Extreme summer temperatures are increasingly common across the Northern Hemisphere and inflict severe socioeconomic and biological consequences. In summer 2021, the Pacific Northwest region of North America (PNW) experienced a 2-week-long extreme heatwave, which contributed to record-breaking summer temperatures. Here, we use tree-ring records to show that summer temperatures in 2021, as well as the rate of summertime warming during the last several decades, are unprecedented within the context of the last millennium for the PNW. In the absence of committed efforts to curtail anthropogenic emissions below intermediate levels (SSP2–4.5), climate model projections indicate a rapidly increasing risk of the PNW regularly experiencing 2021-like extreme summer temperatures, with a 50% chance of yearly occurrence by 2050. The 2021 summer temperatures experienced across the PNW provide a benchmark and impetus for communities in historically temperate climates to account for extreme heat-related impacts in climate change adaptation strategies.

The western United States (US) is a global snow drought hotspot (Huning & AghaKouchak, 2020b), and has experienced significant mountain snowpack declines (∼15%-30%) since the mid-1900s (Mote et al., 2018. In particular, the Cascade Range (Cascades) in the US Pacific Northwest (PNW) has undergone the most dramatic declines during the instrumental era (Mote et al., 2005(Mote et al., , 2018, with the largest climate sensitivities and reduced snowpack predictability (Livneh & Badger, 2020). Oregon has sustained the greatest reductions (Mote, 2003;Mote et al., 2005) and contains approximately half of "at-risk" Cascades snow (Nolin & Daly, 2006). Analysis of observational data sets dating to the mid-20th century suggest a 16% loss in Cascades snowpack independent of internal variability from 1930 to 2007 (Stoelinga et al., 2010).Oregon Cascade snowpacks act as natural reservoirs for water supply that are slowly released in the spring and summer months when demand is highest (Barnett et al., 2005;Siirila-Woodburn et al., 2021). Located adjacent to the state's largest human population centers, they supply up to 75% of annual societal, economic, agriculture, and ecosystem water demands (United States Department of Agriculture, Natural Resources Conservation Service, 2022). The potential impacts of snow drought (Harpold et al., 2017) in Oregon were brought into sharp focus during the 2014-2015 snow drought event (spanning the 2014 and 2015 water years) when near-average winter precipitation was accompanied by exceptionally warm temperatures, resulting in precipitation primarily falling as rain rather than snow (a "warm snow drought") (

The recent intensification of floods and droughts in the Fraser River Basin of British Columbia has had profound cultural, ecological, and economic impacts that are expected to be exacerbated further by anthropogenic climate change. In part due to short instrumental runoff records, the long-term stationarity of hydroclimatic extremes in this major North American watershed remains poorly understood, highlighting the need to use high-resolution paleoenvironmental proxies to inform on past streamflow. Here we use a network of tree-ring proxy records to develop 11 subbasin-scale, complementary flood- and drought-season reconstructions, the first of their kind. The reconstructions explicitly target management-relevant flood and drought seasons within each basin, and are examined in tandem to provide an expanded assessment of extreme events across the Fraser River Basin with immediate implications for water management. We find that past high flood-season flows have been of greater magnitude and occurred in more consecutive years than during the observational record alone. Early 20th century low flows in the drought season were especially severe in both duration and magnitude in some subbasins relative to recent dry periods. Our Fraser subbasin-scale reconstructions provide long-term benchmarks for the natural flood and drought variability prior to anthropogenic forcing. These reconstructions demonstrate that the instrumental streamflow records upon which current management is based likely underestimate the full natural magnitude, duration, and frequency of extreme seasonal flows in the Fraser River Basin, as well as the potential severity of future anthropogenically forced events.

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