Using Large Ensembles to Examine Historical and Projected Changes in Record‐Breaking Summertime Temperatures Over the Contiguous United States.

The frequency and intensity of heat extremes over the United States have increased since the mid‐20th century and are projected to increase with additional anthropogenic greenhouse gas forcing. We define heat extremes as summertime (June–August) daily maximum 2m temperatures that exceed historical records. We examine characteristics of historical and near‐future heat extremes using observations and past and future projections using 100 ensemble members from three coupled global climate models large ensemble simulations. We find that the large ensembles capture the trend and variability of heat extremes over the period 2006–2020 relative to the 1991–2005 climatology but overestimate the frequency at which the heat extremes occur. In future warming scenarios, heat extremes continue to increase over the next 30 years, with high amplitude records in the Northwest and Central US. After 2050, we find there is a spread in the frequency of heat extremes that is dependent on the emissions scenario, with a high emissions until mid‐century followed by a high mitigation scenario showing a decrease in heat extremes by the end of the century. Although the frequency of future heat extremes is likely overestimated in the large ensembles, they are still a powerful tool for researching extreme temperatures in the climate system. Plain Language Summary: Heat extremes have been increasing around the world and the United States, driven largely by human‐caused global warming. With additional warming projected, heat extremes are likely to increase. It is important to understand how heat extremes will change in the future since heat has many societal impacts, especially on human health as it causes more fatalities than any other weather‐related hazard in the US. In this study we use 100 ensemble members from three global climate models simulating past and future climate to evaluate characteristics of record breaking maximum daily temperature records from June to August. We find that the models capture the observed trend and variability of past heat extremes but appear to overestimate the frequency that they occur. The models project an increase in heat extremes in the next 30 years, with high amplitude extremes projected in the Northwest and Central US, and high frequency extremes in the Southwest and Mountain West. The amount of warming directly impacts the frequency of heat extremes past year 2050. In a scenario with rapid reduction of greenhouse emissions after 2040 the frequency of heat extremes declines toward the end of the century. We find that while the frequency of future heat extremes is likely overestimated in the models, the ensembles are nevertheless a powerful tool for researching extreme temperatures in the climate system. Key Points: Large ensembles of climate model simulations capture the observed variability and trend of heat extremes but overestimate the frequency and amplitudeHeat extremes are projected to increase in the next 30 years with additional warming due to greenhouse gas forcingThe frequency of heat extremes toward the end of the century is highly dependent on the emissions scenario and extent of warming [ABSTRACT FROM AUTHOR]