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Compound Drought and Saltwater intrusion Events (CDSEs) refer to hydrologic drought and saltwater intrusion occurring simultaneously or consecutively in estuaries, and exacerbate the negative impacts resulting from an individual extreme event. CDSEs have been drawing increasing attention due to their potential adverse impacts on water resources, crop production, and food security. A new Standardized compound Drought and Saltwater intrusion Index (SDSI) was developed in this study to systematically detect changes in the severity of CDSEs in six estuaries (Little Back, Ebro, Rhine, Orange, Pearl River and Murray). The results illustrated that (1) compared to the Standardized Runoff Index (SRI), SDSI effectively characterizes and quantifies the occurrences and severity of CDSEs in major river estuaries worldwide. (2) Temporally, the SDSI trend varied across estuaries. Specifically, a decreasing trend was observed in the Little Back, Ebro, and Orange estuaries, with corresponding Zs values of −2.43, −3.63, and −3.23. (3) Spatially, moderate CDSEs occurred more frequently among different estuaries, and their frequency, duration and severity varied in different estuaries. Notably, Ebro, Rhine and Murray River estuaries had the highest probability of CDSEs, nearing 60%. Among them, the Murray Estuary had the longest average duration, spanning 7.68 months, and the highest severity was 5.94. (4) According to the contributions analysis, saltwater intrusion plays a dominant role in influencing SDSI severity, accounting for a substantial percentage (54%–95.30%) compared to runoff. Notably, the Orange Estuary experienced the greatest impact from saltwater intrusion (81.54%–95.30%), while the Murray Estuary had relatively equal contributions from hydrological drought and saltwater intrusion. • The simultaneous occurrence of hydrological droughts and saltwater intrusion is considered. • The Little Back, Ebro and Orange estuaries exhibit a significant decreasing trend in SDSI. • Moderate compound drought and saltwater intrusion events occurred more frequently among different estuaries. [ABSTRACT FROM AUTHOR]

Drought is one of the most destructive types of natural disasters, causing great losses of every aspects worldwide. Using a three‐dimensional (longitude, latitude, and time) clustering method based on gridded 3 month standardized precipitation evapotranspiration index data with a resolution of 0.25° collected for East China between 1960 and 2017, we identified and analysed five characteristics of drought events via the Mann–Kendall (MK) test. The results revealed that this method could capture drought events on the basis of their spatiotemporal characteristics. Drought development processes could be visualized via the spatial expansion of drought patches and the transferring path (reconstructed from the centroids of drought patches observed each month). By applying this identification method to the territory of East China, the occurrence of 174 droughts in the last six decades could be determined; furthermore, their objective existence in the spatiotemporal scale was confirmed by comparing the results with historical records and soil moisture data. The increasing moving speed of the centroids and the non‐repetitive rate of the impacted areas suggests that an increasing number of regions have likely been affected over time by large‐scale drought events. The drought patches associated with all of the events identified in the present study tended to move westward in South China and eastward in southwestern China. The remaining four characteristics (i.e., duration, severity, affected area, and intensity), were used to quantify the degree of drought, along with its development, where significant decreasing trends were found among the first three characteristics. In southwestern China, drought events decreased in frequency, but increased spatially, whereas drought events in southeastern China showed an opposite change. Drought events with a small spatial scale occurred more frequently in coastal regions. The majority of drought events mainly occurred in central and North China and moved northward. [ABSTRACT FROM AUTHOR]

• The 1.5 °C, 2 °C, 3 °C, and 4 °C warming scenarios are established by CMIP6 experiments. • The 1.5 °C and 2 °C warming targets avert an expansion of unprecedented drought. • Unprecedented drought tends to emerge earlier in most of the sub-tropics. • The time of emergence of unprecedented drought is modulated by precipitation change. In the context of anthropogenic global warming, unprecedented drought is expected to emerge over some regions where drought conditions exceed the variation range of the natural state. However, it is still unclear how the 1.5 °C and 2 °C warming targets proposed by the Paris Agreement will delay or even prevent the emergence of unprecedented drought. To illustrate the benefits of the two warming targets for mitigating drought conditions, this study investigated the time of emergence (TOE) of unprecedented meteorological drought as indicated by the Standardized Precipitation Evapotranspiration Index (SPEI) using the Coupled Model Intercomparison Project Phase 6 (CMIP6) model ensembles forced by 4 Shared Socioeconomic Pathways (SSP1–2.6, SSP2–4.5, SSP3–7.0, and SSP5–8.5). In particular, the pattern of TOE is explored for 4 pseudo warming scenarios under which global mean surface temperature (GMST) increases by 1.5 °C, 2 °C, 3 °C, and 4 °C above pre-industrial levels, respectively. The results suggested that: (1) Under the 1.5 °C and 2 °C warming scenarios, unprecedented meteorological drought is anticipated to emerge over southern Africa, Arabian Peninsula, central Asia, Mongolia, southwestern South America, and western and eastern Australia. (2) Under the 3 °C and 4 °C warming scenarios, unprecedented meteorological drought will expand to western North America, the Amazon, and the Mediterranean. (3) The comparisons with the exposure under the 3 °C and 4 °C warming scenarios show that the aspirational 1.5 °C and 2 °C Paris Agreement warming targets can prevent at least 0.6 × 107 km2 of land area, 1.4 × 108 people, and 0.6 × 106 km2 of cropland area from suffering the emergence of unprecedented meteorological drought. [Display omitted] [ABSTRACT FROM AUTHOR]

• The drought projection uncertainty in SMILEs mainly arises from internal variability. • Model uncertainty is the major uncertainty source of drought projection in CMIP6. • Model (scenario) uncertainty dominates the near-term (long-term) drought projection. • The reducible model uncertainty is mainly distributed in tropical regions. Reliable projection of future drought conditions is critical for informing adaptation in response to future climate change. However, the uncertainty associated with drought projection impedes the precise estimation of drought risk and consequently efficient adaptation strategy. To better understand the role of uncertainty in drought projection, this study distinguished the total uncertainty of drought projection into scenario uncertainty, model uncertainty, and internal variability uncertainty by variance analysis using realizations from the Single Model Initial-condition Large Ensembles (SMILEs) and the Coupled Model Intercomparison Project Phase 6 (CMIP6). The results showed that SMILEs differ from CMIP6 mainly in the magnitude of internal variability and model uncertainty. The contribution of internal variability to total drought projection uncertainty in SMILEs (31–56 %) is greater than that in CMIP6 (9–21 %), while model uncertainty of SMILEs (36–41 %) is nearly half that of CMIP6 (67–76 %). The estimation of total uncertainty in drought projection is comparable between SMILEs and CMIP6, indicating that total uncertainty attains a minimum in the mid-21st century (the 2060 s for SMILEs and 2050 s for CMIP6). In addition, SMILEs and CMIP6 consistently show that model uncertainty is dominant in tropical regions, and scenario uncertainty is the main uncertainty contributor in western North America, eastern South America, the Mediterranean, and southern Australia. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mild impact of climate extremes on vegetation was found for Hu Line transect in China. • Latitudinal and altitudinal patterns of vegetation responses to climate were analyzed. • Non-woody plants in the mid-to-high latitudes were more sensitive to climate extremes. • Productivity above 45°N increased with drought severity due to favorable environment. • Policy making suggestions were proposed with emphasis on the identified hotspots. Climate projections infer increasing frequency of extreme events in the future which would alter terrestrial hydrologic functioning and ecosystem services. Understanding the responses of vegetation to extreme events can provide prerequisite knowledge to mitigate climate change impacts. The relationships between climate extremes and vegetation phenology and productivity were examined for a transect along the Hu Line across China during 1988–2014. Results show that high frequency and severity of droughts existed in the mid-latitude regions where happened to be mostly occupied by grassland and cropland with relatively low productivity compared to trees. Prolonged growing seasons (4.3 days/decade) resulted mostly from advanced green-up were observed over the transect. No strong impact of extreme droughts on vegetation was observed for any plant functional types, due to the timing of drought occurrences primarily in pre- and post-growing seasons while water supply was likely not so limited during growth indicated by a drought index. Non-woody plants in the mid-to-high latitudes (37-54°N) were identified more sensitive to climate extremes than woody plants in the low latitudes. Meanwhile, grassland in the relatively low-elevation areas (<1000 m.a.s.l) was more responsive than that in the high-elevation areas. Productivity above 45°N increased with drought severity attributable to an intensified favorable environment during frequent and mild droughts with warmer temperature and increased solar radiation. The results may suggest the generally relative stability of the ecosystems along the Hu Line, however, with further deteriorating climate, the stable relationships between climate and ecosystems may be shifted, likely starting from low-elevation grassland. [ABSTRACT FROM AUTHOR]