Process Synchrony a Key Control of Resilience in a Subarctic Freshwater System

Abstract Climate‐induced changes in streamflow and biogeochemistry are occurring across the northern circumpolar region but several key unknowns include (a) the mechanisms responsible among landscapes and permafrost conditions, (b) the resilience and precariousness of hydrological and biogeochemical regimes. Even though it is among the largest physio‐climatic regions of the northern circumpolar, these knowledge gaps are acute in the Taiga Shield. This research aimed to determine if hydrology and biogeochemistry regimes of the Taiga Shield have been resilient to recent climate warming. We apply a recently developed framework of hydrological resilience that shows the first 20 years of the 21st century were the warmest and wettest of the previous 300 years. These conditions altered the catchment such that >50% of the water year streamflow now occurs during winter, shifting the catchment from a nival to a cold season pluvial hydrological regime. This regime shift has significantly changed the fraction of inorganic nitrogen export, but insufficiently to shift the biogeochemical regime. Sustained multi‐year physical process synchronization was the cause of these changes. This behavior is not well simulated by existing Earth system models. The tipping point in local mean annual air temperatures was crossed near the turn of the century well below the warming threshold of the Paris Accord. A one‐size‐fits‐all approach to mitigation targets is not effective at preventing all shifts in Earth systems. This is important to consider as regime changes in small hydrological systems have the potential to trigger cascading effects in the larger catchments to which they contribute. Plain Language Summary We show that since the turn of the century, most runoff in small northwestern Canadian Shield streams that drain this lake‐rich landscape has flowed during the winter rather than after spring snowmelts. This significant departure from the past has not occurred since at least 1700. Recent climate warming means more precipitation in autumn, falling as rain, filling lakes, making runoff more common through winter. The seasonal shift in the streamflow timing has also changed the chemistry of the water. The tipping point coincided with the point in time when global surface temperatures had risen half the 1.5° warming threshold agreed upon in the Paris Accord. These watersheds are not resilient to such an extreme degree of warming because the lake‐rich landscape configuration and relatively dry climate make them vulnerable to regime shifts. Therefore, this mitigation goal did not prevent cold freshwater systems in northern Canada from changing regime. Better coupling of atmospheric and hydrological processes in current climate change models is necessary because currently they are unable to predict such regime shifts and are likely underestimating tipping points. Key Points Small Taiga Shield streams shifted into a new winter‐flow dominated hydrological regime near the turn of the 21st century This shift, caused by the synchrony of several non‐linear physical processes, results from climate warming Current Earth system models do not include process synchrony and cannot predict such regime shifts, likely underestimating tipping points