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1. Observed and projected trends in climate extremes in a tropical highland region: An agroecosystem perspective

   https://doi.org/10.1002/joc.7378  

   Birhan, Dereje Ademe; Zaitchik, Benjamin F.; Fantaye, Kindie Tesfaye; Birhanu, Belay Simane; Damot, Getachew Alemayehu; Tsegaye, Enyew Adgo (October 2021, International Journal of Climatology)

   Abstract Tropical highland environments present substantial challenges for climate projections due to sparse observations, significant local heterogeneity and inconsistent performance of global climate models (GCMs). Moreover, these areas are often densely populated, with agriculture‐based livelihoods sensitive to transient climate extremes not always included in available climate projections. In this context, we present an analysis of observed and projected trends in temperature and precipitation extremes across agroecosystems (AESs) in the northwest Ethiopian Highlands, to provide more relevant information for adaptation. Limited observational networks are supplemented with a satellite‐station hybrid product, and trends are calculated locally and summarized at the adaptation‐relevant unit of the AES. Projections are then presented from GCM realizations with divergent climate projections, and results are interpreted in the context of agricultural climate sensitivities. Trends in temperature extremes (1981–2016) are typically consistent across sites and AES, but with different implications for agricultural activities in the other AES. Trends in temperature extremes from GCM projected data also generally have the same sign as the observed trends. For precipitation extremes, there is greater site‐to‐site variability. Summarized by AES, however, there is a clear tendency towards reduced precipitation, associated with decreases in wet extremes and a tendency towards temporally clustered wet and dry days. Over the retrospective analysis period, neither of the two analysed GCMs captures these trends. Future projections from both GCMs include significant wetting and an increase in precipitation extremes across AES. However, given the lack of agreement between GCMs and observations with respect to trends in recent decades, the reliability of these projections is questionable. The present study is consistent with the “East Africa Paradox” that observations show drying in summer season rainfall while GCMs project wetting. This has an expression in summertime Ethiopian rain that has not received significant attention in previous studies. 

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2. Pre-Industrial and future shifts in seasonal precipitation climatology for large river basins across the continental U.S.A

   https://doi.org/10.1088/2515-7620/ade51f  

   Sung, kyungmin; Stagge, Jim (June 2025, Environmental Research Communications)

   Abstract This study investigates seasonal precipitation trends across major watersheds in the continental United States over the past millennium (850 CE) and into the projected future (2100 CE). Using a non-stationary Standardized Precipitation Index (SPI) model, we quantify shifts in median precipitation climatology relative to an 1850 pre-Industrial baseline, integrating modern observations, tree-ring reconstructions, and climate model simulations. Trends at a gridded scale were then summarized by HUC-2 watershed to produce relevant results for water resources planning. Results show that northern and eastern watersheds have experienced wetting trends in the modern, Industrial era (post 1850), while southwestern regions have undergone intensifying drying trends, particularly during the summer. Trends vary seasonally, the most distinct north-south division occurring in winter and spring, while the region of increased drying expands to cover most of the U.S. during the summer. These trends are projected to continue on their current trajectory through the end of the century. By placing modern and projected precipitation trends into a multi-century historical context, we show that the magnitude of modern trends has exceeded pre-Industrial variability in many watersheds, suggesting the role of anthropogenic climate change in precipitation shifts. This study emphasizes the importance of adapting water management systems to ensure sustainable water availability under evolving climatic conditions. For example, the findings can inform water resources managers when reassessing water infrastructure and operations to mitigate the potential of increased flood risks in wetting regions and heightened water scarcity in drying areas. 

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3. Assessing decadal- to centennial-scale nonstationary variability in meteorological drought trends

   https://doi.org/10.5194/hess-28-2047-2024  

   Sung, Kyungmin; Torbenson, Max_C A; Stagge, James H (January 2024, Hydrology and Earth System Sciences)

   There are indications that the reference climatology underlying meteorological drought has shown nonstationarity at seasonal, decadal, and centennial timescales, impacting the calculation of drought indices and potentially having ecological and economic consequences. Analyzing these trends in meteorological drought climatology beyond 100 years, a time frame which exceeds the available period of observation data, contributes to a better understanding of the nonstationary changes, ultimately determining whether they are within the range of natural variability or outside this range. To accomplish this, our study introduces a novel approach to integrate unevenly scaled tree-ring proxy data from the North American Seasonal Precipitation Atlas (NASPA) with instrumental precipitation datasets by first temporally downscaling the proxy data to produce a regular time series and then modeling climate nonstationarity while simultaneously correcting model-induced bias. This new modeling approach was applied to 14 sites across the continental United States using the 3-month standardized precipitation index (SPI) as a basis. The findings showed that certain locations have experienced recent rapid shifts towards drier or wetter conditions during the instrumental period compared to the past 1000 years, with drying trends generally found in the west and wetting trends in the east. This study also found that seasonal shifts have occurred in some regions recently, with seasonality changes most notable for southern gauges. We expect that our new approach provides a foundation for incorporating various datasets to examine nonstationary variability in long-term precipitation climatology and to confirm the spatial patterns noted here in greater detail. 

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4. Twenty-first century hydroclimate: A continually changing baseline, with more frequent extremes

   https://doi.org/10.1073/pnas.2108124119  

   Stevenson, Samantha; Coats, Sloan; Touma, Danielle; Cole, Julia; Lehner, Flavio; Fasullo, John; Otto-Bliesner, Bette (March 2022, Proceedings of the National Academy of Sciences)

   Variability in hydroclimate impacts natural and human systems worldwide. In particular, both decadal variability and extreme precipitation events have substantial effects and are anticipated to be strongly influenced by climate change. From a practical perspective, these impacts will be felt relative to the continuously evolving background climate. Removing the underlying forced trend is therefore necessary to assess the relative impacts, but to date, the small size of most climate model ensembles has made it difficult to do this. Here we use an archive of large ensembles run under a high-emissions scenario to determine how decadal “megadrought” and “megapluvial” events—and shorter-term precipitation extremes—will vary relative to that changing baseline. When the trend is retained, mean state changes dominate: In fact, soil moisture changes are so large in some regions that conditions that would be considered a megadrought or pluvial event today are projected to become average. Time-of-emergence calculations suggest that in some regions including Europe and western North America, this shift may have already taken place and could be imminent elsewhere: Emergence of drought/pluvial conditions occurs over 61% of the global land surface (excluding Antarctica) by 2080. Relative to the changing baseline, megadrought/megapluvial risk either will not change or is slightly reduced. However, the increased frequency and intensity of both extreme wet and dry precipitation events will likely present adaptation challenges beyond anything currently experienced. In many regions, resilience against future hazards will require adapting to an ever-changing “normal,” characterized by unprecedented aridification/wetting punctuated by more severe extremes. 

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5. A Nonstationary Standardized Precipitation Index (NSPI) Using Bayesian Splines

   https://doi.org/10.1175/JAMC-D-21-0244.1  

   Stagge, James H.; Sung, Kyungmin (July 2022, Journal of Applied Meteorology and Climatology)

   Abstract The standardized precipitation index (SPI) measures meteorological drought relative to historical climatology by normalizing accumulated precipitation. Longer record lengths improve parameter estimates, but these longer records may include signals of anthropogenic climate change and multidecadal natural climate fluctuations. Historically, climate nonstationarity has either been ignored or incorporated into the SPI using a quasi-stationary reference period, such as the WMO 30-yr period. This study introduces and evaluates a novel nonstationary SPI model based on Bayesian splines, designed to both improve parameter estimates for stationary climates and to explicitly incorporate nonstationarity. Using synthetically generated precipitation, this study directly compares the proposed Bayesian SPI model with existing SPI approaches based on maximum likelihood estimation for stationary and nonstationary climates. The proposed model not only reproduced the performance of existing SPI models but improved upon them in several key areas: reducing parameter uncertainty and noise, simultaneously modeling the likelihood of zero and positive precipitation, and capturing nonlinear trends and seasonal shifts across all parameters. Further, the fully Bayesian approach ensures all parameters have uncertainty estimates, including zero precipitation likelihood. The study notes that the zero precipitation parameter is too sensitive and could be improved in future iterations. The study concludes with an application of the proposed Bayesian nonstationary SPI model for nine gauges across a range of hydroclimate zones in the United States. Results of this experiment show that the model is stable and reproduces nonstationary patterns identified in prior studies, while also indicating new findings, particularly for the shape and zero precipitation parameters. Significance StatementWe typically measure how bad a drought is by comparing it with the historical record. With long-term changes in climate or other factors, however, a typical drought today may not have been typical in the recent past. The purpose of this study is to build a model that measures drought relative to a changing climate. Our results confirm that the model is accurate and captures previously noted climate change patterns—a drier western United States, a wetter eastern United States, earlier summer weather, and more extreme wet seasons. This is significant because this model can improve drought measurement and identify recent changes in drought. 

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