Atmospheric CO2 Concentration Research Articles

PyEt v1.3.1: a Python package for the estimation of potential evapotranspiration

Abstract. Evapotranspiration (ET) is a crucial flux of the hydrological water balance, commonly estimated using (semi-)empirical formulas. The estimated flux may strongly depend on the formula used, adding uncertainty to the outcomes of environmental studies using ET. Climate change may cause additional uncertainty, as the ET estimated by each formula may respond differently to changes in meteorological input data. To include the effects of model uncertainty and climate change and facilitate the use of these formulas in a consistent, tested, and reproducible workflow, we present PyEt. PyEt is an open-source Python package for the estimation of daily potential evapotranspiration (PET) using available meteorological data. It allows the application of 20 different PET methods on both time series and gridded datasets. The majority of the implemented methods are benchmarked against literature values and tested with continuous integration to ensure the correctness of the implementation. This article provides an overview of PyEt's capabilities, including the estimation of PET with 20 PET methods for station and gridded data, a simple procedure for calibrating the empirical coefficients in the alternative PET methods, and estimation of PET under warming and elevated atmospheric CO2 concentration. Further discussion on the advantages of using PyEt estimates as input for hydrological models, sensitivity and uncertainty analyses, and hindcasting and forecasting studies (especially in data-scarce regions) is provided.

Stomata Are Driving the Direction of CO2-Induced Water-Use Efficiency Gain in Selected Tropical Trees in Fiji.

Understanding plant physiological response to a rising atmospheric CO2 concentration (ca) is key in predicting Earth system plant-climate feedbacks; however, the effects of long-term rising ca on plant gas-exchange characteristics in the tropics are largely unknown. Studying this long-term trend using herbarium records is challenging due to specimen trait variation. We assessed the impact of a ca rise of ~95 ppm (1927-2015) on the intrinsic water-use efficiency (iWUE) and maximum stomatal conductance (gsmax) of five tropical tree species in Fiji using the isotopic composition and stomatal traits of herbarium leaves. Empirical results were compared with simulated values using models that uniquely incorporated the variation in the empirical gsmax responses and species-specific parameterisation. The magnitude of the empirical iWUE and gsmax response was species-specific, ranging from strong to negligible. Stomatal density was more influential than the pore size in determining the gsmax response to ca. While our simulation results indicated that photosynthesis is the main factor contributing to the iWUE gain, stomata were driving the iWUE trend across the tree species. Generally, a stronger increase in the iWUE was accompanied by a stronger decline in stomatal response. This study demonstrates that the incorporation of variation in the gsmax in simulations is necessary for assessing an individual species' iWUE response to changing ca.

The LGM termination in the southeastern Tibetan plateau: View from high-frequency LGM glacier fluctuations in the Boshula mountain range

Climate processes that operated during the end of the Last Glacial Maximum (LGM) are remarkable for its global synchroneity. Atmospheric CO2 concentrations have been widely seen as its cause. However, the stepwise LGM deglaciation of mountain glaciers in both hemispheres complicates this view, and signifies additional factors that likely prompted the onset of LGM termination. Here, we examine LGM climate change in the Hengduan Mountains (HDM), southeastern Tibetan Plateau (TP), based on 10Be surface exposure dating of moraine boulders (n = 51). The timing of four moraine-building events is constrained to 22.8 ± 1.0 ka, 21.2 ± 0.6 ka, 20.4 ± 0.6 ka, and 19.2 ± 0.6 ka. These precisely-dated events provide convincing evidence of millennial-to centennial-scale glacial activities in the TP during the LGM. We show that these high-frequency glacier fluctuations likely reacted to a combination of changes in regional summer temperature related to sea surface temperatures as well as monsoon precipitation. The pronounced glacial retreat is dated at 19.2 ± 0.6 ka, representing the end of the LGM in the HDM. That is, the onset of LGM termination preceded the rapid CO2 rise at ∼18 ka. We suggest that the LGM termination in the southeastern TP was initiated by ice-sheet shrinkage, which induced changes in summer temperature and monsoon precipitation via ocean-atmosphere interactions.

Air-sea CO2 exchange in the western Pacific influenced by monsoon and giant diatom (Ethmodiscus rex) blooms during the last deglaciation

The concentration of atmospheric CO2 increased rapidly during the last deglaciation due to CO2 outgassing from oceans. However, records of deglacial surface seawater pCO2-sw are sparse, hindering our understanding of the process and mechanism of air-sea CO2 exchange and its influence on glacial-interglacial climate change. Here we reconstructed surface seawater pCO2-sw for the last deglacial period using carbon isotope composition (δ13C) of giant diatom (Ethmodiscus rex) frustules from deep-sea sedimentary core collected in the Philippine Sea, western Pacific. Results showed that air-sea CO2 was fluctuating in the western Pacific during the last deglaciation. The gradients of air-sea CO2 are dominated by monsoon and biological productivity. The enhanced East Asian winter Monsoon and shallow thermocline during late Heinrich Stadial 1 maintained equilibrium in the air-sea CO2 exchange balance. During the Bølling period, enhanced East Asian Summer Monsoon has been observed to accelerate the dissolution of eolian-dust and promoted the growth of Ethmodiscus rex, which has been linked to increased primary productivity and, consequently, the uptake of atmospheric CO2 in the western Pacific. During the Allerød period, continued enhancement of EASM allowed the Philippine Sea to act as a weak CO2 source releasing CO2 to the atmosphere. During the Younger Dryas period, as the EASM weaken and the EAWM strengthen, ΔpCO2(sw-atm) decreased. Our findings highlight the tropical ocean’s role in deglacial air-sea CO2 exchange and provide insights into the monsoonal and biological drivers of the processes.

Holocene aeolian environmental dynamics in fixed and semi-fixed deserts over the arid Central Asia revealed by comprehensive sand-dune records

Fixed and semi-fixed deserts are widely distributed in arid Central Asia (ACA). Environmental changes in these deserts can affect the regional radiation balance, climate dynamics, and global climate through a series of feedback effects. This study analyzed luminescence dating from 37 sites, with a particular focus on dendritic dunes (a pattern of vegetated linear dunes) and the ancient dunes beneath them in the Gurbantunggut Desert and yielded the following insights. (1) Primary ridges of dendritic dunes originated approximately 1.0 ka, whereas secondary ridges predominantly formed at 8.5 ± 0.3 ka. The formation of the underlying ancient dunes commenced at least 11.0 ± 0.5 ka and ended between 8.5 ± 0.3 ka and 8.2 ± 0.4 ka. (2) During the Holocene, the Gurbantunggut Desert underwent two distinct environmental phases. From 11.7 ka to 8.1 ± 0.1 ka, the region was characterized by vigorous aeolian activity, marked by the prevalence of mobile dunes. Conversely, the period from 8.1 ± 0.1 ka to 0 ka exhibited substantial environmental amelioration, with limited aeolian activity and the intermittent partial development of vegetated linear dunes. (3) A comprehensive analysis of the principal factors affecting regional aeolian activity revealed that at 11.7 ka–8.1 ± 0.1 ka, moisture (precipitation) and near-surface wind intensity primarily governed aeolian activity in the Gurbantunggut Desert. At 8.1 ± 0.1 ka–0 ka, aeolian activity was mainly influenced by moisture (precipitation), near-surface wind intensity, and external sand supply. (4) Similar patterns of Holocene aeolian activity were observed in the fixed Bayanbulak, Karakum, and Kizil Kum Deserts within the ACA. On a suborbital scale, winter insolation served as the primary driving factor, along with the ice volume and atmospheric CO2 concentration. These factors collectively controlled regional winter temperatures, which determined moisture (precipitation) and near-surface wind intensity by affecting the winter westerlies and Siberian High, ultimately controlling regional aeolian activity.

The Transcription Factor ZmMYB56 Regulates High CO2-Induced Stomatal Closure in Maize Seedlings by Regulating ZmHLT1 Expression.

Stomata are channels through which plants exchange H2O and CO2 with the external environment. The regulation of stomatal movement has significant impacts on plant growth, development and stress adaptation. Here, we present a maize R2R3 -MYB transcription factor, ZmMYB56, which regulates high CO2-induced stomatal closure in maize seedlings. ZmMYB56 is highly expressed in stomatal guard cells and is negatively regulated by CO2 and HCO3 -. Loss of ZmMYB56 function leads to insensitivity to high CO2. As a transcription factor, ZmMYB56 binds to a cis-acting element in the ZmHLT1 promoter and regulates its expression. ZmHLT1 plays a key role in CO2- induced maize stomatal movement, and its absence causes a severe weakening of maize's response to ambient CO2. ZmHLT1 expression is negatively regulated by bicarbonate, which does not occur in Zmmyb56 mutants, highlighting the significance of this regulatory relationship in plant responses to CO2 and HCO3 -. Taken together, these results show that ZmMYB56-regulated ZmHLT1 expression is important for high CO2-induced stomatal closure in maize. Our findings provide insight into genetic pathways that could be manipulated to improve maize growth and stress tolerance, especially under increasing atmospheric CO2 concentrations.

Unveiling spatial variations in atmospheric CO2 sources: a case study of metropolitan area of Naples, Italy

In the lower atmosphere, CO2 emissions impact human health and ecosystems, making data at this level essential for addressing carbon-cycle and public-health questions. The atmospheric concentration of CO2 is crucial in urban areas due to its connection with air quality, pollution, and climate change, becoming a pivotal parameter for environmental management and public safety. In volcanic zones, geogenic CO2 profoundly affects the environment, although hydrocarbon combustion is the primary driver of increased atmospheric CO2 and global warming. Distinguishing geogenic from anthropogenic emissions is challenging, especially through air CO2 concentration measurements alone. This study presents survey results on the stable isotope composition of carbon and oxygen in CO2 and airborne CO2 concentration in Naples’ urban area, including the Campi Flegrei caldera, a widespread hydrothermal/volcanic zone in the metropolitan area. Over the past 50 years, two major volcanic unrests (1969–72 and 1982–84) were monitored using seismic, deformation, and geochemical data. Since 2005, this area has experienced ongoing unrest, involving the pressurization of the underlying hydrothermal system as a causal factor of the current uplift in the Pozzuoli area and the increased CO2 emissions in the atmosphere. To better understand CO2 emission dynamics and to quantify its volcanic origin a mobile laboratory was used. Results show that CO2 levels in Naples’ urban area exceed background atmospheric levels, indicating an anthropogenic origin from fossil fuel combustion. Conversely, in Pozzuoli's urban area, the stable isotope composition reveals a volcanic origin of the airborne CO2. This study emphasizes the importance of monitoring stable isotopes of atmospheric CO2, especially in volcanic areas, contributing valuable insights for environmental and public health management.

Implementing the iCORAL (version 1.0) coral reef CaCO3 production module in the iLOVECLIM climate model

Abstract. Coral reef development is intricately linked to both climate and the concentration of atmospheric CO2, specifically through temperature and carbonate chemistry in the upper ocean. In turn, the calcification of corals modifies the concentration of dissolved inorganic carbon (DIC) and total alkalinity in the ocean, impacting air–sea gas exchange, atmospheric CO2 concentration, and ultimately the climate. This feedback between atmospheric conditions and coral biogeochemistry can only be accounted for with a coupled coral–carbon–climate model. Here we present the implementation of a coral reef calcification module into an Earth system model. Simulated coral reef production of the calcium carbonate mineral aragonite depends on photosynthetically active radiation, nutrient concentrations, salinity, temperature, and the aragonite saturation state. An ensemble of 210 parameter perturbation simulations was performed to identify carbonate production parameter values that optimize the simulated distribution of coral reefs and associated carbonate production. The tuned model simulates the presence of coral reefs and regional-to-global carbonate production values in good agreement with data-based estimates, despite some limitations due to the imperfect simulation of climatic and biogeochemical fields driving the simulation of coral reef development. When used in association with methods accounting for bathymetry changes resulting from different sea levels, the model enables assessment of past and future coral–climate coupling on seasonal to millennial timescales, highlighting how climatic trends and variability may affect reef development and the resulting climate–carbon feedback.

Different model assumptions about plant hydraulics and photosynthetic temperature acclimation yield diverging implications for tropical forest gross primary production under warming

AbstractTropical forest photosynthesis can decline at high temperatures due to (1) biochemical responses to increasing temperature and (2) stomatal responses to increasing vapor pressure deficit (VPD), which is associated with increasing temperature. It is challenging to disentangle the influence of these two mechanisms on photosynthesis in observations, because temperature and VPD are tightly correlated in tropical forests. Nonetheless, quantifying the relative strength of these two mechanisms is essential for understanding how tropical gross primary production (GPP) will respond to climate change, because increasing atmospheric CO2 concentration may partially offset VPD‐driven stomatal responses, but is not expected to mitigate the effects of temperature‐driven biochemical responses. We used two terrestrial biosphere models to quantify how physiological process assumptions (photosynthetic temperature acclimation and plant hydraulic stress) and functional traits (e.g., maximum xylem conductivity) influence the relative strength of modeled temperature versus VPD effects on light‐saturated GPP at an Amazonian forest site, a seasonally dry tropical forest site, and an experimental tropical forest mesocosm. By simulating idealized climate change scenarios, we quantified the divergence in GPP predictions under model configurations with stronger VPD effects compared with stronger direct temperature effects. Assumptions consistent with stronger direct temperature effects resulted in larger GPP declines under warming, while assumptions consistent with stronger VPD effects resulted in more resilient GPP under warming. Our findings underscore the importance of quantifying the role of direct temperature and indirect VPD effects for projecting the resilience of tropical forests in the future, and demonstrate that the relative strength of temperature versus VPD effects in models is highly sensitive to plant functional parameters and structural assumptions about photosynthetic temperature acclimation and plant hydraulics.

Soil priming effect in the organic and mineral layers regulated by nitrogen mining mechanism in a temperate forest

AbstractSoil organic carbon (SOC) in temperate forests is a crucial part of the global C cycle. The C and nitrogen (N) inputs may greatly increase in forest ecosystems affected by atmospheric CO2 concentration, N deposition, and other climate change, which may further affect SOC dynamics in temperate forests. Nevertheless, how C and N inputs interact to influence the soil priming effect (PE) in the organic and mineral layers of temperate forests remains unclear. Here, we used easily available C and N sources, such as 13C‐glucose with 2% SOC contents and ammonium nitrate (input C:N ratio = 10), to examine the effects and mechanisms of exogenous C and N inputs on soil CO2 production and PE in both soil layers of a temperate forest. Our research revealed that exogenous C input caused a positive PE in both soil layers, with the mineral layer showing a larger PE per unit of SOC than the organic layer (OL). Although C input increased C loss from native SOC, soil net C accumulation still increased. The C and N inputs decreased the soil PE in both soil layers, suggesting that N input alleviates substrate N limitation and weakens microbial N mining in both soil layers. Meanwhile, the C and N inputs increased the exogenous C remaining in the organic layer, which was beneficial for soil C sequestration. Compared to the organic layer, the response of the mineral layer to C and N inputs was weaker. This study suggests that C and N interact to affect PE on SOC decomposition and this interaction should be considered in modeling and prediction of soil C cycling.

A short-term impact of enriched CO2 [eCO2] on select growth performance of Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) and its host plant Gossypium barbadense L. (Malvaceae)

Natural interactions between herbivorous insect pests and their host plants are expected to be altered significantly as atmospheric CO2 concentrations (aCO2) continue to rise according to climate change scenario. The possible effect of enriched CO2 (eCO2) environments on these interactions is under attention. To better understand such effects on select insect growth parameters; early (3rd) and penultimate (6th) instar larvae of the Cotton Leaf Worm Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae), reared on the cotton plant leaves Gossypium barbadense L. (Malvacae) grown under either ambient (aCO2 = 350 ppm) or enriched (eCO2 = 700 ppm) atmospheres were investigated.

Effect of CO2 Elevation on Tomato Gas Exchange, Root Morphology and Water Use Efficiency under Two N-Fertigation Levels.

Atmospheric elevated CO2 concentration (e[CO2]) decreases plant nitrogen (N) concentration while increasing water use efficiency (WUE), fertigation increases crop nutrition and WUE in crop; yet the interactive effects of e[CO2] coupled with two N-fertigation levels during deficit irrigation on plant gas exchange, root morphology and WUE remain largely elusive. The objective of this study was to explore the physiological and growth responses of ambient [CO2] (a[CO2], 400 ppm) and e[CO2] (800 ppm) tomato plant exposed to two N-fertigation regimes: (1) full irrigation during N-fertigation (FIN); (2) deficit irrigation during N-fertigation (DIN) under two N fertilizer levels (reduced N (N1, 0.5 g pot-1) and adequate N (N2, 1.0 g pot-1). The results indicated that e[CO2] associated with DIN regime induced the lower N2 plant water use (7.28 L plant-1), maintained leaf water potential (-5.07 MPa) and hydraulic conductivity (0.49 mol m-2 s-1 MPa-1), greater tomato growth in terms of leaf area (7152.75 cm2), specific leaf area (223.61 cm2 g-1), stem and total dry matter (19.54 g and 55.48 g). Specific root length and specific root surface area were increased under N1 fertilization, and root tissue density was promoted in both e[CO2] and DIN environments. Moreover, a smaller and denser leaf stomata (4.96 µm2 and 5.37 mm-2) of N1 plant was obtained at e[CO2] integrated with DIN strategy. Meanwhile, this combination would simultaneously reduce stomatal conductance (0.13 mol m-2 s-1) and transpiration rate (1.91 mmol m-2 s-1), enhance leaf ABA concentration (133.05 ng g-1 FW), contributing to an improvement in WUE from stomatal to whole-plant scale under each N level, especially for applying N1 fertilization (125.95 µmol mol-1, 8.41 µmol mmol-1 and 7.15 g L-1). These findings provide valuable information to optimize water and nitrogen fertilizer management and improve plant water use efficiency, responding to the potential resource-limited and CO2-enriched scenario.

Natural carbonation boost for hardened cement fines by dripping technique

This study proposes a dripping technique to improve the carbonation efficiency of hardened cement fine (HCF), which mimicked the binder part of waste concrete, sized between 0.6 mm and 1.18 mm under atmospheric CO2 concentration. After a 7-day carbonation using the dripping technique, the carbonation degree doubled compared with that at 85 % relative humidity (RH). This improved carbonation efficiency can be attributed to two factors: 1) The pores created during drying accelerate the infiltration of the solution, releasing more calcium ions; 2) The difference in calcium ion concentration between the surface aqueous film and pore solution causes calcium ions to migrate to the surface, accelerating the dissolution of calcium-bearing hydrates in the pore solution and releasing more calcium ions. Adjusting the dripping interval and drying conditions according to the particle size and amount of carbonatable calcium improves the carbonation efficiency. This strategic approach offers a practical means to contribute to the sustainable recycling of waste concrete.

CDK8 of the mediator kinase module connects leaf development to the establishment of correct stomata patterning by regulating the levels of the transcription factor SPEECHLESS (SPCH).

The components of the mediator kinase module are highly conserved across all eukaryotic lineages, and cyclin-dependent kinase 8 (CDK8) is essential for correct cell proliferation and differentiation in diverse eukaryotic systems. We show that CDK8 couples leaf development with the establishment of correct stomata patterning for prevailing CO2 conditions. In Arabidopsis, the basic helix-loop-helix(bHLH) transcription factor SPEECHLESS(SPCH) controls cellular entry into the stomatal cell lineage, and CDK8 interacts with and phosphorylates SPCH, controlling SPCH protein levels and thereby also expression of the SPCH target genes encoding key regulators of cell fate and asymmetric cell divisions. The lack of the CDK8-mediated control of SPCH results in an increased number of meristemoid and guard mother cells, and increased stomata index in the cdk8 mutants. Increasing atmospheric CO2 concentrations trigger a developmental programme controlling cell entry into stomatal lineage by limiting the asymmetric divisions. In cdk8, the number of meristemoids and guard mother cells remains the same under ambient and high CO2 concentrations, as the accumulated levels of SPCH caused by the lack of CDK8 appear to override the negative regulation of increased CO2. Thus, our work provides novel mechanistic understanding of how plants alter critical leaf properties in response to increasing atmospheric CO2.

Getting allometry right at the Oak Ridge free‐air CO2 enrichment experiment: Old problems and new opportunities for global change experiments

Societal Impact StatementFree‐air CO2 enrichment (FACE) experiments provide essential data on forest responses to increasing atmospheric CO2 for evaluations of climate change impacts on humanity. Understanding and reducing the uncertainty in the experimental results is critical to ensure scientific and public confidence in the models and policy initiatives that derive therefrom. One source of uncertainty is the estimation of tree biomass using mathematical relationships between biomass and easily obtained and non‐destructive measurements (allometry). We evaluated the robustness of the allometric relationships established at the beginning of a FACE experiment and discuss the challenges and opportunities for the new generation of FACE experiments.Summary Long‐term field experiments to elucidate forest responses to rising atmospheric CO2 concentration require allometric equations to estimate tree biomass from non‐destructive measurements of tree size. We analyzed whether the allometric equations established at the beginning of a free‐air CO2 enrichment (FACE) experiment in a Liquidambar styraciflua plantation were still valid at the end of the 12 year experiment. Aboveground woody biomass was initially predicted by an equation that included bole diameter, taper, and height, assuming that including taper and height as predictors would accommodate changes in tree structure that might occur over time and in response to elevated CO2. At the conclusion of the FACE experiment, we harvested 23 trees, measured dimensions and dry mass of boles and branches, and extracted and measured the woody root mass of 10 trees. Although 10 of the harvested trees were larger than the trees used to establish the allometric relationship, measured aboveground woody biomass was well predicted by the original allometry. The initial linear equation between bole basal area and woody root biomass underestimated final root biomass by 28%, but root biomass was just 21% of total wood mass, and errors in aboveground and belowground estimates were offsetting. The allometry established at the beginning of the experiment provided valid predictions of tree biomass throughout the experiment. New allometric approaches using terrestrial laser scanning should reduce an important source of uncertainty in decade‐long forest experiments and in assessments of centuries‐long forest biomass accretion used in evaluating carbon offsets and climate mitigation.

CFD modeling investigation of oxy-fuel combustion application in an industrial-scale FCC regenerator

The increase in atmospheric CO2 concentration and its consequential impact on climate change have elicited increased public concern. The refinery units including fluid catalytic cracking (FCC) generate substantial quantities of CO2. To mitigate the emission from the FCC process, oxy-fuel combustion has emerged as a prospective carbon capture and storage technology. This study presents the first trial for the modeling investigation of a 70 kt/a industrial FCC regenerator under the scenario of retrofitting it with oxy-fuel combustion technology. Employing the Eulerian-Eulerian model, a CFD model integrating heat transfer and coke combustion reactions has been established. The detailed hydrodynamics, temperature, and species concentration distribution inside the regenerator are obtained under both air-firing and oxy-firing conditions, which are further compared to exploit the possibility of oxy-fuel combustion retrofitting. As has been found, decreases in gas temperature and carbon conversion rate were observed for 21 % O2/79 % CO2 atmosphere in comparison to the air reference case due to the differences in gas properties between N2 and CO2. This discrepancy resulted in a drop of 17 K in dilute phase temperature and 2 K in dense phase temperature. The bed density also exhibited a large with the oxy-firing conditions, with notable observations revealing a lower bed density below a height of 4.2 m, transitioning to a higher density above said height. Sensitivity analysis was also conducted for three principal operating parameters, including superficial gas velocity, oxygen partial pressure, and catalyst circulation rate. An increase of oxygen partial pressure to 27 % or a decrease of the catalyst circulation rate to 20.7 kg/s proved effective in achieving the same temperature profile and even a slightly better carbon conversion in comparison to air-firing regeneration.

Recent warming and increasing CO2 stimulate growth of dominant trees under no water limitation in South Korea.

Increases in temperatures and atmospheric CO2 concentration influence the growth performance of trees worldwide. The direction and intensity of tree growth and physiological responses to changing climate do, however, vary according to environmental conditions. Here we present complex, long-term, tree-physiological responses to unprecedented temperature increase in East Asia. For this purpose, we studied radial growth and isotopic (δ13C and δ18O) variations using tree-ring data for the past 100yr of dominant Quercus mongolica trees from the cool-temperate forests from Hallasan, South Korea. Overall, we found that tree stem basal area increment, intercellular CO2 concentration and intrinsic water-use efficiency significantly increased over the last century. We observed, however, short-term variability in the trends of these variables among four periods identified by change point analysis. In comparison, δ18O did not show significant changes over time, suggesting no major hydrological changes in this precipitation-rich area. The strength and direction of growth-climate relationships also varied during the past 100yr. Basal area increment (BAI) did not show significant relationships with the climate over the 1924-1949 and 1975-1999 periods. However, over 1950-1974, BAI was negatively affected by both temperature and precipitation, while after 2000, a temperature stimulus was observed. Finally, over the past two decades, the increase in Q. mongolica tree growth accelerated and was associated with high spring-summer temperatures and atmospheric CO2 concentrations and decreasing intrinsic water-use efficiency, δ18O and vapour pressure deficit, suggesting that the photosynthetic rate continued increasing under no water limitations. Our results indicate that the performance of dominant trees of one of the most widely distributed species in East Asia has benefited from recent global changes, mainly over the past two decades. Such findings are essential for projections of forest dynamics and carbon sequestration under climate change.

Global reduction in sensitivity of vegetation water use efficiency to increasing CO2

Ecosystem water use efficiency (WUE), which measures the ratio of carbon gain to water loss, is crucial to the global water, energy, and carbon cycles. It is an important metric in understanding how efficiently an ecosystem uses water to assimilate carbon and is affected by various environmental factors, including CO2 levels. However, the temporal dynamics of CO2-induced changes in WUE (CO2_WUE) remain uncertain. After isolating CO2_WUE from total changes in WUE, we assessed historical changes in global CO2_WUE and future projected trends. We found that the increasing atmospheric CO2 concentration enhanced global WUE by 5.65 ± 1.14 % 100 ppm−1 during 1982–2015. However, CO2-driven gains in WUE exhibit a significant declining trend across most of the Earth’s land surface during 1982–2015 (−1.38 ± 0.19 % 100 ppm−1 yr−1). This widespread CO2_WUE decline is projected to continue throughout the remainder of the 21st century. The decline in CO2_WUE implies that the global land surface is likely to witness a higher water cost to maintain the land carbon sink. This finding suggests a heightened difficulty of mitigating global warming and an elevated risk of global water scarcity. The global CO2_WUE decline is primarily driven by rising vapor pressure deficit (VPD) reducing photosynthesis and increasing total evaporation. Our findings highlight the key role of VPD in the coupling of global water and carbon cycles and reveal a continuous increase in the water cost of the global land carbon sink.

Gradual and abrupt increase in atmospheric CO2 concentrations trigger divergent responses of microbial necromass accumulation in paddy soils

Microbial necromass C (MNC) plays a critical role in promoting soil organic C (SOC) formation and stabilization, particularly in the context of climate change. Most investigations on the impact of elevated CO2 concentrations (eCO2) on MNC have been performed by exposing ecosystems to an abrupt increase in CO2. However, the atmospheric CO2 increase is a gradual process, and knowledge about the response of necromass to gradual increase in CO2 is lacking. We tested the hypothesis that microbial necromass would show different responses to abrupt and gradual CO2 increases. Furthermore, it is still unknown whether eCO2 will trigger similar responses between surface and subsurface soil layers. Here, we determined the MNC concentrations and their proportions in SOC in surface and subsurface soil layers via open-top chambers. CO2 treatments included ambient control, abrupt CO2 increase by 200 ppm above control, and gradual CO2 increase by 40 ppm each year until reaching 200 ppm over five years. Overall, compared with the ambient control, abrupt eCO2 induced a significantly stronger decrease in necromass (20.3 %) when averaged across three soil layers, while gradual eCO2 led to insignificant variations in necromass (6.7 %). The necromass proportion in SOC decreased with depth under both eCO2 treatments with a significant decline occurring in abrupt eCO2 treatment. The observed greater magnitude of eCO2 effects on fungal necromass relative to bacterial necromass in subsurface layers illustrates a distinct microbial community response to climate change. Taken together, abrupt and gradual eCO2 approaches differed in their impact on MNC, and its response to eCO2 may be somewhat overestimated by abrupt eCO2 approach. Furthermore, MNC in subsurface soil witnessed a more sensitivity or vulnerability to eCO2 than that of topsoil. We suggest vigilant attention will need to be paid to the feedback of necromass C in deep soil poised by future climate change.

Modelling Mediterranean ocean biogeochemistry of the Last Glacial Maximum

Abstract. We present results of simulations performed with a physical–biogeochemical ocean model of the Mediterranean Sea for the Last Glacial Maximum (LGM) and analyse the differences in physical and biochemical states between the historical period and the LGM. Long-term simulations with an Earth system model based on ice sheet reconstructions provide the necessary atmospheric forcing data, oceanic boundary conditions at the entrance to the Mediterranean Sea, and river discharge to the entire basin. Our regional model accounts for changes in bathymetry due to ice sheet volume changes, reduction in atmospheric CO2 concentration, and an adjusted aeolian dust and iron deposition. The physical ocean state of the Mediterranean during the LGM shows a reduced baroclinic water exchange at the Strait of Gibraltar, a more sluggish zonal overturning circulation, and the relocation of intermediate and deep-water-formation areas – all in line with estimates from palaeo-sediment records or previous modelling efforts. Most striking features of the biogeochemical realm are a reduction in the net primary production, an accumulation of nutrients below the euphotic zone, and an increase in the organic matter deposition at the seafloor. This seeming contradiction of increased organic matter deposition and decreased net primary production challenges our view of possible changes in surface biological processes during the LGM. We attribute the origin of a reduced net primary production to the interplay of increased stability of the upper water column, changed zonal water transport at intermediate depths, and lower water temperatures, which slow down all biological processes during the LGM. Cold water temperatures also affect the remineralisation rates of organic material, which explains the simulated increase in the organic matter deposition, which is in good agreement with sediment proxy records. In addition, we discuss changes in an artificial tracer which captures the surface ocean temperature signal during organic matter production. A shifted seasonality of the biological production in the LGM leads to a difference in the recording of the climate signal by this artificial tracer of up to 1 K. This could be of relevance for the interpretation of proxy records like, e.g., alkenones. Our study not only provides the first consistent insights into the biogeochemistry of the glacial Mediterranean Sea but will also serve as the starting point for transient simulations of the last deglaciation.