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1. Increasing prevalence of hot drought across western North America since the 16th century

   https://doi.org/10.1126/sciadv.adj4289  

   King, Karen E; Cook, Edward R; Anchukaitis, Kevin J; Cook, Benjamin I; Smerdon, Jason E; Seager, Richard; Harley, Grant L; Spei, Benjamin (January 2024, Science Advances)

   Across western North America (WNA), 20th-21st century anthropogenic warming has increased the prevalence and severity of concurrent drought and heat events, also termed hot droughts. However, the lack of independent spatial reconstructions of both soil moisture and temperature limits the potential to identify these events in the past and to place them in a long-term context. We develop the Western North American Temperature Atlas (WNATA), a data-independent 0.5° gridded reconstruction of summer maximum temperatures back to the 16th century. Our evaluation of the WNATA with existing hydroclimate reconstructions reveals an increasing association between maximum temperature and drought severity in recent decades, relative to the past five centuries. The synthesis of these paleo-reconstructions indicates that the amplification of the modern WNA megadrought by increased temperatures and the frequency and spatial extent of compound hot and dry conditions in the 21st century are likely unprecedented since at least the 16th century. 

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2. Six hundred years of South American tree rings reveal an increase in severe hydroclimatic events since mid-20th century

   https://doi.org/10.1073/pnas.2002411117  

   Morales, Mariano S.; Cook, Edward R.; Barichivich, Jonathan; Christie, Duncan A.; Villalba, Ricardo; LeQuesne, Carlos; Srur, Ana M.; Ferrero, M. Eugenia; González-Reyes, Álvaro; Couvreux, Fleur; et al (July 2020, Proceedings of the National Academy of Sciences)

   South American (SA) societies are highly vulnerable to droughts and pluvials, but lack of long-term climate observations severely limits our understanding of the global processes driving climatic variability in the region. The number and quality of SA climate-sensitive tree ring chronologies have significantly increased in recent decades, now providing a robust network of 286 records for characterizing hydroclimate variability since 1400 CE. We combine this network with a self-calibrated Palmer Drought Severity Index (scPDSI) dataset to derive the South American Drought Atlas (SADA) over the continent south of 12°S. The gridded annual reconstruction of austral summer scPDSI is the most spatially complete estimate of SA hydroclimate to date, and well matches past historical dry/wet events. Relating the SADA to the Australia–New Zealand Drought Atlas, sea surface temperatures and atmospheric pressure fields, we determine that the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are strongly associated with spatially extended droughts and pluvials over the SADA domain during the past several centuries. SADA also exhibits more extended severe droughts and extreme pluvials since the mid-20th century. Extensive droughts are consistent with the observed 20th-century trend toward positive SAM anomalies concomitant with the weakening of midlatitude Westerlies, while low-level moisture transport intensified by global warming has favored extreme rainfall across the subtropics. The SADA thus provides a long-term context for observed hydroclimatic changes and for 21st-century Intergovernmental Panel on Climate Change (IPCC) projections that suggest SA will experience more frequent/severe droughts and rainfall events as a consequence of increasing greenhouse gas emissions. 

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3. Paleohydrological context for recent floods and droughts in the Fraser River Basin, British Columbia, Canada

   https://doi.org/10.1088/1748-9326/ac3daf  

   Brice, Becky L; Coulthard, Bethany L; Homfeld, Inga K; Dye, Laura A; Anchukaitis, Kevin J (December 2021, Environmental Research Letters)

   Abstract 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. 

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4. Recent Increases in Missouri River Streamflow Driven by Combined Effects of Climate Variability, Land‐Use Change, and Elevated CO ₂

   https://doi.org/10.1029/2024AV001432  

   Dannenberg, Matthew P; McCabe, Gregory J; Wise, Erika K; Johnston, Miriam R; Huntzinger, Deborah N; Williams, A Park (April 2025, AGU Advances)

   Abstract Missouri River streamflow increased substantially during the 20th century, with multiple large floods occurring since 1990. Using land surface models and water budget simulations, we examined the extent to which increased flow was driven by natural climate variability, anthropogenic climate trends, land‐use and land‐cover change (LULCC), and ecological effects of elevated atmospheric CO₂. Natural climate variability (arising largely from coupled ocean‐atmosphere circulation systems in both the Pacific and North Atlantic) accounted for ∼765 m³ s⁻¹of the ∼900 m³ s⁻¹increase in flow since mid‐century, while anthropogenic climate trends negatively forced flow by increasing evapotranspiration more than precipitation. LULCC and elevated CO₂further increased simulated mean streamflow by ∼550 and ∼70 m³ s⁻¹, respectively, relative to pre‐Industrial conditions and ∼100 and ∼65 m³ s⁻¹relative to mid‐20th century conditions. The LULCC effect was especially large in wet years, implying that current land cover is ill‐suited for buffering against extreme precipitation, likely in large part due to replacement of forest by cropland in the lower basin. Because increases in Missouri River flow over the past century were driven mostly by a recent (and likely transient) pluvial, our results suggest that flow in the basin could revert to a drier mean state when that pluvial ends, likely made worse by increased evaporative demand from anthropogenic warming. 

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5. Nonlinear Growth and Physiological Responses of White Spruce at North American Arctic Treeline

   https://doi.org/10.1029/2022JG007096  

   Lévesque, Mathieu; Andreu‐Hayles, Laia; D’Arrigo, Rosanne; Oelkers, Rose; Buckley, Brendan M (April 2023, Journal of Geophysical Research: Biogeosciences)

   Abstract Much is still unknown about the growth and physiological responses of trees to global change at the northern treeline. We combined tree‐ring width data with century‐long stable carbon and oxygen isotope records to investigate growth and physiological responses of white spruce at two treeline sites in the Canadian Arctic to concurrent increases in temperature, atmospheric CO₂concentration (c_a), and decline in sea ice extent over the past century. The tree‐ring records were assessed during three periods with contrasting climatic conditions: (a) the early 20th century warming, (b) the 1940–1970 cooling period, and (c) the anthropogenic late 20th century warming period. We found opposing growth trends between the two sites, but similar carbon isotope discrimination (Δ¹³C) and intrinsic water‐use efficiency (_iWUE) trajectories. While tree growth (defined as basal area increment) increased at the site nearer to the Arctic Ocean during the 20th century following the rise in temperature and sea ice loss, growth declined after 1950 at the more interior site. At both sites, Δ¹³C slightly increased over these periods. However, trees showed a nonlinear response to increasedc_a, shifting after 1970 from a passive stomatal response (i.e., no changes in_iWUE) to an active response (i.e., a moderate ∼12% increase in_iWUE). Further, our isotope‐based findings do not support the idea that temperature‐induced drought stress caused the divergent growth trends at our treeline sites. This study thus highlights nonlinear and complex physiological and growth adjustments to concomitant changes in temperature, sea ice extent, andc_aover the last century at the northern treeline. 

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