Land surface phenology–the seasonal rhythm of leaf emergence and senescence–is shifting in response to climatic changes. These shifts modify how the land and atmosphere exchange energy, water, and carbon, feeding back onto the climate system. This article synthesizes current understanding of phenology-climate interactions and provides a perspective on how to address key uncertainties across biogeophysical and biogeochemical pathways, including changes in surface albedo, turbulent heat fluxes, carbon cycling, and cloud formation. To bridge synthesis and perspective, we complement the review with new fully coupled Earth system model simulations in which satellite-derived leaf area index (LAI) changes serve as an observationally constrained proxy for recent phenological trends, corresponding to approximately 2.1 days per decade earlier spring onset and 1.8 days per decade later autumn senescence. Under these idealized pre-industrial conditions, a 10-day growing-season extension triggers a global surface cooling of -0.10 +/- 0.03 °C, strongest in northern high latitudes. These results identify a potentially important feedback mechanism; as outputs of an idealized single-model setup, however, these simulations only serve as hypothesis-generating tools, not definitive predictions. This first-order quantification motivates a research pathway forward focused on: (1) coordinated integration of multi-stream observations and experimental networks; (2) model-data fusion via causal and hybrid approaches; and (3) a hierarchy of mechanistic models, from single-column frameworks to next-generation Earth system models capable of resolving phenology-climate feedbacks across scales. Emerging priorities include phenological saturation and acclimation, improved representation of autumn phenology and legacy effects, and characterization of non-linear compensation mechanisms. Phenology thus emerges not only as a climate responder but as an active regulator of the Earth system.

Actual evapotranspiration (ETa) is a vital terrestrial ecosystem process that links water, energy, and carbon cycles. ETa can be limited by either energy or water availability. The transition between water-and energy-limited regimes is related to soil moisture and is often characterized as a threshold, denoted as critical soil moisture threshold (Ocrit). However, the determination of Ocrit is subject to uncertainties due to the different methods used to evaluate the relationship between ETa and soil moisture (SM), such as SM depths, definitions of ETa and curve fitting functions. Typically, surface SM is used to identify Ocrit as it is easily accessible and assumed to represent root zone SM status. Weighable lysimeter technology provides a unique opportunity to assess the role of root zone SM on the transition between water and energy limited ETa. It is widely regarded as the gold standard for measuring in-situ ET, and at the same time allows for in-situ SM measurements at different depths. In this study, we estimated Ocrit using in situ SM measurements at 10 cm depth and root zone SM by vertically integrating in situ SM (0-60 cm) observations. In addition, we applied three different definitions of relative evapotranspiration (evaporative fraction, the ratio of ETa to grass reference evapotranspiration and the ratio of actual ETa to calculated potential evapotranspiration) as well as two different fitting curves to investigate the sensitivities of Ocrit. We found robust Ocrit estimates across different definitions and fitting curve methods, but the estimates were significantly higher for root zone than for surface Ocrit. Our results also highlight the high correlation (0.83) between root zone and surface Ocrit. However, the relation between both values is not unique since it depends on the actual moisture profile and plant root system and, herewith, on the soil type and previous weather conditions. We further observed that both surface and root zone Ocrit decreased with increasing sand fraction. Under changing climatic conditions but with identical soil and ecosystem types, both surface and root zone Ocrit decreased with increasing aridity. Additionally, we found that using the midpoint between field capacity and wilting point provides a reliable range of root zone Ocrit for a given soil texture.

Clouds and aerosols can increase canopy photosynthesis relative to clear-sky values through changes in total and diffuse solar radiation: the diffuse fertilization effect (DFE). DFE varies across observational sites due to (a) inconsistent definitions and quantifications of DFE, (b) unexplored relationships between DFE and cloudiness type, and (c) insufficient knowledge of the effect of site characteristics. We showed that: DFE definitions vary, DFE quantifications do not connect to existing definitions or do not isolate the causal factor, and a systematic protocol to quantify DFE is lacking. A new theoretical framework served to clarify the relation between DFE definitions, and showed how DFE varies with cloudiness types and site characteristics. We proposed guidelines for a systematic DFE quantification across studies, and which aim to isolate the causal factor of DFE.Applying our framework to observations of canopy photosynthesis, solar radiation and cloudiness types we quantified DFE at daily and sub-daily time scales. We showed for the first time how DFE varies with cloudiness type, due to the varying trade-off between diffuse radiation and total solar radiation. Using an observation-driven canopy photosynthesis model, we showed that the DFE varies with site characteristics and time of day. The DFE responded strongly to leaf area index, canopy nitrogen distribution, leaf orientation and leaf transmittance, with leaf area index and leaf orientation driving DFE occurrences at our site. Our study emphasizes the importance of quantifying the DFE systematically and accurately across observational sites and highlights the need for information on cloudiness climatology and site characteristics.

This study combines the method of climate analogue regions with a bioclimatic approach. Bioclimate analogue regions were determined for the Rhenish lignite mining area in western Germany, which will face a major structural change in the following decades. These analogue regions currently experience a similar number of days with heat stress compared to the projected future (RCP8.5) at the end of the century in the investigation area. The method is based on the Universal Thermal Climate Index (UTCI) parameters temperature, solar radiation, wind and relative humidity in 3-h temporal resolution while taking day- and night-time values into account. The analogues were calculated for an ensemble of 15 GCM-RCM model combinations from EURO-CORDEX data. The results suggest that analogue regions of the Rhenish lignite mining area are most likely to be found in southern Europe. The highest similarities for the whole ensemble can be found around the Gulf du Lion in southern France. However, some other regions, e.g. around the Black Sea, north of the Balkan Mountains or south of Bordeaux are good fits in some individual model results. While some of these regions are in accordance with previous studies on climate analogue regions, some others were unexpected. The study further shows advantages of using full-coverage instead of punctual data for climate analogue determination, as the results in this study exhibit a high level of spatial detail. For areas facing major structural changes, knowledge of possible climate futures and their present examples can be key aspects for regional planning.

Evapotranspiration (ET) is a crucial terrestrial ecosystem process that links water, energy, and carbon cycles. ET can be limited by either energy or water availability. The transition between water- and energy-limited regimes is associated with the soil moisture content, and can be postulated as the soil moisture content reaching a threshold, denoted as critical soil moisture (θcrit). Knowledge of θcrit is important for improving land surface, hydrological and crop models and predicting hydroclimate extremes such as droughts and heatwaves. However, the quantification of θcrit and the factors that impact θcrit are still not well understood. Here we used precise lysimeter observations to quantify θcrit by analyzing the relationship between soil moisture content and evaporative fraction (EF), as well as the relationship between soil moisture content and the actual ET/ potential ET ratio during drydowns. We estimated θcrit not only at the surface layer using in situ soil moisture measurements at 10 cm depth, but also for the root zone using vertically integrated in situ soil moisture (0–50 cm) observations. We estimated θcrit across various soil textures (e.g., sandy loam, silty loam, clay loam), vegetation types (grass, crop), as well as weather conditions from western and eastern Germany (spatial distances: 10 ~ 600 km). Especially, with some lysimeters that were taken from their original environment and translocated to other regions, we can identify the shift of θcrit with the same soil and vegetation but under different weather conditions, which can provide implication on changes of θcrit under global warming. We would expect a dependence of θcrit on soil texture and weather condition. We found for example that at the same site with the same crop rotation on the lysimeters but different soils, the sandy loamy soil experienced a lower θcrit (approximate 0.15 m3/m3) than the silty loamy soil (approximate 0.17 m3/m3), indicating that the higher content of sand would lead to the lower θcrit. In addition, an increase in θcrit was observed when the lysimeter was translocated from a site with a lower potential ET to a site with a higher potential ET.

We analyze the surface energy budget from four climate model ensembles and its future changes in the twenty-first century under the RCP8.5 or shared socioeconomic pathway (SSP) 5-8.5 scenario. High-resolution European domain of the Coordinated Regional Climate Downscaling Experiment (EURO-CORDEX) regional climate models (RCMs) and their driving CMIP5 global climate models (CMIP5-D) are first tested in central Europe against observational datasets. Evaluation reveals the added value of RCMs in terms of spatial variability and smaller biases. CMIP5-D are affected by the positive bias of global irradiance that propagates into other radiation and heat fluxes. There are strong differences in the projected surface energy budget components between RCMs and CMIP5-D. There is an increase in global irradiance for most of the year in CMIP5-D and other GCM ensembles that is translated into a year-round enhancement of the absorbed solar energy and balanced by higher latent heat flux, except in summer, when the sensible heat flux rises strongly. Together with strong warming and reduced precipitation in summer, this leads to warm, sunny, and dry conditions with reduced evapotranspiration and higher drought stress for vegetation. In the RCMs, the reduction in global irradiance dominates, and it is translated into a round-year reduction in the net balance of longwave radiation and stronger latent heat flux. The first months of the growing season show weaker warming associated with higher evapotranspiration and precipitation. In summer, precipitation drops and global irradiance and warming rise, but they fall behind the changes in the GCMs. Compared to GCMs, there are less visible signs of conditions leading to a reduction in evapotranspiration or a shortage of soil water in the RCMs in summer.

In September 2013, 8.6 hectares of a 70-year old Norway spruce (picea abies) monoculture were cleared in the newly founded national park Eifel (Germany) and left to spontaneous regrowth of the expected deciduous forest matching the site’s climate and soil conditions. The site is part of the 38.5 hectare experimental catchment “Wüstebach” (50° 30’N, 6° 19’E, 595 to 630 m a.s.l.), one of the core investigation sites of TERENO (TERrestrial ENvironmental Observatories, https://www.tereno.net). Most of the rest of the catchment is still covered by the original spruce monoculture. Its energy and matter exchange with the atmosphere, most notably of CO2, is monitored by an ICOS associated eddy-covariance station (DE-RuW) since 2010. In 2013 after the partial deforestation, a second flux station was installed near the centre of the clearcut. Due to an overpressure of game (boar and deer) in the area, 2 hectares of the central clearcut area are protected against grazing by a fence. CO2 budget and albedo results from the first four growing periods after the clearcut were presented by Ney et al. in 2019 (https://doi.org/10.1016/j.agrformet.2019.04.009). Here, we will give an update covering the first ten growing periods after deforestation (2014-2023). Most notably, regrowing vegetation on the initially almost bare clearcut turned it from a source back into a sink of atmospheric CO2 eight years after the deforestation. We will give an overview on how flux components (soil) respiration and gross primary productivity, season length and peak fluxes contributed to the difference between the spruce forest and the early and recent stages of the regrowing forest. For the last eight years, we recorded the species, height and partly the diameter of all spontaneous regrowing trees in the deforested area in a 10 m corridor both inside and outside the fence. Regrowth was strongly dominated by rowan (sorbus aucuparia, >1200 trees), a pioneer species propagated through their berries by birds that was present with at least one adult tree already before the deforestation beside further trees in distant surroundings. The next two important species were spruce and birch (betula pendula), whose seeds are propagated by wind. Rowan and birch grew in height approximately twice as fast as spruce. The presence of the protective fence affected all species, especially rowan, which grew more than twice as fast on the inside of the fence.

In considerations about land management and global climate, biophysical effects like those of albedo are known to modify biochemical effects of greenhouse gas release or uptake. In particular, the cooling effect of afforestation via creation of carbon sinks has been shown to be partly offset by the low albedo and snow-masking effect of tree canopies.In this presentation, we give a global overview on the relationship between albedo and CO2 uptake (net ecosystem productivity NEP and net biome productivity NBP). We focus on a recent study (Graf et al. 2023, https://doi.org/10.1038/s43247-023-00958-4) and the questions:(i) Do ecosystems sequestering more CO2 have a lower albedo as a rule?(ii) How close would such a relation be and how much room does it leave for climate-smart land use?(iii) Given the different immediacy of albedo and NBP based radiative forcing, are there different mitigation policies to be preferred at different points in time?To empirically investigate these questions with direct in-situ measurements, we identified 176 FLUXNET stations with sufficient coverage of NEP, incoming and outgoing shortwave radiation and ancillary data. A method to fill gaps in outgoing shortwave radiation and identify snow cover periods was developed and validated against available data and PI-provided snow statistics. We found a hyperbola-like decrease in maximum achievable effective (flux-weighted) long-term albedo as NEP increases, and vice versa. Apart from this joint limit, which also applied to non-forest and snow-free sites, the relation scattered strongly, indicating some room for climate-smart land use considering both albedo and carbon sequestration.A conceptual model based on a paired-site permutation approach showed that maximizing each site’s NEP without considering albedo, leads to albedo-based positive radiative forcing (warming) during the first approximately 20 years, before being offset by an even stronger NBP-based cooling. However, the fact that most sites are currently far below their possible maximum albedo-NEP combination also allows for a balanced scenario in which both parameters are improved simultaneously. It avoids warming on all timescales, but provides less cooling than pure NEP maximization in the long term. We discuss how these timelines would interact with current emission reduction policies, the reasons underlying the relationship and real-world examples of joint NEP and albedo change.

This study quantifies the accuracy of evapotranspiration (ET) estimates from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) geostationary sensor onboard the Meteosat Second Generation (MSG) satellites, along seven key dimensions, i.e., diurnal cycle, daily, intra-annual, inter-annual, ecosystem, climate zone, and products intercomparison. In situ measurements were collected at 54 eddy covariance (EC) sites to evaluate the accuracy of SEVIRI actual ET products (diurnal and daily SEVIRI-ETa) as well as reference ET (daily SEVIRI-ET0) covering the period from 2004 to 2018 across Europe. SEVIRI-ETa is produced by the Tiled ECMWF Surface Scheme of Exchange processes at the Land surface (TESSEL) model, while SEVIRI-ET0 is estimated by a combination of a thermodynamically-based and an atmospheric boundary layer model. This evaluation is further separated according to the land cover heterogeneity of the SEVIRI pixels across all 54 EC sites, using MODIS land cover data. The Root Mean Squared Error (RMSE), along with the Kling-Gupta efficiency (KGE) and their respective decompositions, were employed to quantify the errors. For diurnal SEVIRI-ETa estimates, we found that the KGE (RMSE [mm hour- 1]) varied between -1.6 (0.04) to 0.8 (0.14), with a median value of 0.26 (0.07). Higher accuracy for diurnal SEVIRI-ETa was obtained in the summer and during the mid-day time. For daily SEVIRI-ETa, the KGE (RMSE [mm day- 1]) varied between -0.88 (0.43) to 0.93 (1.79), with a median value of 0.6 (0.77) and for daily SEVIRI-ET0 the KGE (RMSE [mm day- 1]) varied between 0.51 (0.40) to 0.94 (1.50), with a median value of 0.77 (0.57). For daily SEVIRI-ETa, intra-annual accuracy was low from January to March, increased in the mid-year, and then began to decline from November to December. Although accuracy remained relatively stable during the middle of the year, it varied considerably in the winter period. In the inter-annual dimension, the mid-year positive KGE values and distributions changed over time from 2004 to 2018. In spatial dimensions, the highest accuracy was in peat and grassland ecosystems, and the lowest in cropland ecosystem, with similar patterns observed in the boreal snow fully humid warm summer and warm temperate fully humid hot summer climate zones. Regarding SEVIRI-ET0 results, similar to SEVIRI-ETa, intra-annual accuracy was low in the first quarter of the year and the last one but high in the midyear. In the inter-annual dimension, unlike SEVIRI-ETa, almost an identical pattern was observed for the midyear positive KGE values, demonstrating only a slight change in SEVIRI-ET0 accuracy during 2004-2018. However, the highest accuracy was found in crop ecosystem, while the lowest was in forest ecosystem, reflecting similar trends in the warm temperate fully humid hot summer and warm temperate summer dry hot summer climate zones. The observed range of median RMSE changed between 0.4 and 1.5 mm day-1, also suggests a reasonable accuracy for SEVIRI-ET estimates in all spatial domains. Our results showed that the main trends in the accuracies (median KGEs) of SEVIRI-ET (both ETa and ET0) remained similar in separated homogeneous and heterogeneous sites and were comparable to combined sites among the dimensions. Through error decomposition, we discerned that SEVIRI-ET estimates performed particularly well in explaining inter-annual and spatial variabilities. Furthermore, the intercomparison of ET products revealed that SEVIRI satellite-derived ETa exhibited the strongest correlation with in situ ET measurements across all ecosystem types and climate zones, outperforming other products (such as MODIS, PML, GLEAM, and BESS). The ET estimates from other products exhibited lower standard deviation errors and were in closer agreement with the in situ measurements. This study provides the first comprehensive evaluation of the accuracy of SEVIRI diurnal and daily ET products across Europe, which may serve as a stimulus for further optimized selection of these products by potential users for various applications.

This study assesses the accuracy of Spinning Enhanced Visible and Infrared Imager (SEVIRI) geostationary sensor-derived evapotranspiration (ET) estimates from Meteosat Second Generation (MSG) satellites across Europe. Evaluation encompasses seven dimensions: diurnal cycle, daily, intra-annual, inter-annual, ecosystem, and climate zone. Using in situ measurements from 54 eddy covariance (EC) sites spanning 2004 to 2018, SEVIRI actual ET products (diurnal and daily SEVIRI-ET a ) and reference ET (daily SEVIRI-ET 0 ) were examined. Results indicate varying accuracies based on diurnal and daily assessments, with seasonal fluctuations. Notably, SEVIRI-ET a demonstrated better accuracy during summer and midday. Intra-annual accuracy for daily SEVIRI-ET a showed improvement during mid-year. Peat and grassland ecosystems exhibited higher accuracy than cropland ecosystems. SEVIRI-ET 0 mirrored similar patterns with the highest accuracy in crop ecosystems. Overall, the SEVIRI-ET products showed stable accuracy trends across different spatial domains, effectively capturing both inter-annual and spatial variations. This study comprehensively evaluates SEVIRI diurnal and daily ET products in Europe, offering insights for optimized product selection.

Accurate determination of actual evapotranspiration (ETa) is important in various research fields like hydrology, meteorology, ecology and agriculture. In situ ETa can be determined using weighing lysimeters and eddy covariance. However, despite being regarded as the most precise in situ method for measuring ETa, the information content of lysimeter measurements remains poorly understood. Here we examined the spatial correlations between ETa measured at different locations by lysimeter (ET-LYS) and at different locations by eddy covariance (ET-EC). This was done for the period 2015 - 2020 and the analysis was made for different spatial (range: 0 to 500 km) and temporal scales (range: 1 day to 1 year) using 23 lysimeters and 4 eddy covariance towers. We found that: (a) Same lysimeters at the plot scale show very high correlations of ET-LYS; (b) The Pearson correlation of daily standardized anomalies of ET-LYS between sites exhibit moderate to high correlations and were similar to that of ET-EC, indicating that lysimeter is generally as representative as EC regarding ETa, and can provide certain information at the landscape and larger regional scale. During winter, the spatial correlations for ET-LYS were smaller; (c) Wavelet analysis indicated that temporal correlations in ETa were strongest for distances in time around 12 months (yearly cycle) and less than three months. Spatial correlations were smaller under drought conditions (in the year 2018). Furthermore, combination of multiple ET-LYS from different sites improved the predictability of ET-LYS for another site, suggesting that ET-LYS can be predicted well using ET-LYS from different neighboring sites. Overall, lysimeter measurements can provide information at much larger scales compared to their small measurement area.

It is known from arid and semi-arid ecosystems that atmospheric water vapor can directly be adsorbed by the soil matrix. Soil water vapor adsorption was typically neglected and only recently received attention because of improvements in measurement techniques. One technique rarely explored for the measurement of soil water vapor adsorption is eddy covariance (EC). Soil water vapor adsorption may be detectable as downwardly directed (i.e., negative) EC latent heat (λE) flux measurements under dry conditions, but a systematic assessment of the use of negative λE fluxes from EC flux stations to characterize adsorption is missing. We propose a classification method to characterize soil water vapor adsorption, excluding conditions of dew and fog when λE derived from EC is not trustworthy due to stable atmospheric conditions. We compare downwardly directed λE fluxes from EC with measurements from weighing lysimeters for 4 years in a Mediterranean savanna ecosystem and 3 years in a temperate agricultural site. Our aim is to assess if overnight water inputs from soil water vapor adsorption differ between ecosystems and how well they are detectable by EC. At the Mediterranean site, the lysimeters measured soil water vapor adsorption each summer, whereas at the temperate site, soil water vapor adsorption was much rarer and was measured predominantly under an extreme drought event in 2018. During 30 % of nights in the 4-year measurement period at the Mediterranean site, the EC technique detected downwardly directed λE fluxes of which 88.8 % were confirmed to be soil water vapor adsorption by at least one lysimeter. At the temperate site, downwardly directed λE fluxes were only recorded during 15 % of the nights, with only 36.8 % of half hours matching simultaneous lysimeter measurement of soil water vapor adsorption. This relationship slightly improved to 61 % under bare-soil conditions and extreme droughts. This underlines that soil water vapor adsorption is likely a much more relevant process in arid ecosystems compared to temperate ones and that the EC method was able to capture this difference. The comparisons of the amounts of soil water vapor adsorption between the two methods revealed a substantial underestimation of the EC compared to the lysimeters. This underestimation was, however, comparable with the underestimation in evaporation by the eddy covariance and improved in conditions of higher turbulence. Based on a random-forest-based feature selection, we found the mismatch between the methods being dominantly related to the site's inherent variability in soil conditions, namely soil water status, and soil (surface) temperature. We further demonstrate that although the water flux is very small with mean values of 0.04 or 0.06 mm per night for EC or lysimeter, respectively, it can be a substantial fraction of the diel soil water balance under dry conditions. Although the two instruments substantially differ with regard to the measured ratio of adsorption to evaporation over 24 h with 64 % and 25 % for the lysimeter and EC methods, they are in either case substantial. Given the usefulness of EC for detecting soil water vapor adsorption as demonstrated here, there is potential for investigating adsorption in more climate regions thanks to the greater abundance of EC measurements compared to lysimeter observations.

Abstract We analyze the surface energy budget from two ensembles of climate models and its future changes in the 21st century under the RCP8.5 scenario. High-resolution Euro-CORDEX regional climate models (RCMs) and their driving CMIP5 global climate models (GCMs) are first tested in Central Europe against gridded observational datasets. Evaluation reveals the added value of RCMs in terms of spatial variability and smaller biases. GCMs are affected by the positive bias of global irradiance that propagates into other radiation and heat fluxes. There are strong differences in the projected surface energy budget components between GCMs and RCMs. There is an increase in global irradiance for most of the year in GCMs that is translated into a year-round enhancement of the absorbed solar energy and balanced by higher latent heat flux, except in summer, when the sensible heat flux rises strongly. Together with strong warming and reduced precipitation in summer, this leads to warm, sunny, and dry conditions with reduced evapotranspiration and higher drought stress for vegetation. In the RCMs, the reduction in global irradiance dominates, and it is translated into a round-year reduction in the net balance of longwave radiation and stronger latent heat flux. The first months of the growing season show weaker warming associated with higher rainfall and evapotranspiration. In summer, precipitation drops, and global irradiance and warming rise, but they fall behind the changes in the GCMs. There are no visible signs of conditions leading to a reduction in evapotranspiration or a shortage of soil water in the RCMs in summer.

In this study, we describe a methodology to derive climate analogue cities for spatially highly resolved future climate scenarios. For the computation, a reduced and in hindsight bias-adjusted EURO-CORDEX EUR-11 dataset is used based on two climate scenarios (RCP4.5 and RCP8.5). A total of 389 European cities are processed by the algorithm, which uses five statistical climate variables (2-m air temperature average and amplitude, precipitation sum and amplitude, correlation between 2-m air temperature average and precipitation sum). Additionally, extreme weather events (hot days, summer days, tropical nights, extreme precipitation events) are calculated for further comparison and validation. Finding an appropriate analogue permits a more accurate derivation and depiction of necessary climate adaptation efforts and therefore assist decision-making in city planning. As an example of our method, we searched for plausible climate twins for the mid-sized city of Aachen (Germany) at the end of the twenty-first century. Our results show that the French city of Dijon is highly likely to become Aachen’s climate twin by the end of the century for RCP4.5. As for the scenario RCP8.5, no clear European analogue city could be determined, indicating that the city might enter a novel climate. The nearest match suggests the cities of Florence and Prato in Tuscany. However, considering climate indices, the encompassing region of the French–Spanish city triangle Bordeaux–Toulouse–Bilbao is a better fit. The developed algorithm can be applied to any of the cities included in the dataset.

Both carbon dioxide uptake and albedo of the land surface affect global climate. However, climate change mitigation by increasing carbon uptake can cause a warming trade-off by decreasing albedo, with most research focusing on afforestation and its interaction with snow. Here, we present carbon uptake and albedo observations from 176 globally distributed flux stations. We demonstrate a gradual decline in maximum achievable annual albedo as carbon uptake increases, even within subgroups of non-forest and snow-free ecosystems. Based on a paired-site permutation approach, we quantify the likely impact of land use on carbon uptake and albedo. Shifting to the maximum attainable carbon uptake at each site would likely cause moderate net global warming for the first approximately 20 years, followed by a strong cooling effect. A balanced policy co-optimizing carbon uptake and albedo is possible that avoids warming on any timescale, but results in a weaker long-term cooling effect.

Accurate measurements of actual evapotranspiration (ETa) play an important role in understanding land surface processes and agricultural management. Two of the most commonly used and established methods for quantifying ETa are eddy covariance (EC) and weighable lysimeters measurements. Previous studies on hourly or daily basis indicated sometimes large differences between the ETa of the two methods (Δly-EC). It is still unclear which factors influence these differences. Here, we examine and compare half-hourly ETa measurements from EC (ETEC) and weighable lysimeters (ETly) at four different sites. The four sites span a climatic gradient from humid conditions at a pre-alpine (Fendt, DE) and a mid-mountain grassland (Rollesbroich, DE) to semi-arid conditions at two sites with a natural grass and shrub (Els Plans, ES) and a tree-grass ecosystem (Majadas de Tiétar, ES). We used a boosted regression tree method to identify environmental drivers of Δly-EC during day and night at the half-hourly resolution.Our results revealed that substantial differences were found with a mean annual Δly-EC of 117 mm, and Δly-EC displayed obvious spatiotemporal variabilities across the sites. Energy balance non-closure of EC was found to be the most important factor contributing to the large annual Δly-EC, especially at Majadas de Tiétar site. With the distinct climatic gradient, Δly-EC was negatively correlated with mean annual wind speed and vapor pressure deficit when they reached a specific level. Monthly ETEC and ETly agreed well with Δly-EC peaking in summer at the sites in Germany, while Δly-EC peaked earlier due to the different climate in Spain. Differences in grass height caused by field management and EC footprint also affected Δly-EC, especially at daily timescales for the pre-alpine and the mid-mountain grassland ecosystem. The relative impacts of different environmental variables to half-hourly ETEC and ETly were almost the same with soil water content (SWC) being more important for nighttime ETly. Meanwhile, we found that the dominant controlling factors of daytime Δly-EC changed with climatic conditions, but nighttime Δly-EC were mainly regulated by SWC. These findings provide a critical evaluation for the roles of climatic and land surface conditions on turbulent flux dynamics from different measurements, which has important implications for ecosystem water and energy balance.