Extreme Event Reconstructions for the Upper Fraser River Basin, British Columbia, Canada

Spring freshets and summer droughts have recently worsened in the Fraser River Basin, British Columbia, Canada, with significant impacts to the keystone Pacific salmon populations, the food and economic sovereignty of over eighty First Nations, and the western Canadian economy. These extreme events present a potential risk since, unlike many large and less hydroclimatically-complex and/or empounded watersheds, the Fraser River Basin is susceptible to a combination of unregulated spring freshet and summer drought events even within the same year. A major limitation for understanding past and future extreme event risk in the Fraser River Basin is that observational streamflow datasets are both short in duration and potentially forced by anthropogenic warming. They therefore provide a potentially incomplete record of natural hydrological variability and inaccurate benchmarks of long-term natural runoff extremes. While longer-term, highly-resolved (annual), tree ring (TR) based paleohydrological reconstructions are increasingly being used worldwide by water managers and stakeholders to extend short observational streamflow records, this approach is difficult in complex temperate watersheds like the Fraser. For this study I developed the first multi-century, sub-annual resolution (seasonal), paired freshet and drought reconstructions within a single watershed. I targeted the Upper Fraser Basin since it represents the headwaters and primary runoff source of the greater Fraser Basin. By focusing on sub-annual streamflow seasons, I was able to both independently reconstruct seasonal extreme flow events, and also overcome methodological limitations that precluded prior attempts to reconstruct total water-year runoff in this watershed. Newly developed and existing TR chronologies from multiple species were used as proxies for seasonal temperature and cool-season precipitation which are, in turn, drivers of streamflow in each reconstruction season. I analyzed the magnitudes, durations, and statistical probabilities of high freshets and droughts over the past 140 years. My results suggest that the instrumental records do not accurately reflect the full variability of high freshet events or drought events as both duration and magnitudes of past events are higher than any during the observed time period. There is also a change in the frequency of high freshet events towards more frequent occurrences since 1950 AD. The new extreme event reconstructions presented here provide paleoenvironmental benchmarks that can be used by water managers and stakeholders to significantly change and improve water management-relevant statistical analyses such as frequency analysis and return periods calculation, and adapt to future freshets and droughts on the Fraser River under climate change

Paleohydrological Context for Recent Floods and Droughts in the Fraser River Basin, British Columbia, Canada

The recent intensification of floods and droughts in the Fraser River Basin (FRB) 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 FRB 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 FRB, as well as the potential severity of future anthropogenically forced events

Unprecedented 21st century heat across the Pacific Northwest of North America

Abstract: 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