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In the 2022 summer, western–central Europe and several other regions in the northern extratropics experienced substantial soil moisture deficits in the wake of precipitation shortages and elevated temperatures. Much of Europe has not witnessed a more severe soil drought since at least the mid-20th century, raising the question whether this is a manifestation of our warming climate. Here, we employ a well-established statistical approach to attribute the low 2022 summer soil moisture to human-induced climate change using observation-driven soil moisture estimates and climate models. We find that in western–central Europe, a June–August root zone soil moisture drought such as in 2022 is expected to occur once in 20 years in the present climate but would have occurred only about once per century during preindustrial times. The entire northern extratropics show an even stronger global warming imprint with a 20-fold soil drought probability increase or higher, but we note that the underlying uncertainty is large. Reasons are manifold but include the lack of direct soil moisture observations at the required spatiotemporal scales, the limitations of remotely sensed estimates, and the resulting need to simulate soil moisture with land surface models driven by meteorological data. Nevertheless, observation-based products indicate long-term declining summer soil moisture for both regions, and this tendency is likely fueled by regional warming, while no clear trends emerge for precipitation. Finally, our climate model analysis suggests that under 2 ∘ C global warming, 2022-like soil drought conditions would become twice as likely for western–central Europe compared to today and would take place nearly every year across the northern extratropics. [ABSTRACT FROM AUTHOR]

Drought events have multiple adverse impacts on environment, society, and economy. It is thus crucial to monitor and characterise such events. Here, we compare the ability of selected state-of-the-art long-term reanalysis and remote-sensing products to represent major seasonal and multi-year drought events in the 2000–2020 period globally. We focus on soil moisture (or agroecological) drought and place the results in the context of trends in dry-season soil moisture. We consider surface and root-zone soil moisture from ERA5, the related ERA5-Land, and MERRA-2 reanalysis products, the ESA CCI remote-sensing surface soil moisture products (encompassing an ACTIVE, a PASSIVE and a COMBINED product), as well as its near real-time counterpart produced within C3S. In addition, we use a new root-zone soil moisture dataset derived from the ESA CCI COMBINED product. Except for ESA CCI surface and root-zone soil moisture, the considered products offer opportunities for drought monitoring since they are available in near real-time. We analyse 18 documented drought events within predefined spatial and temporal bounds derived from scientific literature. Based on standardised daily anomalies of surface and root-zone soil moisture, the drought events are characterised by their severity (the time accumulated standardised anomalies), magnitude (the minimum of the standardised anomalies over time), duration, and spatial extent. Product deviations in drought severity and magnitude are then placed in the context of trends in dry-season soil moisture, and potential reasons for diverging global soil moisture trends in the products are further investigated. All investigated products capture the considered drought events. Overall, responses of surface soil moisture tend to be weakest for the ACTIVE remote-sensing products in all metrics, but most pronounced in the drought magnitudes. Also, MERRA‑2 shows lower magnitudes than the other products. Except for the COMBINED products, the remote-sensing products tend to underestimate the spatial extents of larger droughts. Product differences in drought severity and magnitude for single events are consistent with the differences in dry-season soil moisture trends. These trends are globally diverse and partly contradictory between products. ERA5, ERA5-Land and the COMBINED products show larger fractions of drying trends, MERRA-2 and the C3S ACTIVE and PASSIVE products more widespread wetting trends. MERRA-2 surface air temperature shows regionally negative biases in trends compared to a ground observational product, which suggests that this reanalysis product underestimates drought trends. Also, the comparison with trends in selected land-surface characteristics and bioclimatic indicators shows that dry-season soil moisture trends may be affected by retrieval or modelling artifacts in some cases. In the root zone (based on the reanalysis products and the ESA CCI root-zone soil moisture dataset), the droughts are dampened in magnitude and smaller in spatial extent but show a tendency to prolonged durations. Based on the overall observational evidence and the consideration of the respective limitations of the included products, the present analyses suggest a consistent tendency towards drying during the last two decades in some regions, namely in parts of central Europe, in a region north of the Black Sea/Caspian Sea, in southern Africa, and in parts of Australia, Siberia and South America. [ABSTRACT FROM AUTHOR]

In the 2022 summer, West-Central Europe and several other northern-hemisphere mid-latitude regions experienced substantial soil moisture deficits in the wake of precipitation shortages and elevated temperatures. Much of Europe has not witnessed a more severe soil drought since at least the mid-20th century, raising the question whether this is a manifestation of our warming climate. Here, we employ a well-established statistical approach to attribute the low 2022 summer soil moisture to human-induced climate change, using observation-driven soil moisture estimates and climate models. We find that in West-Central Europe, a June-August root-zone soil moisture drought such as in 2022 is expected to occur once in 20 years in the present climate, but would have occurred only about once per century during pre-industrial times. The entire northern extratropics show an even stronger global warming imprint with a 20-fold soil drought probability increase or higher, but we note that the underlying uncertainty is large. Reasons are manifold, but include the lack of direct soil moisture observations at the required spatiotemporal scales, the limitations of remotely sensed estimates, and the resulting need to simulate soil moisture with land surface models driven by meteorological data. Nevertheless, observationbased products indicate long-term declining summer soil moisture for both regions, and this tendency is likely fueled by regional warming, while no clear trends emerge for precipitation. Finally, our climate model analysis suggests that in a 2 °C world, 2022-like soil drought conditions would become twice as likely for West-Central Europe compared to today, and would take place nearly every year across the northern extratropics. [ABSTRACT FROM AUTHOR]

Climatological drought monitoring in Switzerland relies heavily on station-based precipitation and temperature data. Due to the high spatial variability and complexity of droughts, it is important to complement station-based drought indices with gridded information and to couple multiple drought indicators within the monitoring system. Here, long-term satellite-based drought parameters from the EUMETSAT SAF network are analyzed in terms of dry anomalies within their climatology's, namely ASCAT soil water index (SWI), CM SAF land surface temperature (LST), complemented with NOAA vegetation data, and LSA SAF Meteosat evapotranspiration data. The upcoming EUMETSAT SAF climate data records on land surface temperature and evapotranspiration will cover for the first time the WMO climatological 30-year reference period. This study is the first study investigating the potential of those long-term data records for climate monitoring of droughts in Europe. The satellite datasets are compared with the standardized precipitation index (SPI), soil moisture observations from the SwissSMEX measurement network, with a modelled soil moisture index (SMI) based on observations, and with evapotranspiration measurements, focusing on the temporal dynamics of the anomalies. For vegetation and surface temperature, the dry years of 2003, 2015, and 2018 are clearly visible in the satellite data. CM SAF LSTs show strong anomalies at the beginning of the drought period. The comparison of in situ and modelled soil moisture and evapotranspiration measurements with the satellite parameters shows strong agreement in terms of anomalies. The SWI indicates high anomaly correlations of 0.56 to 0.83 with measurements and 0.63 to 0.76 with the SMI at grassland sites. The Meteosat evapotranspiration data strongly agree with the measurements, with anomaly correlations of 0.63 and 0.67 for potential and actual evapotranspiration, respectively. Due to the prevailing humid climate conditions at the considered sites, evapotranspiration anomalies during the investigated dry periods were mostly positive and thus not water limited, but were also a driver for soil moisture drought. The results indicate that EUMETSAT SAF satellite data can well complement the station-based drought monitoring in Switzerland with spatial information. [ABSTRACT FROM AUTHOR]

Crop producers face significant and increasing drought risks. We evaluate whether insurances based on globally and freely available satellite-retrieved soil moisture data can reduce farms' financial drought risk exposure. We design farm individual soil moisture index insurances for wheat, maize and rapeseed production using a case study for Eastern Germany. We find that the satellite-retrieved soil moisture index insurances significantly decrease risk exposure for these crops compared to the situation where production is not insured. The satellite-retrieved index also outperforms one based on soil moisture estimates derived from meteorological measurements at ground stations. Important implications for insurers and policy makers are that they could and should develop better suited insurances. Available satellite-retrieved data can be used to increase farmers' resilience in a changing climate. [ABSTRACT FROM AUTHOR]

Terrestrial climate is influenced by various land-atmosphere interactions that involve numerous land surface state variables. In several regions on Earth, soil moisture plays an important role for climate through its control on the partitioning of net radiation into sensible and latent heat fluxes and, consequently, its impact on temperature and precipitation. The Global Land-Atmosphere Climate Experiment-Coupled Model Intercomparison Project phase 5 (GLACE-CMIP5) aims to quantify the impact of soil moisture on these important climate variables and to trace the individual coupling mechanisms. GLACE-CMIP5 provides experiments with different soil moisture prescriptions that can be used to isolate the effect of soil moisture on climate. Using a theoretical approach that relies on the distinct relation of soil moisture with evaporative fraction (the ratio of latent heat flux over net radiation) and daily maximum near-surface air temperature in different soil moisture regimes, the climate impact of the soil moisture prescriptions in the GLACE-CMIP5 experiments can be emulated and quantified. The theoretical estimation of the soil moisture effect on evaporative fraction agrees very well with estimations obtained directly from the GLACE-CMIP5 experiments (pattern correlation of 0.85). Moreover, the soil moisture effect on daily maximum temperature is well captured in those regions where soil moisture exerts a strong control on latent heat fluxes. The theoretical approach is further applied to quantify the soil moisture contribution to the projected change of the temperature on the hottest day of the year, confirming recent estimations by other studies. Finally, GLACE-style soil moisture prescriptions are emulated in an extended set of CMIP5 models. The results indicate consistency between the soil moisture-climate coupling strength estimated with GLACE-CMIP5 and CMIP5 models. Although the theoretical approach is designed to capture only the local soil moisture-climate coupling strength, it can also help to distinguish non-local from local soil moisture-atmosphere feedbacks where sensitivity experiments (such as GLACE-CMIP5) are available. Overall, the presented theoretical approach constitutes a simple and powerful tool to quantify local soil moisture-climate coupling in both GLACE-CMIP5 and CMIP5 models that can be applied in the absence of dedicated sensitivity experiments. [ABSTRACT FROM AUTHOR]

Soil moisture plays a crucial role for the energy partitioning at Earth's surface. Changing fractions of latent and sensible heat fluxes caused by soil moisture variations can affect both near-surface air temperature and precipitation. In this study, a simple framework for the dependence of evaporative fraction (the ratio of latent heat flux over net radiation) on soil moisture is used to analyze spatial and temporal variations of land-atmosphere coupling and its effect on near-surface air temperature. Using three different data sources (two reanalysis datasets and one combination of different datasets), three key parameters for the relation between soil moisture and evaporative fraction are estimated: 1) the frequency of occurrence of different soil moisture regimes, 2) the sensitivity of evaporative fraction to soil moisture in the transitional soil moisture regime, and 3) the critical soil moisture value that separates soil moisture- and energy-limited evapotranspiration regimes. The results show that about 30%-60% (depending on the dataset) of the global land area is in the transitional regime during at least half of the year. Based on the identification of transitional regimes, the effect of changes in soil moisture on near-surface air temperature is analyzed. Typical soil moisture variations (standard deviation) can impact air temperature by up to 1.1-1.3 K, while changing soil moisture over its full range in the transitional regime can alter air temperature by up to 6-7 K. The results emphasize the role of soil moisture for atmosphere and climate and constitute a useful benchmark for the evaluation of the respective relationships in Earth system models. [ABSTRACT FROM AUTHOR]

Many countries rely on regional climate model (RCM) simulations to quantify the projectedimpacts of climate change and design their national adaptation plans. The accuracy andreliability of RCM simulations is thus of fundamental importance and, in particular, it iscrucial to include all relevant climate processes in RCMs. One of the processes notconsidered in most RCMs is the response of plants to elevated atmospheric CO2concentrations (the plant physiological CO2 effect); a process which is however includedin most global climate models (GCMs). Here we show that due to this missingprocess, RCMs underestimate the future increase of extreme temperatures compared toGCMs. The plant physiological CO2 effect takes into account that at higher CO2 concentrationsplants increase their water use efficiency, i.e. the fraction of CO2 uptake to transpiration. As aconsequence, plants reduce the openings of their leaf stomata at elevated CO2 concentrations.This causes a reduction of plant transpiration, resulting in a decrease of latent heat flux and anincrease of sensible heat flux. Ultimately, the plant physiological CO2 effect can thusalter near-surface air temperatures. Using a state-of-the-art regional climate model(COSMO-CLM2), we perform two model simulations: one that includes the plantphysiological CO2 effect and one where plants are insensitive to the atmospheric CO2increase. The simulations cover the European domain and the time span 1971-2099. In thesimulation that includes the plant physiological CO2 effect, evapotranspiration in central andnorthern Europe at the end of the 21st century is significantly lower than in the simulationwith insensitive plants (calculated as changes with respect to 1971-2000). In contrast, theevapotranspiration difference between both simulations is small in southern Europe,suggesting that other factors, such as soil moisture limitations, prevent a strongimpact of the plant physiological CO2 effect on evapotranspiration in this region. Theresults obtained with COSMO-CLM2 are in line with the fact that RCMs fromthe EURO-CORDEX multi-model ensemble also exhibit lower evapotranspirationcompared to GCMs in central and northern Europe at the end of the 21st century (withrespect to 1971-2000). Additionally, the results from COSMO-CLM2 agree verywell with estimations of the plant physiological CO2 effect from dedicated CMIP5experiments. Including the plant physiological CO2 effects in COSMO-CLM2 also affectsnear-surface air temperatures. In central and northern Europe, the annual maximumtemperature (TXx) is about 1 K higher in the COSMO-CLM2 simulation with plantphysiological CO2 effects compared to the simulation with insensitive plants at theend of the 21st century. This result can partially explain the discrepancy betweenthe TXx evolution in GCMs and RCMs in central and northern Europe where atthe end of the 21st century GCMs exhibit a TXx increase that is about 2 K higherthan the TXx increase in RCMs. According to the COSMO-CLM2 simulations,the plant physiological CO2 effect accounts for about 50 % of the TXx differencebetween GCMs and RCMs. Neglecting the plant physiological CO2 effect can thusresult in a substantial underestimation of future temperature increases in RCMs. [ABSTRACT FROM AUTHOR]

Climate Data Records of soil moisture are fundamental for improving our understanding of long-term dynamics in the coupled water, energy, and carbon cycles over land. To respond to this need, in 2012 the European Space Agency (ESA) released the first multi-decadal, global satellite-observed soil moisture (SM) dataset as part of its Climate Change Initiative (CCI) program. This product, named ESA CCI SM, combines various single-sensor active and passive microwave soil moisture products into three harmonised products: a merged ACTIVE, a merged PASSIVE, and a COMBINED active + passive microwave product. Compared to the first product release, the latest version of ESA CCI SM includes a large number of enhancements, incorporates various new satellite sensors, and extends its temporal coverage to the period 1978–2015. In this study, we first provide a comprehensive overview of the characteristics, evolution, and performance of the ESA CCI SM products. Based on original research and a review of existing literature we show that the product quality has steadily increased with each successive release and that the merged products generally outperform the single-sensor input products. Although ESA CCI SM generally agrees well with the spatial and temporal patterns estimated by land surface models and observed in-situ, we identify surface conditions (e.g., dense vegetation, organic soils) for which it still has large uncertainties. Second, capitalising on the results of > 100 research studies that made use of the ESA CCI SM data we provide a synopsis of how it has contributed to improved process understanding in the following Earth system domains: climate variability and change, land-atmosphere interactions, global biogeochemical cycles and ecology, hydrological and land surface modelling, drought applications, and meteorology. While in some disciplines the use of ESA CCI SM is already widespread (e.g. in the evaluation of model soil moisture states) in others (e.g. in numerical weather prediction or flood forecasting) it is still in its infancy. The latter is partly related to current shortcomings of the product, e.g., the lack of near-real-time availability and data gaps in time and space. This study discloses the discrepancies between current ESA CCI SM product characteristics and the preferred characteristics of long-term satellite soil moisture products as outlined by the Global Climate Observing System (GCOS), and provides important directions for future ESA CCI SM product improvements to bridge these gaps. [ABSTRACT FROM AUTHOR]

Abstract: Soil moisture is a key variable of the climate system. It constrains plant transpiration and photosynthesis in several regions of the world, with consequent impacts on the water, energy and biogeochemical cycles. Moreover it is a storage component for precipitation and radiation anomalies, inducing persistence in the climate system. Finally, it is involved in a number of feedbacks at the local, regional and global scales, and plays a major role in climate-change projections. In this review, we provide a synthesis of past research on the role of soil moisture for the climate system, based both on modelling and observational studies. We focus on soil moisture–temperature and soil moisture–precipitation feedbacks, and their possible modifications with climate change. We also highlight further impacts of soil moisture on climate, and the state of research regarding the validation of the relevant processes. There are promises for major advances in this research field in coming years thanks to the development of new validation datasets and multi-model initiatives. However, the availability of ground observations continues to be critical in limiting progress and should therefore strongly be fostered at the international level. Exchanges across disciplines will also be essential for bridging current knowledge gaps in this field. This is of key importance given the manifold impacts of soil moisture on climate, and their relevance for climate-change projections. A better understanding and quantification of the relevant processes would significantly help to reduce uncertainties in future-climate scenarios, in particular with regard to changes in climate variability and extreme events, as well as ecosystem and agricultural impacts. [Copyright &y& Elsevier]

Quality assurance of satellite-derived soil moisture datasets is key for ensuring users are provided with datasets of sufficient quality to their applications. Currently, such activities are undertaken on an ad-hoc basis or using a protocol designed for a specific product. These results are useful, but differ in terms of output validation data, plots and reports and therefore are difficult to inter-compare.The aim of the Quality Assurance for Soil Moisture (QA4SM) (www.qa4sm.eodc.eu) project (supported by the Austrian Space Application Programme) is to bring together methodologies and protocols used for the validation and quality control of soil moisture data products and develop this into a coherent methodology allowing the provision of standardised, and above all traceable validation results to the user community. QA4SM is driven by validation best practice guidelines resulting from a collaborative effort of the entire satellite soil moisture validation community, including the Committee on Earth Observation Satellites, the Global Climate Observing System, and the validation teams of the individual satellite missions.An easy-to-use functional, web-based test application has been made available to the user community. The application currently allows users to compare medium coarse resolution satellite datasets to in-situ validation data (made available through the International Soil Moisture Network (ISMN) (http://ismn.geo.tuwien.ac.at)) and global land surface models. Later updates of the system will allow the upload and comparison of user's own datasets as well as the validation of high resolution (1 km) products. The web-based graphical front-end allows users to easily select specific criteria for the comparison process including control of the masking applied to each dataset, the metrics calculated and so on. The validation results are provided as a netCDF file which includes traceability data in the global attributes as well as plots of each metric.Various satellite-derived soil moisture products are available for validation within the QA4SM system including SMAP L3 and the SMOS IC soil moisture products as well as the European Space Agency (ESA) Climate Change Initiative (CCI) and EU Copernicus Climate Changes Services (C3S) soil moisture products. These latter two products are produced by systematically combining Level-2 datasets from separate missions. The combination of individual Level 2 datasets into a single product gives us the opportunity to profit from the advantages of individual missions, and to obtain homogenised CDRs with improved spatial and temporal coverage.The accuracy of the ESA CCI and C3S products are routinely assessed (each time a new version is generated) through comparison to in-situ (ISMN) and global models. The quality assessment also includes consideration of the stability of the product and a completeness / consistency check. The aim of these assessments is to ensure that the highest quality product is provided to the data users. With the availability of the QA4SM system, this can now be achieved in a robust and traceable manner.The goal of this presentation is to present the QA4SM system, show validation results for both the ESA CCI and C3S soil moisture products using the system, and collect feedback from the wider satellite soil moisture validation/calibration community. [ABSTRACT FROM AUTHOR]