Synthesis of Publications on the Anomalous June 2021 Heat Wave in the Pacific Northwest of the United States and Canada

Abstract We synthesized more than 70 articles, most peer reviewed, that addressed the heat wave that occurred across the Pacific Northwest of the United States and Canada in late June 2021, breaking hundreds of daily and all-time daily maximum temperature records across the region. A persistent, extraordinarily strong ridge of high pressure was a primary driver of the heat wave. Contributing mechanisms were moisture originating in the tropical western Pacific Ocean, high solar radiation, low pressure offshore, large-scale subsidence over land, and unusually dry soils. Climate change contributed to the heat wave’s magnitude by increasing mean temperature, although it is unclear whether the trend in extreme temperature is steeper than the trend in mean temperature. Mortality, heat-induced illness, and the number of visits to emergency departments during the 2021 heat wave were anomalously high. Individual and compounded social determinants of adverse outcomes included older age, living alone, lower income, and lack of functioning air conditioning. Browning or scorch of tree leaves and needles following the heat wave was extensive, although the extent of long-term tree mortality is not yet clear. Following the heat wave, Oregon, Washington, and British Columbia established new regulations and programs to reduce the risk of heat-related illness in the workplace. It is not yet feasible to rigorously evaluate the effectiveness of these new initiatives. Significance Statement We synthesized more than 70 publications that addressed the causes and consequences of the extreme heat wave across the Pacific Northwest of the United States and Canada in late June 2021 and the potential for similar future heat waves. Interest in the heat wave among scientists, policymakers, and the public continues to be intense. Climate change contributed to the heat’s magnitude, and many publications indicated that a similarly intense heat wave was extremely unlikely prior to the Industrial Revolution. Mortality and heat-induced illness during the heat wave were anomalously high, especially among older adults. Extensive scorch of tree leaves and needles followed the heat wave. In response to the heat wave, Oregon, Washington, and British Columbia established new protective regulations.