Jessica Keune

Extreme precipitation events can have severe impacts on society and the environment. Understanding what causes these events is a vital step toward better prediction and improved disaster preparedness. One research direction is to answer the question: Where did the moisture that rained here come from? The moisture sources for precipitation (i.e., where the moisture originally evaporated) cannot be measured directly and, therefore, a variety of different moisture-tracking methods have been developed and evolved over time. To better understand the uncertainty of these methods, we unite the community to advance common understanding and guidelines. As the first step, in this study, we quantify moisture sources of three extreme precipitation events using methods obtained from 14 different research groups. These three events cover different meteorological conditions: monsoon precipitation in Pakistan, convective precipitation in Australia, and atmospheric river-associated precipitation over Scotland. We find that for the three cases, the different moisture-tracking methods qualitatively agree in moisture source patterns, but there are regional and quantitative differences. For example, for the Pakistan case, the recycling ratio shows a multimethod spread of 2%–20%. We also find similar behavior across methods for the three different events, where methods consistently show either more recycling or more sources further away from the precipitation region. This coordinated model intercomparison facilitates the explanation and quantification of uncertainty, acting as a point of reference and inspiration for future work and literature on moisture tracking.

Droughts are increasingly intensified by human-induced climate change and pose a growing threat to society. Thus, enhancing our capabilities to monitor drought occurrence and intensity is crucial. This paper introduces a new dataset of drought indices derived from the 5th generation ECMWF reanalysis system (ERA5), which offers long-term monitoring of the global climate in both deterministic and probabilistic forms. This global dataset is freely accessible through an ECMWF-hosted data store, and it entails two prominent drought indices: the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI). Both indices are calculated over a range of accumulation periods from 1 month to 4 years and are available for the full ERA5 climatology from 1940 to today. It also contains validation data that indicates the quality of these drought indices. The ERA5–Drought dataset serves as a valuable tool for environmental agencies and supports sectors such as water management and agriculture, thus contributing to efforts that monitor water and food security.

The catastrophic Los Angeles Fires of January 2025 underscore the urgent need to understand the complex interplay between hydroclimatic variability and wildfire behavior. This study investigates how sequential wet and dry periods, hydroclimatic rebound events, create compounding environmental conditions that culminate in extreme fire events. Our results show that a cascade of moisture anomalies, from the atmosphere to vegetation health, precedes these fires by around 6–27 months. This is followed by a drying cascade 6 months before ignition that results in anomalously high and dry fuel loads conducive to fires. These patterns are confirmed when analyzing recent (2012–2025) extreme fire events in Mediterranean and Desert Californian biomes. We find hydroclimatic rebound as a key mechanism driving extreme wildfire risk, where moisture accumulation fuels vegetation growth that later dries into highly flammable fuel. In contrast, extreme fires in the fuel-rich Forested Mountain regions are less influenced by the moistening cascade and more impacted by prolonged drought conditions, which typically persist up to 11 months prior to fire occurrence. These insights improve fuel-informed operational fire forecasts for the January 2025 Los Angeles fires, particularly when year-specific fuel conditions are included. This underscores the value of incorporating long-memory variables to better anticipate extreme events in fuel-limited regions.

Irrigation has been investigated as an important historical climate forcing, but there is no study exploring its future climatic impacts considering possible changes in both extent and efficiency. Here, we address these issues via developing irrigation efficiency scenarios in line with the Shared Socioeconomic Pathways (SSPs), implementing these in the Community Earth System Model, and applying them to generate projections over the period 2015–2074. We project that annual irrigation water withdrawal decreases under SSP1-2.6 (from  ~2100 to  ~1700 km3 yr-1) but increases under SSP3-7.0 (to  ~2400 km3 yr-1), with some new irrigation hot spots emerging, especially in Africa. Irrigation is projected to reduce the occurrence of dry-heat stress under both scenarios, but cannot reverse the warming trend due to greenhouse gas emission (e.g., increasing from  ~90 to around 600 and 1200 hours yr-1 in intensely irrigated areas, under two scenarios). Moreover, moist-heat extreme event frequency increases more substantially (by ≥1600 hours yr-1 under SSP3-7.0 in tropical regions), and irrigation further amplifies the hours of exposure (for example, by ≥100 hours yr-1 in South Asia), thereby raising the risk of moist-heat-related illnesses and mortality for exposed communities. Our results underscore the importance of reducing greenhouse gas emissions, limiting irrigation expansion and improving irrigation efficiency to preserve water resources and decelerate escalating exposure to dry- and moist-heat stress.

Dryland expansion causes widespread water scarcity and biodiversity loss. Although the drying influence of global warming is well established, the role of existing drylands in their own expansion is relatively unknown. In this work, by tracking the air flowing over drylands, we show that the warming and drying of that air contributes to dryland expansion in the downwind direction. As they dry, drylands contribute less moisture and more heat to downwind humid regions, reducing precipitation and increasing atmospheric water demand, which ultimately causes their aridification. In ~40% of the land area that recently transitioned from a humid region into a dryland, self-expansion accounted for >50% of the observed aridification. Our results corroborate the urgent need for climate change mitigation measures in drylands to decelerate their own expansion.