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Increases in burned area and large fire occurrence are widely documented over the western United States over the past half century. Here, we focus on the elevational distribution of forest fires in mountainous ecoregions of the western United States and show the largest increase rates in burned area above 2,500 m during 1984 to 2017. Furthermore, we show that high-elevation fires advanced upslope with a median cumulative change of 252 m (-107 to 656 m; 95% CI) in 34 y across studied ecoregions. We also document a strong interannual relationship between high-elevation fires and warm season vapor pressure deficit (VPD). The upslope advance of fires is consistent with observed warming reflected by a median upslope drift of VPD isolines of 295 m (59 to 704 m; 95% CI) during 1984 to 2017. These findings allow us to estimate that recent climate trends reduced the high-elevation flammability barrier and enabled fires in an additional 11% of western forests. Limited influences of fire management practices and longer fire-return intervals in these montane mesic systems suggest these changes are largely a byproduct of climate warming. Further weakening in the high-elevation flammability barrier with continued warming has the potential to transform montane fire regimes with numerous implications for ecosystems and watersheds., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

Terrestrial evapotranspiration (ET a ) reflects the complex interactions of climate, vegetation, soil and terrain and is a critical component in water and energy cycles. However, the manner in which climate change and vegetation greening influence ET a remains poorly understood, especially in alpine regions. Drawing on the Global Land Evaporation Amsterdam Model (GLEAM) ET a data, the interannual variability of ET a and its ties to precipitation (P), potential evaporation (ET p ) and vegetation (NDVI) were analysed. The Budyko framework was implemented over the period of 1982 to 2015 to quantify the response of ET a to climate change's direct (P and ET p ) and indirect (NDVI) impacts. The ET a , P, ET p and NDVI all showed significant increasing trends from 1981 to 2015 with rates of 1.52 mm yr -1 , 3.18 mm yr -1 , 0.89 mm yr -1 and 4.0 × 10 -4  yr -1 , respectively. At the regional level, the positive contribution of increases in P and NDVI offset the negative contribution of ET p to the change in ET a (∆ET a ). The positive ∆ET a between 1982 and 2001 was strongly linked with the concomitant increase in NDVI. Increases in vegetation contributing to a positive ∆ET a differed among landscape types: for shrub, meadow and steppe they occurred during both periods, for alpine vegetation between 1982 and 2001, and for desert between 2002 and 2015. Climate change directly contributed to a rise in ET a, with P as the dominant factor affecting forested lands during both periods, and alpine vegetation between 2002 and 2015. Moreover, ET p was a dominant factor for the desert between 1982 and 2001, where the variation of P was not significant. The contributions of factors having an impact on ∆ET a are modulated by both the sensitivity of impact factors acting on ET a as well as the magnitudes of factor changes. The greening of vegetation can influence ET a by increasing vegetation transpiration and rainfall interception in forest, brush and meadow landscapes. These findings can help in developing a better understanding of the interaction of ecosystems and hydrology in alpine regions., Competing Interests: Declaration of competing interest The authors declare no competing financial interests., (Crown Copyright © 2020. Published by Elsevier B.V. All rights reserved.)

Derived from realistic global warming scenarios, long‐term projections of spatial patterns in precipitation and temperature in hydrology and climatology can serve to evaluate climate risk, explore sources of renewable energies and allow local‐scale data to inform decisions regarding agricultural, ecosystem, social, recreational and economic activities. Under the CORDEX‐EA project, the precipitation and temperature projections (2020–2045) for the economically and socially important region of Northwestern China were derived from high‐resolution regional climate model (RCM) simulations for RCP 4.5 and 8.5 scenarios, and compared against a historical period or baseline of 1980–2005. Drawing data from four key RCMs [Weather Research and Forecasting (WRF); Regional Spectral Model (RSM); Regional Climate Model version 4.0 (RegCM4); Mesoscale Model version 5 (MM5)], reliable local‐scale projections were generated by applying suitable bias correction that accords with the multivariate bias correction (MBC) approach. To validate this approach and then evaluate climate change impacts, the adjusted precipitation and temperature estimated from bias‐corrected models for the historical period were compared to the observed data. The results showed that the simulated spatiotemporal distribution of multiyear average precipitation and temperature appear to fit relatively well with the observations, however, the wet‐cold and dry‐warm climate‐related biases were still evident for the high altitude regions. Bias‐corrected future projections of RCMs indicated spatially averaged annual precipitation is expected to rise by about 23.6 and 35.3 mm under the RCP 4.5 and 8.5 scenarios, respectively, while spatially averaged annual temperature is expected to rise by about 1.95 and 1.10°C. Precipitation is projected to increase in all seasons, albeit, more so in the cold season (i.e., boreal winter and spring) than the warm season (i.e., boreal summer and autumn). The annual increase is expected to be about 56.7% under the RCP 4.5 scenario, compared to 67.6% under the RCP 8.5 scenario. The changes of mean temperature in winter are expected to be significant, −37.8% under both RCP scenarios. For spring, the mean temperature will rise by 23.1% (28.9%) under the RCP 4.5 (8.5) scenario. A MBC approach was found to be effective in yielding reliable projected changes in precipitation and temperature variables. The proposed approach has important implications for climatological studies and evaluation of climate change impacts on localized regions, not only in China, but also in other similar areas of the world, where decisions for managing climate risk must be implemented by policy makers, government, industry and stakeholders. [ABSTRACT FROM AUTHOR]

The Rocky Mountains in North America are comprised of headwater snow catchments that provide sustained seasonal flow downstream. Changes in streamflow over the last half century in these basins may be associated with changing climate with increased temperature and variable precipitation, shifting seasonal hydrology. We investigated potential changes in future hydrology in a Rocky Mountain headwater catchment by simulating water budgets of the Athabasca River located in Jasper National Park, Canada. Potential hydrologic changes were predicted using a calibrated version of the Soil and Water Assessment Tool (SWAT). Future discharge and other parts of the catchment water budget were projected based on the global circulation model (GCM) derived from the Special Report on Emission Scenarios (SRES) for the latter part of the century (2081-2099). A projected decrease in future precipitation resulted in reduced mean annual streamflow, by up to 86%, compared to the baseline period for the catchment. Projected summer streamflow decreased from 58 to 39%. Streamflow increased from 13 to 26% during the spring, dampening the dominance of summer peak-flow hydrology. Colder winters for the future scenarios increase the overall proportion of precipitation as winter snowfall. However, dramatically lower precipitation estimated for this basin will drive water limits for the future. [ABSTRACT FROM AUTHOR]

Through its effects on water and energy cycles, elevation plays an important role in modulating the spatial distribution of climatic changes in mountainous regions. A key hydrological indicator, reference evapotranspiration (ET0) reflects the maximum amount of water transferred to the atmosphere from the land surface. The current scarcity of information regarding elevation's impact on variation in ET0 under climate change limits our understanding of the extent to which elevation modulates interactions between ET0 and climate change and of the attendant processes involved. Drawing upon long-term (1960–2017) meteorological observations from 84 stations in Northwest China (NWC), we examined (i) spatial and temporal variations in ET0; (ii) the sensitivity and contribution of air temperature (T), sunshine duration (SD), relative humidity (RH), and wind speed (WS) to ET0; (iii) the existence of a relationship between elevation and ET0 trends; and (iv) the major factor in controlling this relationship by using attribution analysis. Overall, annual ET0 in NWC showed a declining trend between 1960 and 2017, though at a change point in 1993, the trend shifted from a decline to a rise. A significant correlation between temporal change in ET0 and elevation confirmed the existence of a relationship between elevation and ET0 variation. The effect of elevation on changes in ET0 depended mainly on the elevation-based tradeoff between the contributions of T and WS: WS was the primary factor contributing to the decrease in ET0 below 2000 m, and T was the dominant factor contributing to the increase of ET0 above 2000 m. The rate of reduction in WS declined as elevation increased, thereby diminishing its contribution to variation in ET0. The present study's results can serve to guide agricultural irrigation in different elevation zones under NWC's evolving climatic conditions. [ABSTRACT FROM AUTHOR]

The development of methods for improving the assessment of resilience of socio-ecological systems has become increasingly important as the severity and variability of global climate patterns continue to compound and intensify. The present study was conducted with the aim of using a dynamically-coupled bio-physical and socioeconomic model to test the efficacy of stakeholder-defined policy measures in conferring resilience to two agroecological variables, farm income and water-table depth, experiencing both socio-ecological shocks and climatic stress in the Rechna Doab basin of northeastern Pakistan. The objective was accomplished by using a dynamically-coupled physical and group-built system dynamics modelling framework for scenario testing and output generation, including three NASA Earth Exchange Global Daily Downscaled Climate Projections (NEX-DGGP), two relevant socio-ecological shock scenarios (market inflation and canal water supply variability), and three stakeholder-defined policy measures. Resilience was assessed using the following system functionality metrics: 1) The degree of return for each variable after a perturbation, i.e., the extent to which the observed variable returns to baseline functioning; 2) The return time of the variable to baseline functioning; 3) The rate of variable return to baseline; 4) Overall perturbation of the system post-disturbance; and 5) The corrective impact of the shock on system functionality. Differences in the resilience metrics were subsequently compared based on the behavioral change(s) of the study variables in response to the application of the three selected policy scenarios. The results presented here indicate that rainwater harvesting is the most effective stakeholder-defined policy measure for improving or maintaining resilience of the tested study variables in the Rechna Doab agricultural basin; this holds true for every climate and shock scenario with the exception of water-table depth in the upper and mid-watershed regions under canal supply shock conditions, for which canal lining is the most effective policy measure. The irrigation improvement and canal lining policies were more effective in improving the resilience metrics of water-table depth than those of the farm income variable, as the water-table depth variable is not bound so tightly by economic constraints; although there were regional differences in policy efficacy, these trends hold true, on average, for the entire watershed. The results presented herein align well with previous studies conducted in the region which note similar patterns in spatial resilience of agroecosystem variables (i.e., upper to lower watershed) in the Recha Doab basin as well as regional differences in the efficacy of the stakeholder-defined policy measures. This unique approach for assessing socio-environmental policies can help improve decision-making processes with respect to agro-infrastructure and climate change mitigation strategies in vulnerable agroecosystems. [Display omitted] • Improving the resilience of socio-ecological systems has become crucial with the intensification of global climate patterns • Quantify resilience of climate-stressed socio-agroecological variables via application of stakeholder-defined policies • Rainwater harvesting is the most effective stakeholder-defined policy measure for achieving variable-level resilience • Methods can be utilized in single-system analyses as well as comparative studies across spatial and temporal boundaries • The methods used herein can help improve socio-ecological policy development and climate change mitigation strategy [ABSTRACT FROM AUTHOR]

• Grassland biomass allocation is sensitive to natural conditions and grazing practices. • Differences in grassland biomass allocation existed among continents. • Grazing practice affected aboveground part and the ratio of it to below-ground part. • Climate has a stronger role in regulating grassland biomass allocation than altitude. • Warming may exert a more detrimental effect on grazed grasslands than non-grazed ones. Biomass allocation in grasslands is key to understanding plant response to environmental changes and grazing management. Yet, global studies on how this split between above-ground and below-ground biomass varies across continents and grazing practices are notably scarce. We employ a comprehensive field-oriented grassland database to examine differences in total net primary productivity (TNPP), above-ground net primary productivity (ANPP), below-ground net primary productivity (BNPP), and the BNPP to ANPP ratio across continents and between grazed and non-grazed grasslands. Oceania showed the greatest ANPP (916 g·m−2·a−1), while Asia had the lowest (192 g·m−2·a−1). BNPP values were similar among Oceania, Africa, and Asia (∼600 g·m−2·a−1), significantly exceeding those in Europe (289 g·m−2·a−1) and North America (408 g·m−2·a−1). South America, Africa, and Oceania had the highest TNPP (around 1418 g·m−2·a−1 to 1466 g·m−2·a−1), while Europe had the lowest TNPP (344 g·m−2·a−1). The BNPP to ANPP ratio was highest in South America (4.17) and Asia (3.46). Global differences in TNPP and BNPP between grazed and non-grazed grasslands were minimal, but ANPP (169 g·m−2·a−1 and 198 g·m−2·a−1, respectively) and the BNPP to ANPP ratio (4.87 and 2.40, respectively) differed significantly. Across continents and grazing practices, climate had a greater role in regulating grassland biomass allocation than altitude, and warming may have a more detrimental impact on grazed grasslands than non-grazed grasslands. Distinct biomass allocation trends under various continental and grazing conditions exert effects on the economic and ecological functions of grasslands. Our study underscores the need for balanced grassland utilization strategies at a global scale. This involves optimizing grazing in high ANPP grasslands and protecting those with significant BNPP, thereby contributing to a sustainable and ecologically sound use of grasslands in the future. [ABSTRACT FROM AUTHOR]

• WUE showed a significant increasing trend with a break point in 2002 in NWC. • A robust coupled attribution analysis with elasticity method was employed. • GPP contributed 88%, while ET a contributed 12%, to variation in WUE. • Precipitation was the primary factor driving the 66% increase in WUE. • A structure diagram was developed to present the driving mechanism of WUE variation. Besides contributing to a broader understanding of ecosystem-hydrology interactions, ecosystem water use efficiency (WUE) is a key factor operating in an ecosystem's balance between carbon assimilation and water transpiration. However, the effects of climate, elevated atmospheric CO 2 concentration and land surface changes on variation in regional WUE are sparsely documented. To elucidate the driving mechanisms of spatiotemporal variation in WUE in northwest China (NWC) between 1982 and 2015, the contribution of a changing environment — represented by the WUE's sensitivity to Gross Primary Productivity (GPP) and evapotranspiration (ET a) — was examined under different landscape types. Annual WUE series for NWC showed a significant increasing trend and a break point in 2002 with an increasing rate (7.40 mg C kg−1 H 2 O yr−1) prior to the break, and a slightly increasing rate (1.1 mg C kg−1 H 2 O yr−1) thereafter in NWC. A robust (Bias = 2.6%) attribution analysis revealed that GPP was the dominant factor contributing 88% variation in WUE, while ET a contributed 12%. Precipitation was the principal indirect driving factor that affected WUE variation in NWC, while the secondary factor was different among different landscapes. Regionally, precipitation drove 66% of the increase in WUE, wind speed contributed −15% of variation in WUE, and temperature contributed 12% of increase in WUE, while the remaining four factors contributed <3%. Compared to energy-related factors, water-related factors were the more sensitive drivers to WUE variation. The results can guide agricultural irrigation, integrated water resources management and ecological restoration under the changes in climate coming to Northwest China. [ABSTRACT FROM AUTHOR]

• A significant rise in ET a occurred over 75% of the vegetated area in NWC. • Results of ET a variation attribution by different methods were compared. • Partial correlation coefficient-based elasticity method was robust in capturing ET a variation. • Precipitation was the dominant variable driving the variation in ET a in NWC. Actual evapotranspiration ( ET a) has significantly increased under recent climate and vegetation changes. Discriminating the primary driver of ET a variation would improve our knowledge of the interaction between regional hydrological and ecological systems. In this study, a reliable ET a dataset was obtained by validating three ET a products from different sources with in-situ observations. We modified the elasticity-coefficient method using partial correlation, and compared it with the traditional regression-based method in estimating ET a variation in northwest China (NWC). On the basis of the magnitude and the sensitivity of ET a variation, results revealed that annual ET a significantly increased (2.32 mm/yr, p ≤ 0.05) in NWC over 1982–2015 with over 75% of the vegetated area showing an increasing trend. Robust in capturing spatial variation patterns in ET a , the partial correlation coefficient-based elasticity method was able to explain 83% of the annual variation in ET a. Precipitation, temperature, and NDVI were the most important factors controlling the increase in annual ET a from 1982 to 2015. Regionally, precipitation was the dominant factor and contributed 73% to the variation of annual ET a in the vegetated NWC. However, the contribution rates of precipitation varied across the different land cover types and ranged from 31% in irrigated cropland to 81% in the steppe. Potential uncertainty in the attribution of causative factors could arise in selecting the separation method and potential driving factors. Establishing a reliable relationship between ET a and potential driving factors should be confirmed with observations. The present study's results can guide sustainable water resources management and ecological restoration in water limited regions. [ABSTRACT FROM AUTHOR]