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Irrigation is an essential practice for sustaining global food production and many regional economies. Emerging scientific evidence indicates that irrigation substantially affects mean climate conditions in different regions of the world. Yet how this practice influences climate extremes is currently unknown. Here we use ensemble simulations with the Community Earth System Model to assess the impacts of irrigation on climate extremes. An evaluation of the model performance reveals that irrigation has a small yet overall beneficial effect on the representation of present‐day near‐surface climate. While the influence of irrigation on annual mean temperatures is limited, we find a large impact on temperature extremes, with a particularly strong cooling during the hottest day of the year (−0.78 K averaged over irrigated land). The strong influence on extremes stems from the timing of irrigation and its influence on land‐atmosphere coupling strength. Together these effects result in asymmetric temperature responses, with a more pronounced cooling during hot and/or dry periods. The influence of irrigation is even more pronounced when considering subgrid‐scale model output, suggesting that local effects of land management are far more important than previously thought. Our results underline that irrigation has substantially reduced our exposure to hot temperature extremes in the past and highlight the need to account for irrigation in future climate projections.

Although the African Great Lakes are important regulators for the East African climate, their influence on atmospheric dynamics and the regional hydrological cycle remains poorly understood. This study aims to assess this impact by comparing a regional climate model simulation that resolves individual lakes and explicitly computes lake temperatures to a simulation without lakes. The Consortium for Small-Scale Modelling model in climate mode (COSMO-CLM) coupled to the Freshwater Lake model (FLake) and Community Land Model (CLM) is used to dynamically downscale a simulation from the African Coordinated Regional Downscaling Experiment (CORDEX-Africa) to 7-km grid spacing for the period of 1999-2008. Evaluation of the model reveals good performance compared to both in situ and satellite observations, especially for spatiotemporal variability of lake surface temperatures (0.68-K bias), and precipitation (2116 mm yr 21 or 8% bias). Model integrations indicate that the four major African Great Lakes almost double the annual precipitation amounts over their surface but hardly exert any influence on precipitation beyond their shores. Except for Lake Kivu, the largest lakes also cool the annual near-surface air by 20.6 to 20.9 K on average, this time with pronounced downwind influence. The lake-induced cooling happens during daytime, when the lakes absorb incoming solar radiation and inhibit upward turbulent heat transport. At night, when this heat is released, the lakes warm the near-surface air. Furthermore, Lake Victoria has a profound influence on atmospheric dynamics and stability, as it induces circular airflow with over-lake convective inhibition during daytime and the reversed pattern at night. Overall, this study shows the added value of resolving individual lakes and realistically representing lake surface temperatures for climate studies in this region.

<p>Weather- and climate-related extreme events such as droughts, heatwaves and storms arise from interactions between complex sets of physical processes across multiple spatial and temporal scales, often overwhelming the capacity of natural and/or human systems to cope. In many cases, the greatest impacts arise through the ‘compounding’ effect of weather and climate-related drivers and/or hazards, where the scale of the impacts can be much greater than if any of the drivers or hazards occur in isolation; for instance, when a heavy precipitation falls on an already saturated soil causing a devastating flood. Compounding in this context refers to the amplification of an impact due to the occurrence of multiple drivers and/or hazards either because multiple hazards occur at the same time, previous climate conditions or weather events have increased a system’s vulnerability to a successive event, or spatially concurrent hazards lead to a regionally or globally integrated impact. More generally, compound weather and climate events refer to a combination of multiple climate drivers and/or hazards that contributes to societal or environmental risk.</p><p>Although many climate-related disasters are caused by compound events, our ability to understand, analyse and project these events and interactions between their drivers is still in its infancy. Here we review the current state of knowledge on compound events and propose a typology to synthesize the available literature and guide future research. We organize the highly diverse event types broadly along four main themes, namely preconditioned, multivariate, temporally compounding, and spatially compounding events. We highlight promising analytical approaches tailored to the different event types, which will aid future research and pave the way to a coherent framework for compound event analysis. We further illustrate how human-induced climate change affects different aspects of compound events, such as their frequency and intensity through variations in the mean, variability, and the dependence between their climatic drivers. Finally, we discuss the emergence of new types of events that may become highly relevant in a warmer climate.</p>

The concentration of dissolved oxygen in aquatic systems helps regulate biodiveristy 1, 2 , nutrient biogeochemistry 3 , greenhouse gas emissions 4 , and drinking water quality 5 . The long-term declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity 6, 7 , but little is known about changes in dissolved oxygen concentrations in lakes. While dissolved oxygen solubility decreases with increasing water temperatures, long-term lake trajectories are not necessarily predictable. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification 8, 9 or they may increase as a result of enhanced primary production 10 . Here we analyse 45,148 dissolved oxygen and temperature profiles from 393 temperate lakes spanning 1941-2017. We find that a decline in dissolved oxygen is widespread in surface and deep-water habitats. The decline in surface waters is primarily associated with reduced solubility under warmer water temperatures, although surface dissolved oxygen increased in a subset of highly-productive warming lakes, likely due to increasing phytoplankton production. In contrast, the decline in deep waters is associated with stronger thermal stratification and water clarity losses, but not with changes in gas solubility. Our results suggest that climate change and declining water clarity have altered the physical and chemical environment of lakes. Freshwater dissolved oxygen losses are 2.5-10 times greater than observed in the world's oceans 6, 7 and could threaten essential lake ecosystem services 2,3,5,11 .

Terrestrial water storage (TWS) strongly modulates the hydrological cycle and is a key determinant of water availability and an indicator of drought. While historical TWS variations have been studied, future changes in TWS and the linkages to droughts remain unexamined. Here, using ensemble hydrological simulations, we show that climate change could reduce TWS in many regions, especially in the southern hemisphere. A strong inter-ensemble agreement indicates high confidence in the projected changes that are driven primarily by climate forcing, rather than land-water management activities. Declines in TWS translate to increase in future droughts. By the late-21 st century global land area and population in extreme-to-exceptional TWS drought could more than double, each increasing from 3% during 1976-2005 to 7% and 8%, respectively. Our findings highlight the importance of climate change mitigation to avoid adverse impacts on TWS and related droughts, and the need for adaptation to improve water resource management. TWS-the sum of continental water stored in canopies, snow and ice, rivers, lakes and 51 reservoirs, wetlands, soil, and groundwater-is a critical component of the global water and energy budget. It plays key roles in determining water resource availability 1 and modulating water flux interactions among various Earth system components 2 . Further, observed changes in TWS are inherently linked to droughts 2-6 , floods 7 , and global sea level change [8][9][10][11] . Despite such importance, global TWS remains less studied relative to hydrological fluxes (e.g., river discharge, evapotranspiration, and groundwater flow) owing to the lack of large-scale observations and challenges in explicitly resolving all TWS components in hydrological modeling 12 . This generally holds true for historical analyses; crucially, no study has to date examined the potential impacts of future climate change on global TWS. Recent modeling advancements 13 have improved the representation of TWS in global hydrological models 14,15 (GHMs) and land surface models 12 (LSMs). The Gravity Recovery and Climate Experiment (GRACE) satellite mission provided added opportunities to improve and validate TWS simulations in these models. GRACE TWS data and model simulations, often in combination, have been used for wide ranging applications including the assessment of water resources and impacts of human activities on the water cycle 14,16 , quantifying aquifer depletion 12,14,[17][18][19] , monitoring drought [3][4][5][6]20 , and assessing flood potential 7 . These studies have advanced the understanding of global TWS systems that are continually changing under natural hydro-climatic variability and accelerating human land-water management activities, but the 70 focus has been on historical variabilities in TWS. Further, future projections from general 71 circulation models (GCMs) have been used to quantify climate change impacts on hydrological 72 fluxes [21][22][23] and storages, but the projections of storages are limited to a subset of T...

One of the most important physical characteristics driving lifecycle events in lakes is stratification. Already subtle variations in the timing of stratification onset and break-up (phenology) are known to have major ecological effects, mainly by determining the availability of light, nutrients, carbon and oxygen to organisms. Despite its ecological importance, historic and future global changes in stratification phenology are unknown. Here, we used a lake-climate model ensemble and long-term observational data, to investigate changes in lake stratification phenology across the Northern Hemisphere from 1901 to 2099. Under the high-greenhouse-gas-emission scenario, stratification will begin 22.0 ± 7.0 days earlier and end 11.3 ± 4.7 days later by the end of this century. It is very likely that this 33.3 ± 11.7 day prolongation in stratification will accelerate lake deoxygenation with subsequent effects on nutrient mineralization and phosphorus release from lake sediments. Further misalignment of lifecycle events, with possible irreversible changes for lake ecosystems, is also likely.

Anthropogenic climate change is expected to affect global river flow. Here, we analyze time series of low, mean, and high river flows from 7250 observatories around the world covering the years 1971 to 2010. We identify spatially complex trend patterns, where some regions are drying and others are wetting consistently across low, mean, and high flows. Trends computed from state-of-the-art model simulations are consistent with the observations only if radiative forcing that accounts for anthropogenic climate change is considered. Simulated effects of water and land management do not suffice to reproduce the observed trend pattern. Thus, the analysis provides clear evidence for the role of externally forced climate change as a causal driver of recent trends in mean and extreme river flow at the global scale.