While the 20th century saw mostly dry heat waves in L.A., humid events have been on the rise since 1950, partly due to warming sea surface temperatures. The researchers discovered that both coastal and inland areas are now experiencing increased nighttime heat stress during humid heat waves, a notable change particularly for valley regions which historically had drier nights.

The findings highlight the importance of considering microclimate variations when designing emergency plans and policies to deal with heat waves, now and in the future.