From mild to extreme heatwaves: Examining trends in North America

Extreme heat is associated with negative consequences. Previous research has shown an increase in the frequency and intensity of extreme heat in the 20 th and early 21 st centuries in most of North America. Similar trends are expected in the next decades. These increases are primarily driven by a shift of the temperature distribution towards warmer temperatures. Despite this rich literature, few studies have considered how past trends of milder heatwaves may differ from those of more extreme heatwaves. Here we quantify recent intensity and duration trends of North American heatwaves according to their severity, a novel metric which measures the intensity of a heatwave relative to other local contemporaneous heatwaves. We measure heatwave intensity using three different metrics (cumulative, average and maximum). These metrics are based on the anomaly of the daily maximum temperature relative to the local non-stationary 90 th percentile. Heatwaves are then categorized as either mild, moderate or extreme in their severity. Our findings indicate that heatwave temperatures have been increasing in most of North America between 1940 and 2019 for every season. However, heatwave temperature anomalies have remained stable over this same period. Additionally, higher heatwave severity is linked to less noisy intensity trends. This cannot be explained solely by changes in the mean of the temperature distribution over time. Our results have important implications for the current estimation of heatwave intensity trends and suggest that the impact of their severity should be considered.