A warming-induced reduction in snow fraction amplifies rainfall extremes.

The intensity of extreme precipitation events is projected to increase in a warmer climate1-5, posing a great challenge to water sustainability in natural and built environments. Of particular importance are rainfall (liquid precipitation) extremes owing to their instantaneous triggering of runoff and association with floods6, landslides7-9 and soil erosion10,11. However, so far, the body of literature on intensification of precipitation extremes has not examined the extremes of precipitation phase separately, namely liquid versus solid precipitation. Here we show that the increase in rainfall extremes in high-elevation regions of the Northern Hemisphere is amplified, averaging 15 per cent per degree Celsius of warming-double the rate expected from increases in atmospheric water vapour. We utilize both a climate reanalysis dataset and future model projections to show that the amplified increase is due to a warming-induced shift from snow to rain. Furthermore, we demonstrate that intermodel uncertainty in projections of rainfall extremes can be appreciably explained by changes in snow-rain partitioning (coefficient of determination 0.47). Our findings pinpoint high-altitude regions as 'hotspots' that are vulnerable to future risk of extreme-rainfall-related hazards, thereby requiring robust climate adaptation plans to alleviate potential risk. Moreover, our results offer a pathway towards reducing model uncertainty in projections of rainfall extremes.

Enabling FAIR data in Earth and environmental science with community-centric (meta)data reporting formats.

Research can be more transparent and collaborative by using Findable, Accessible, Interoperable, and Reusable (FAIR) principles to publish Earth and environmental science data. Reporting formats-instructions, templates, and tools for consistently formatting data within a discipline-can help make data more accessible and reusable. However, the immense diversity of data types across Earth science disciplines makes development and adoption challenging. Here, we describe 11 community reporting formats for a diverse set of Earth science (meta)data including cross-domain metadata (dataset metadata, location metadata, sample metadata), file-formatting guidelines (file-level metadata, CSV files, terrestrial model data archiving), and domain-specific reporting formats for some biological, geochemical, and hydrological data (amplicon abundance tables, leaf-level gas exchange, soil respiration, water and sediment chemistry, sensor-based hydrologic measurements). More broadly, we provide guidelines that communities can use to create new (meta)data formats that integrate with their scientific workflows. Such reporting formats have the potential to accelerate scientific discovery and predictions by making it easier for data contributors to provide (meta)data that are more interoperable and reusable.

Urbanization and aridity mediate distinct salinity response to floods in rivers and streams across the contiguous United States.

Salinity is an important water quality parameter that affects ecosystem health and the use of freshwaters for industrial, agricultural, and other beneficial purposes. Although a number of studies have investigated the variability and trends of salinity in rivers and streams, the effects of floods on salinity across a wide range of watersheds have not been determined. Here, we examine this question by utilizing long-term observational records of daily streamflow and specific conductance (SC; a proxy for salinity) in addition to catchment characteristics for 259 United States Geological Survey (USGS) monitoring sites in the contiguous United States spanning a wide range of climatic, geologic and hydrologic conditions. We used a combination of statistical methods, random forest machine learning models, and information-theoretic causal inference algorithms to determine the response of SC to floods and the factors that impact salinity changes within sites (intra-site variability) and across sites (inter-site variability). Our results show that changes to SC during flood events exhibited substantial variability ranging from a 100% decrease to 34% increase relative to the long-term mean. We found that dilution is the prevailing mechanism that decreases SC levels during floods for most sites, but other mechanisms caused an increase of SC for 6.1% (n = 5521) of flood events. Our analysis revealed that antecedent conditions of SC in the few days preceding the flood are the most important factor in explaining intra-site variability. The response of salinity to floods also varied considerably across sites with different characteristics, with a notable effect of urbanization in temperate climates resulting in increased dilution of SC, and mining in arid climates, which adversely increases SC levels. Overall, we find that the combined effect of aridity and anthropogenic factors is of primary importance in determining how salinity responds to floods, and it bears strongly on water quality conditions in a future world - one in which floods are expected to increase in frequency and intensity, concurrent with shifting aridity patterns and increasing urbanization.

Species-Specific Shifts in Diurnal Sap Velocity Dynamics and Hysteretic Behavior of Ecophysiological Variables During the 2015-2016 El Niño Event in the Amazon Forest.

Current climate change scenarios indicate warmer temperatures and the potential for more extreme droughts in the tropics, such that a mechanistic understanding of the water cycle from individual trees to landscapes is needed to adequately predict future changes in forest structure and function. In this study, we contrasted physiological responses of tropical trees during a normal dry season with the extreme dry season due to the 2015-2016 El Niño-Southern Oscillation (ENSO) event. We quantified high resolution temporal dynamics of sap velocity (Vs), stomatal conductance (gs) and leaf water potential (ΨL) of multiple canopy trees, and their correlations with leaf temperature (Tleaf) and environmental conditions [direct solar radiation, air temperature (Tair) and vapor pressure deficit (VPD)]. The experiment leveraged canopy access towers to measure adjacent trees at the ZF2 and Tapajós tropical forest research (near the cities of Manaus and Santarém). The temporal difference between the peak of gs (late morning) and the peak of VPD (early afternoon) is one of the major regulators of sap velocity hysteresis patterns. Sap velocity displayed species-specific diurnal hysteresis patterns reflected by changes in Tleaf. In the morning, Tleaf and sap velocity displayed a sigmoidal relationship. In the afternoon, stomatal conductance declined as Tleaf approached a daily peak, allowing ΨL to begin recovery, while sap velocity declined with an exponential relationship with Tleaf. In Manaus, hysteresis indices of the variables Tleaf-Tair and ΨL-Tleaf were calculated for different species and a significant difference (p < 0.01, α = 0.05) was observed when the 2015 dry season (ENSO period) was compared with the 2017 dry season ("control scenario"). In some days during the 2015 ENSO event, Tleaf approached 40°C for all studied species and the differences between Tleaf and Tair reached as high at 8°C (average difference: 1.65 ± 1.07°C). Generally, Tleaf was higher than Tair during the middle morning to early afternoon, and lower than Tair during the early morning, late afternoon and night. Our results support the hypothesis that partial stomatal closure allows for a recovery in ΨL during the afternoon period giving an observed counterclockwise hysteresis pattern between ΨL and Tleaf.

Reoxidation of Chromium(III) Products Formed under Different Biogeochemical Regimes.

Hexavalent chromium, Cr(VI), is a widespread and toxic groundwater contaminant. Reductive immobilization to Cr(III) is a treatment option, but its success depends on the long-term potential for reduced chromium precipitates to remain immobilized under oxidizing conditions. In this unique long-term study, aquifer sediments subjected to reductive Cr(VI) immobilization under different biogeochemical regimes were tested for their susceptibility to reoxidation. After reductive treatment for 1 year, sediments were exposed to oxygenated conditions for another 2 years in flow-through, laboratory columns. Under oxidizing conditions, immobilized chromium reduced under predominantly denitrifying conditions was mobilized at low concentrations (≪1 µM Cr(VI); ∼ 3% of Cr(III) deposited) that declined over time. A conceptual model of a limited pool of more soluble Cr(III), and a larger pool of relatively insoluble Cr(III), is proposed. In contrast, almost no chromium was mobilized from columns reduced under predominantly fermentative conditions, and where reducing conditions persisted for several months after introduction of oxidizing conditions, presumably due to the presence of a reservoir of reduced species generated during reductive treatment. The results from this 3-year study demonstrate that biogeochemical conditions present during reductive treatment, and the potential for buildup of reducing species, will impact the long-term sustainability of the remediation effort.

Identifying chemicals of concern in hydraulic fracturing fluids used for oil production.

Chemical additives used for hydraulic fracturing and matrix acidizing of oil reservoirs were reviewed and priority chemicals of concern needing further environmental risk assessment, treatment demonstration, or evaluation of occupational hazards were identified. We evaluated chemical additives used for well stimulation in California, the third largest oil producing state in the USA, by the mass and frequency of use, as well as toxicity. The most frequently used chemical additives in oil development were gelling agents, cross-linkers, breakers, clay control agents, iron and scale control agents, corrosion inhibitors, biocides, and various impurities and product stabilizers used as part of commercial mixtures. Hydrochloric and hydrofluoric acids, used for matrix acidizing and other purposes, were reported infrequently. A large number and mass of solvents and surface active agents were used, including quaternary ammonia compounds (QACs) and nonionic surfactants. Acute toxicity was evaluated and many chemicals with low hazard to mammals were identified as potentially hazardous to aquatic environments. Based on an analysis of quantities used, toxicity, and lack of adequate hazard evaluation, QACs, biocides, and corrosion inhibitors were identified as priority chemicals of concern that deserve further investigation.

Surveys, simulation and single-cell assays relate function and phylogeny in a lake ecosystem.

Much remains unknown about what drives microbial community structure and diversity. Highly structured environments might offer clues. For example, it may be possible to identify metabolically similar species as groups of organisms that correlate spatially with the geochemical processes they carry out. Here, we use a 16S ribosomal RNA gene survey in a lake that has chemical gradients across its depth to identify groups of spatially correlated but phylogenetically diverse organisms. Some groups had distributions across depth that aligned with the distributions of metabolic processes predicted by a biogeochemical model, suggesting that these groups performed biogeochemical functions. A single-cell genetic assay showed, however, that the groups associated with one biogeochemical process, sulfate reduction, contained only a few organisms that have the genes required to reduce sulfate. These results raise the possibility that some of these spatially correlated groups are consortia of phylogenetically diverse and metabolically different microbes that cooperate to carry out geochemical functions.

Characterization of Chromium Bioremediation Products in Flow-Through Column Sediments Using Micro-X-ray Fluorescence and X-ray Absorption Spectroscopy.

Microbially mediated reductive immobilization of chromium is a possible remediation technique for sites contaminated with Cr(VI). This study is part of a broader effort investigating the biogeochemical mechanisms for Cr(VI) reduction in Hanford 100H aquifer sediments using flow-through laboratory columns. It had previously been shown that reduced chromium in the solid phase was in the form of freshly precipitated mixed-phase Cr(III)-Fe(III) (hydr)oxides, irrespective of the biogeochemical conditions in the columns. In this study, the reduced Cr phases in the columns were investigated further using spectroscopy to understand the structure and mechanisms involved in the formation of the end products. Several samples representing potential processes that could be occurring in the columns were synthesized in the laboratory and characterized using X-ray absorption near edge structure (XANES) and X-ray scattering. The XANES of Cr(III) particles in the columns most closely resembled those from synthetic samples produced by the abiotic reaction of Cr(VI) with microbially reduced Fe(II). Microbially mediated Cr-Fe reduction products were distinct from abiotic Cr-Fe (hydr)oxides [CrFe(OH)] and organically complexed Cr(III) sorbed onto the surface of a mixed ferrihydrite-goethite mineral phase. Furthermore, analyses of the abiotically synthesized samples revealed that even the end products of purely abiotic, iron-mediated reduction of Cr(VI) are affected by factors such as the presence of excess aqueous Fe(II) and cellular matter. These results suggest that CrFe(OH) phases made under realistic subsurface conditions or in biotic cultures are structurally different from pure Cr(OH) or laboratory-synthesized CrFe(OH). The observed structural differences imply that the reactivity and stability of biogenic CrFe(OH) could potentially be different from that of abiotic CrFe(OH).

Divergent aquifer biogeochemical systems converge on similar and unexpected Cr(VI) reduction products.

In this study of reductive chromium immobilization, we found that flow-through columns constructed with homogenized aquifer sediment and continuously infused with lactate, chromate, and various native electron acceptors diverged to have very different Cr(VI)-reducing biogeochemical regimes characterized by either denitrifying or fermentative conditions (as indicated by effluent chemical data, 16S rRNA pyrotag data, and metatranscriptome data). Despite the two dramatically different biogeochemical environments that evolved in the columns, these regimes created similar Cr(III)-Fe(III) hydroxide precipitates as the predominant Cr(VI) reduction product, as characterized by micro-X-ray fluorescence and micro-X-ray absorption near-edge structure analysis. We discuss two conflicting scenarios of microbially mediated formation of Cr(III)-Fe(III) precipitates, each of which is both supported and contradicted by different lines of evidence: (1) enzymatic reduction of Cr(VI) to Cr(III) followed by coprecipitation of Cr(III) and Fe(III) and (2) both regimes generated at least small amounts of Fe(II), which abiotically reduced Cr(VI) to form a Cr-Fe precipitate. Evidence of zones with different levels of Cr(VI) reduction suggest that local heterogeneity may have confounded interpretation of processes based on bulk measurements. This study indicates that the bulk redox status and biogeochemical regime, as categorized by the dominant electron-accepting process, do not necessarily control the final product of Cr(VI) reduction.

Effect of dissolved CO2 on a shallow groundwater system: a controlled release field experiment.

Capturing carbon dioxide (CO(2)) emissions from industrial sources and injecting the emissions deep underground in geologic formations is one method being considered to control CO(2) concentrations in the atmosphere. Sequestering CO(2) underground has its own set of environmental risks, including the potential migration of CO(2) out of the storage reservoir and resulting acidification and release of trace constituents in shallow groundwater. A field study involving the controlled release of groundwater containing dissolved CO(2) was initiated to investigate potential groundwater impacts. Dissolution of CO(2) in the groundwater resulted in a sustained and easily detected decrease of ~3 pH units. Several trace constituents, including As and Pb, remained below their respective detections limits and/or at background levels. Other constituents (Ba, Ca, Cr, Sr, Mg, Mn, and Fe) displayed a pulse response, consisting of an initial increase in concentration followed by either a return to background levels or slightly greater than background. This suggests a fast-release mechanism (desorption, exchange, and/or fast dissolution of small finite amounts of metals) concomitant in some cases with a slower release potentially involving different solid phases or mechanisms. Inorganic constituents regulated by the U.S. Environmental Protection Agency remained below their respective maximum contaminant levels throughout the experiment.