Moderation of Extreme Heat along the Coasts of the Eastern United States

Climate projections suggest a rapid increase of extremely hot days in the coming decades, portending major consequences for human and ecosystem health. But such projections are based on models that poorly resolve coastal regions, which is where about 25% of the global population resides (including some 50 million in the eastern United States). Will cooler marine air ventilate this extreme heat in coastal regions? If so, by how much and over what spatial scales? There is no observational basis for understanding present-day coastal ventilation of extreme heat — its length scale, magnitude, and trends — which is necessary to evaluate model projections. Here we present a survey of the impact of this moderation of extreme heat in the eastern United States over 1979-2016 and find that it has a significant tempering effect, leading to a median cooling of 2-4°C extending 25-40 km inland. Regional variations in the amount of average cooling and in its inland extent are large, with New England and Texas having the greatest cooling, a result of two different sets of geophysical processes. Dynamically and statistically downscaled global climate model representations of this cooling are very muted, despite having spatial resolutions sufficient to capture it. We apply a mean bias correction to an ensemble of downscaled GCM projections for the late 21^st century and find little expected change in coastal cooling, with the exception of Texas, where models agree that the coastal cooling effect will increaseand partially ameliorate extreme heat there. Our results suggest the importance of a regional perspective for assessing both coastal cooling and projections of extreme heat more generally.