Flux Measurements Research Articles

Estimation of Urban Greenhouse Gas Fluxes from Mole Fraction Measurements Using Monin–Obukhov Similarity Theory

Abstract The purpose of this study is to determine whether urban greenhouse gas (GHG) fluxes can be quantified from tower-based mole fraction measurements using Monin–Obukhov similarity theory (MOST). Tower-based GHG mole fraction networks are used in many cities to quantify whole-city GHG emissions. Local-scale, micrometeorological flux estimates would complement whole-city estimates from atmospheric inversions. CO2 mole fraction and eddy-covariance flux data at an urban site in Indianapolis, Indiana, from October 2020 through January 2022 are analyzed. Using MOST flux–variance and flux–gradient relationships, CO2 fluxes were calculated using these mole fraction data and compared to the eddy-covariance fluxes. MOST-based fluxes were calculated using varying measurement heights and methods of estimating stability. The MOST flux–variance relationship method showed good temporal correlation with eddy-covariance fluxes at this site but overestimated flux magnitudes. Fluxes calculated using flux–gradient relationships showed lower temporal correlation with eddy-covariance fluxes but closer magnitudes to eddy-covariance fluxes. Measurement heights closer to ground level produce more precise flux estimates for both MOST-based methods. For flux–gradient methods, flux estimates are more accurate and precise when low-altitude measurements are combined with a large vertical separation between measurement heights. When stability estimates based on eddy-covariance flux measurements are replaced with stability estimates based on the weather station or net radiation data, the MOST-based fluxes still capture the temporal patterns measured via eddy covariance. Based on these results, MOST can be used to estimate the temporal patterns in local GHG fluxes at mole fraction tower sites, complementing the small number of eddy-covariance flux measurements available in urban settings.

Electrocaloric effect in chemically modified barium titanate ferroelectric ceramics

Electrocaloric (EC) refrigeration offers superior energy-conversion efficiency, miniaturization, and environmental benefits compared to compression refrigeration. However, its practical application is limited by the challenge of aligning the adiabatic temperature change (ΔT) with the operational temperature range. In this study, we have tailored the EC characteristics of BaTiO3 (BT)-based Ba(Ti0.9Sn0.1)O3 (BTS) ferroelectric ceramics using Bi(Mg0.5Ti0.5)O3 (BMT). We provide a comprehensive analysis of the microstructure, electrical properties, and EC behavior of the (1–x)BTS-xBMT system. Our results indicate that the incorporation of BMT establishes a broad platform in the dielectric spectrum while maintaining high polarization. This improvement potentially increases the electrocaloric effect (ECE) and expands the operating temperature range. Direct heat flux measurements reveal that the x = 0.02 composition achieves a maximum ΔT (ΔTmax) of 0.41 K with a temperature span (Tspan) of 68 °C under an intermediate electric field of 50 kV/cm. Moreover, the x = 0.01 sample exhibits a room-temperature ΔT of 0.33 K and relatively good temperature stability within 30–130 °C. These findings indicate that chemical modification methods have the potential to optimize the cooling capacity of refrigerants.

Comprehensive Analysis of Magnetic Flux Density and RF-EMF Exposure in Electric Buses: A Case Study from Samsun, Turkey.

This study investigates magnetic flux density (B) and radiofrequency electromagnetic field (RF-EMF) measurements on electric buses operating in Samsun, Turkey, focusing on two bus routes (called E1 and E4) during the morning and evening hours. Measurements were taken under diverse operational conditions, including acceleration, cruising, and braking, at locations of peak passenger density. Along the E1 route, the magnetic field intensity varied significantly based on the bus position, road slope, and passenger load, with notable increases during braking. In contrast, the E4 route showed a lower magnetic field intensity and RF-EMF values due to its straighter trajectory and reduced operational stops. The highest RF-EMF measurement recorded was 6.01 V/m, which is below the maximum levels established by the ICNIRP guidelines. In 11 out of the 12 different band-selective RF-EMF measurements, the highest contribution came from the downlink band of the base stations, while in only one measurement, the highest contribution originated from the uplink bands of the base stations. All data were subject to the Anderson-Darling test, confirming the generalized extreme value distribution as the best fit for both B and RF-EMF measurements. Additionally, the study assessed B levels inside and outside the bus during charging, revealing heightened readings near the pantograph. These findings significantly contribute to our understanding of electromagnetic field exposure in electric bus environments, highlighting potential health implications and informing the development of targeted mitigation strategies.

Optimization of geometric configurations for enhanced thermal performance in microchannel finned plates

This study investigates the thermal performance of microchannel finned plates by comparing various geometric configurations and operating conditions. Twelve models (a to l) were evaluated by varying parameters such as fin height, fin spacing, and channel width, as well as operating factors like inlet velocity and heat flux. The results reveal that increasing the fin height and decreasing the fin spacing enhance heat transfer by expanding the surface area, though at the cost of increased pressure drops and higher pumping power requirements. Variations in channel width impact flow characteristics, with wider channels reducing temperature but potentially lowering convective efficiency. Higher inlet velocities improve convective cooling but increase pressure drops, while elevated heat fluxes lead to steeper temperature gradients. Model k was found to provide an optimal balance between heat transfer efficiency and pressure drop, while Model l’s novel fin geometry showed unique temperature distributions warranting further exploration. Temperature contours and volume renders demonstrated air temperature increases across the models, ranging from 4.78 K to 17.36 K, and surface heat flux measurements confirmed the significant influence of fin configuration on thermal performance. The findings offer valuable insights for optimizing the design of microchannel finned plates for enhanced heat transfer and effective thermal management in various applications.

Surface energy fluxes in a drip-irrigated agroecosystem: Unique advection effect of oasis

Surface energy fluxes, mainly encompassing the net radiation (Rn), latent heat flux (LE), sensible heat flux (Hs), and soil heat flux (Gs), play an important role in the land-atmosphere interactions. However, almost all sites face the problem of energy imbalance, and advection fluxes associated with large inhomogeneous surfaces have been ignored, especially in arid oasis areas. In this study, a three-year continuous measurement of energy fluxes with an eddy covariance system was conducted in a drip-irrigated oasis agroecosystem in Northwest China. Reanalysis data including air temperature (Ta), soil moisture (θ), and leaf area index (LAI) in our cropland and surrounding deserts were also collected. The results showed that multi-year mean turbulent fluxes (LE+Hs) accounted for 75 ± 8 % (mean ± standard deviation) of available energy (Rn–Gs). To be precise, LE took up 72 ± 10 % of available energy, and 7.8 ± 2.8 % of it was induced by higher sensible heat advection, proving a pronounced advection effect in this study. When advection was present, most likely during the heading stage, the threshold value for the Priestley–Taylor parameter α, an indicator to reflect the advection effect, fell in the range of 0.88–1.34. Additionally, after a significant irrigation event, α showed a good linear relationship with differences in air temperature (ΔTa), soil moisture (Δθ), and leaf area index (ΔLAI) between our cropland and surrounding deserts. It's worth mentioning that Δθ was the most significant factor, showing a negative correlation with the advection effect. This study has deepened our understanding of the energy balance in oasis agriculture, emphasizing that the advection effect should not be overlooked.

Development of an integrated online deposition and corrosion monitoring system in a full-scale solid waste CFB boiler

Solid waste incineration is a clean and sustainable approach for solid waste management. However, ash deposition and corrosion remain a critical issue due to fuel’s inherent enrichment of alkali chlorine. This study develops an integrated online deposition and corrosion monitoring system to enhance the operational safety and efficiency of solid waste incineration boilers. This system combines linear polarization resistance (LPR) for corrosion rate estimation with heat flux measurements for ash deposition analysis. It can offer a novel approach for real-time monitoring of heat exchangers’ safety during solid waste combustion. It was deployed in a full-scale circulating fluidized bed (CFB) boiler that purely combust solid wastes. Key findings demonstrate the system’s capability to deliver continuous, real-time data, crucial for the dynamic control of combustion processes and the maintenance of heat transfer surfaces. Its robust diagnostic capabilities were evident across various scenarios. Specially, initial corrosion rates sharply increase with deposition rates due to the enrichment of alkali chlorine on inner deposit layer, in which chlorine serves as a catalyst, facilitating the rapid penetration and aggravation of corrosion by other agents. As deposit further buildup, the corrosion rate steadily decreases along with surface temperature, highlighting a dynamic interaction. Moreover, measured corrosion rates can quickly response to temperature variations. Such multi-process online monitoring system provide more possibilities to investigate the inherent interaction between deposition and corrosion. Therefore, this work offers insights that could significantly influence operational strategies, maintenance protocols, and the overall reliability of waste-to-energy technologies.

Development of Readily Available & Robust High Heat Flux Gardon Gauges

Concentrated solar power (CSP) technologies deliver concentrated solar energy as a heat source to industrial processes, power generation cycles, and chemical cycles. CSP systems require accurate and reliable high flux measurements, and next generation CSP systems will require flux measurement up to 1000 W/cm2. Existing flux measurement devices do not comprehensively meet the flux rating, cycle life, cost, and lead-time needs of stakeholders, necessitating the development of an improved flux sensor. In this study, Sandia National Laboratories (SNL) partnered with Hukseflux Thermal Sensors to develop a low-cost, short lead-time, and robust flux sensor rated to 250 W/cm2. Three prototype circular foil gauge designs were assessed for performance at the National Solar Thermal Test Facility (NSTTF) at SNL. Each gauge design measured flux up to 250 W/cm2 with <5% measurement error. Following baseline error quantification, gauges were exposed to flux above 500 W/cm2 to assess gauge failure mechanisms. Gauges physically survived >500 W/cm2 flux exposure, but measurement error was found to increase after foil coatings reached 400 °C. The results of this study suggest that coating optical properties change at excessive temperatures and that foil coating temperature, rather than heat flux level, dictates the acceptable gauge measurement range.

Advanced Feedforward Controller for a Molten Salt Receiver in Star Design Based on Real-Time Flux Measurement

This paper presents a control system with a panel-wise applied feedforward temperature controller for a molten salt receiver in star design utilizing temperature measurements in the connecting pipes between the panels and a real-time flux density measurement as inputs. It is tested in realistic cloud passage scenarios and the results are compared to the results from an earlier developed control system for the same receiver, in which the temperature controller is based on a simple feedback controller (PID) supported by a simple feedforward controller. The results show that the performance of the new feedforward controller is outstanding and could be applied to common cylindrical receiver designs as well. Nevertheless, the highly increased actuator movement of the control valves is to be further investigated.

Pulsar Population Synthesis with Magnetorotational Evolution: Constraining the Decay of the Magnetic Field

We present a population synthesis model for normal radio pulsars in the Galaxy incorporating the latest developments in the field and the magnetorotational evolution processes. Our model considers spin-down with a force-free magnetosphere and the decay of the magnetic field strength and its inclination angle. The simulated pulsar population is fit to a large observation sample that covers the majority of radio surveys using the Markov Chain Monte Carlo technique. We compare the distributions of four major observables—spin period (P), spin-down rate ( Ṗ ), dispersion measure, and radio flux density—using accurate high-dimensional Kolmogorov–Smirnov statistics. We test two B-field decay scenarios, an exponential model motivated by ohmic dissipation and a power-law model motivated by the Hall effect. The former clearly provides a better fit, and it can successfully reproduce the observed pulsar distributions with a decay timescale of 8.3−3.0+3.9 Myr. The result suggests that significant B-field decay in aged pulsars and ohmic dissipation could be the dominant process.

The Universe SPHEREx Will See: Empirically Based Galaxy Simulations and Redshift Predictions

We simulate galaxy properties and redshift estimation for SPHEREx, the next NASA Medium Class Explorer. To make robust models of the galaxy population and test the spectrophotometric redshift performance for SPHEREx, we develop a set of synthetic spectral energy distributions based on detailed fits to COSMOS2020 photometry spanning 0.1–8 μm. Given that SPHEREx obtains low-resolution spectra, emission lines will be important for some fraction of galaxies. Here, we expand on previous work, using better photometry and photometric redshifts from COSMOS2020 and tight empirical relations to predict robust emission-line strengths and ratios. A second galaxy catalog derived from the GAMA survey is generated to ensure the bright (m AB < 18 in the i band) sample is representative over larger areas. Using template fitting to estimate photometric continuum redshifts, we forecast the recovery of 19 million galaxies over 30,000 deg2 with redshifts better than σ z < 0.003(1 + z), 445 million with σ z < 0.1(1 + z), and 810 million with σ z < 0.2(1 + z). We also find through idealized tests that emission-line information from spectrally dithered flux measurements can yield redshifts with accuracy beyond that implied by the naive SPHEREx channel resolution, motivating the development of a hybrid continuum–line redshift estimation approach.

Characterization of carbon dioxide emissions from late stage windrow composting

As organic waste is converted to usable amendments via composting, there are large CO2 emissions associated with the decomposition of organic matter via microorganisms. While the active composting phase produces the largest emissions over a short duration, compost can often be stored during and after the maturation phase for much longer periods of time, increasing cumulative emissions. As such, the objectives of this study were to examine the spatial and temporal variability associated with in situ emissions sampling while identifying the environmental and chemical controls on emissions in windrow composting facilities during and after the maturation phase. A total of 665 flux measurements were taken from four windrows representing different ages and compositions between June and November 2020. Factorial analysis of covariance (ANOVA) was used to determine the variability between sampling locations, while multiple linear regression was used to identify those parameters which had the most influence on CO2 flux. Emissions showed significant variability over time that were attributed to ambient temperatures. During the summer, each windrow reached peak emissions between 5.0 and 32.3 g CO2 m-2 hr-1. As temperatures cooled, the windrows saw a 62%–86% decline in emissions, generally falling below 2 g CO2 m-2 hr-1. Significant differences occurred between the top-most sampling location and all others on the windrow, emitting between 33%–100% more CO2. The environmental controls of surface temperature, moisture content, and internal temperature showed the highest influence on emissions (R2 = 0.62). Chemical properties including organic nitrogen, carbon, pH, magnesium, and nitrate also showed significant influence (R2 = 0.43). This research has shown that environmental factors including temperature and moisture show the strongest influence over emission rates in mature compost. A significant negative effect of organic nitrogen on CO2 flux was found, indicating that increased presence of organic nitrogen would aid in the retention of carbon after the maturation phase, acting to lower total emissions.

Temporal Variations in Methane Emissions from a Restored Mangrove Ecosystem in Southern China

The role of coastal mangrove wetlands in sequestering atmospheric carbon dioxide has been increasingly investigated in recent years. While studies have shown that mangroves are weak sources of methane (CH4) emissions, measurements of CH4 fluxes from these ecosystems remain scarce. In this study, we examined the temporal variation and biophysical drivers of ecosystem-scale CH4 fluxes in China’s northernmost mangrove ecosystem based on eddy covariance measurements obtained over a 3-year period. In this mangrove, the annual CH4 emissions ranged from 6.15 to 9.07 g C m−2 year−1. The daily CH4 flux reached a peak of over 0.07 g C m−2 day−1 during the summer, while the winter CH4 flux was negligible. Latent heat, soil temperature, photosynthetically active radiation, and tide water level were the primary factors controlling CH4 emissions. This study not only elucidates the mechanisms influencing CH4 emissions from mangroves, strengthening the understanding of these processes but also provides a valuable benchmark dataset to validate the model-derived carbon budget estimates for these ecosystems.

Effects of Fuel Bed Slope and Wind Direction on Flame Spread Characteristics Over a Bed of Pinus Silvestris Needles

ABSTRACT Important factors influencing the flame characteristics over a bed of dry pine needles from Siberian Boreal forests (Pinus silvestris) in lab-scale is provided. Factors such as slope of the fuel bed, wind velocity, and direction of spread (concurrent flow or opposed flow), which play significant roles in determining the flame spread rate, have been investigated. Systematic lab-scale experiments have been carried out in a wind tunnel with varying air velocities and on an inclined fuel bed in still air. Numerical simulations have been carried out using a three-dimensional physics-based flame spread model available in Fire Dynamics Simulator, and the results are validated against a wide range of measured flame characteristics. The validated numerical model is then used to simulate concurrent flow and opposed flow flame spread over a sloping fuel bed in the presence of wind. Gas phase temperature on and above the bed surface and total and radiative heat flux measurements for concurrent flow and opposed flow flame spread under the influence of wind as well as for upslope flame spread in still air are presented. A stationary flame propagation mode is observed for a flat fuel surface. Predicted temperature, flow, and species concentration fields have been reported for selected cases to understand the flow and flame dynamics in gas phase and within the fuel beds. These data serve as a validation of the numerical model used for simulating the experimental cases. Further, it can be used to validate popular fire models such as Fire Foam and Fire-Star 3D.

Spatial variability of CH4 uptake in aerated soils of Yellow River Delta

The widespread occurrence of aerated plain soils underscores their significant role in the global soil methane (CH4) sink budget. However, plain soils are poorly characterized in terms of spatial variability of CH4 uptake and the relevant control. We investigated the intra- and inter-site spatial variability of CH4 uptake through flux measurements in intact soil cores from five non-wetland sites within the Yellow River Delta, each representing a distinct land use/cover type. Methane uptake rates were highest in undisturbed forest cores. The rates were very low, often falling below the detection limit, in cores from the other four sites. The significant correlation between CH4 uptake and bulk density across sites suggests the integrative role of bulk density for the effects of different disturbances (including salt stress and succession) on CH4 uptake. Methane uptake was heterogeneous at the within-site scale as indicated by large coefficients of variations (CVs). Soil texture variation manipulated the within-site pattern of CH4 uptake in the low-salinity sites. Salt affected the spatial variation of CH4 uptake only at high level of salinity. Neither Potter's nor Ridgwell's models effectively captured the within-site variation of CH4 uptake due to a texture-associated bias in the models. Establishing a quantitative relationship between CH4 uptake and clay content at the field scale in alluvial plain soils will facilitate the refinement of model parameters linked to texture and rectify biases in CH4 estimation. These results provide an insight for the biogeochemical control of CH4 uptake in alluvial plain soils and have important application for improving CH4 models.

The influence of bioturbator activity on sediment bacterial structure and function is moderated by environment

Bioturbation in coastal sediments plays a crucial role in biogeochemical cycling. However, a key knowledge gap is the extent to which bioturbation influences bacterial community diversity and ecosystem processes, such as nitrogen cycling. This study paired bacterial diversity, bioturbation activity and in situ flux measurements of oxygen and nitrogen from bioturbated sediments at six estuaries along the East coast of Australia. Bacterial community diversity, composition and predicted functional profiles were similar across burrow and surface sediments but were significantly influenced by bioturbator activity (measured as number of burrows) at sites with higher fine grain content. Sediment oxygen demand increased with bioturbator activity but changes in nitrogen cycling (as measured by fluxes and predicted bacterial functional gene analysis) were more spatially variable and were unrelated to bioturbator activity and bacterial community shifts. This study highlights how bioturbator activity influences bacterial community structure and functioning and what implications this has for biogeochemical cycles in estuarine sediments.

Mito-kaede photoactivation and chase experiment for mitophagy: optimizing flux measurement via fluid exchange system.

Modulating autophagy and mitophagy, vital cellular quality control systems, offer therapeutic potential for critical illnesses. However, limited drug screening options hinder progress. We present a novel assay using the photoswitchable fluorescent reporter, mito-Kaede, to quantify mitophagy flux. Mito-Kaede's superior UV-induced photoconversion and brightness post-conversion make it ideal for prolonged mitochondrial dynamics tracking. Its specificity in responding to mitophagy, confirmed by parkin-knockout cells, adds value. When coupled with a custom fluid exchange system, enabling efficient medium changes, precise mitophagy observations become feasible. This mitophagy assay, alongside our methodological insights, can decipher mitophagy's role in pathology and supports drug screening efforts.

Low nitrous oxide fluxes from mineral affected peatland soils in Iceland

Nitrous oxide (N2O) is a potent greenhouse gas, and soil is one of its most substantial sources. Emissions are influenced by both biotic and abiotic processes, as well as the physiochemical properties of the medium. Organic soils fully drained in boreal climates are a potentially significant source of nitrous oxide. Lower, drainage levels and nutrient availability have been shown to reduce N2O emissions. Predicting N2O emissions from agriculturally managed peatlands is challenging. In situ measurements are crucial for addressing these difficulties. This study presents three years of in situ field measurements of nitrous oxide fluxes and soil hydrological status in peatland soils under different agricultural management regimes: cultivated drained (CD), uncultivated drained (UCD), and wet (W) in Western Iceland. Fluxes were quantified by analysing a time-concentration series obtained from permanent chambers and subsequently evaluated using gas chromatography in the laboratory. The minimum potential N2O consumption rate (mean ± 95 % Cl) was estimated as 0.76 ± 0.38, 0.56 ± 0.18 and 0.56 ± 0.14 mg N2O-N m⁻² day⁻¹ for CD, UCD, and W sites, respectively. Yearly accumulated emissions were estimated as 2.24 ± 1.47, 1.26 ± 0.64 and 0.26 ± 0.44 kg N2O-N ha−1 yr−1 for CD, UCD, and W sites, respectively. The relatively low emission rates observed at drained sites can be explained by the stable water content, and by limited availability of soil phosphorus, which hampers N2O production and potentially leads to more active N2O consumption. Both the stable water content and low phosphorus availability is explained by mineral inputs to the peatlands, acting through decreased soil unsaturated hydraulic conductivity and increased phosphorus retention. The effects of mineral content offer potential mitigation option to decrease N2O emissions. This study highlights the complexity of N2O emissions from peatland soils and emphasizes the significance of site-specific factors in managing greenhouse gas emissions. Further studies investigating the underlying processes behind N2O fluxes in the peatland and andic soils are recommended.

Nitrate and silicate fluxes at the sediment–water interface of the deep North Pacific Ocean illuminated by 226Ra/230Th disequilibria

By taking advantage of recent analytical advances, we herein develop the 226Ra/230Th isotope systematics as a novel tool for quantifying nitrate and dissolved silicate fluxes across the sediment–water interface of the deep-ocean floor. Sediment cores were retrieved from the seabed between 4927 m and 5951 m in the North Pacific Ocean. Downcore profiles of 230Th and both dissolved and total 226Ra were measured using a high-sensitivity inductively coupled plasma mass spectrometer. At all study sites, a marked deficit of total 226Ra with respect to 230Th was observed between 0 and 20 cm, indicating active migration of soluble 226Ra from the sediment into the overlying seawater. By constructing the mass balance of 226Ra in the sediment column, the flux of dissolved 226Ra across the sediment–water interface was estimated to range from 461 to 1320 dpm m−2 yr−1. When coupled to a diffusive transport model as developed by early investigators, these flux values of 226Ra enabled us to calculate the flux of any dissolved constituent of interest by measuring their bottom water concentrations and pore water “saturation” concentrations. Based on the 226Ra/230Th disequilibrium approach, the derived fluxes vary between 4.1 and 10.5 mmol m−2 yr−1 for nitrate and between 11 and 49 mmol m−2 yr−1 for dissolved silicate. A compilation of nitrate and silicate fluxes from the seabed in the deep Pacific Ocean shows that these values are consistent with historical flux measurements based on the conventional core incubation method in the same study region. In addition, both nitrate and silicate fluxes exhibit a clear depth-dependent trend. Overall, our results suggest that sedimentary diagenetic alterations at the North Pacific Ocean floor below ∼ 5000 m are efficient so that only < 2 % of the particulate organic carbon and < 12 % of the biogenic opal raining to the seafloor are ultimately preserved in the sediment.

Real-time portable muography with Hankuk Atmospheric-muon Wide Landscaping : HAWL

Cosmic ray muons prove valuable across various fields, from particle physics experiments to non-invasive tomography, thanks to their high flux and exceptional penetrating capability. Utilizing a scintillator detector, one can effectively study the topography of mountains situated above tunnels and underground spaces. The Hankuk Atmospheric-muon Wide Landscaping (HAWL) project successfully charts the mountainous region of eastern Korea by measuring cosmic ray muons with a detector in motion. The real-time muon flux measurement shows a tunnel length accuracy of 6.0%, with a detectable overburden range spanning from 8 to 400 meter-water-equivalent depth. This is the first real-time portable muon tomography.

Water limitation regulates positive feedback of increased ecosystem respiration.

Terrestrial ecosystem respiration increases exponentially with temperature, constituting a positive feedback loop accelerating global warming. However, the response of ecosystem respiration to temperature strongly depends on water availability, yet where and when the water effects are important, is presently poorly constrained, introducing uncertainties in climate-carbon cycle feedback projections. Here, we disentangle the effects of temperature and precipitation (a proxy for water availability) on ecosystem respiration by analysing eddy covariance CO2 flux measurements across 212 globally distributed sites. We reveal a threshold precipitation function, determined by the balance between precipitation and ecosystem water demand, which separates temperature-limited and water-limited respiration. Respiration is temperature limited for precipitation above that threshold function, whereas in drier areas water limitation reduces the temperature sensitivity of respiration and its positive feedback to global warming. If the trend of expansion of water-limited areas with warming climate over the last decades continues, the positive feedback of ecosystem respiration is likely to be weakened and counteracted by the increasing water limitation.