Pathways for balancing CO2 emissions and sinks

The environmental burden of inhalation

Inhalation systems, mostly metered dose inhalers (MDIs) and dry powder inhalers (DPIs), are currently submitted to a critical assessment for their carbon footprint (CF) and environmental impact. They are related to greenhouse gas (GHG) emissions and they produce waste of used devices with withheld drug residues and unused doses. However, with estimated contributions to anthropogenic GHG-emissions of 0.03% for MDIs and 0.0012% for DPIs globally, it may not be expected that mitigating the GHG emissions from inhalers will have a meaningful effect on the current climate change and global warming, notwithstanding that nationally these percentages may be somewhat higher, depending on the ratio of MDIs to DPIs and the total national CF. MDIs are particularly the preferred type of inhalers over DPIs in the USA and UK with ratios of 9: 1 and 7: 3 respectively. In such countries, a partial switch from MDIs to DPIs is to be recommended, providing that such a switch does not jeopardize the therapy. Using renewable energy only for the production and waste management of DPIs will make this type of inhaler almost climate neutral. A greater concern exists about inhaler waste, more particularly about the residual drug and unused doses in discarded devices. Inhalers contribute less than 0.02% to global plastic waste annually and most plastic inhalers end in the domestic waste bin and not as litter polluting the environment with plastic. However, they do contain retained drug and unused doses, whereas even full inhalers are disposed. Because globally most municipal waste (70%) ends up in dumps and landfills, leakage of the drugs into the soil and surface waters is a serious problem. It pollutes drinking water and endangers species and biodiversity. Therefore, a good collection system and an adequate waste management program for used inhalers seems to be the most meaningful measure to take for the environment, as this will stop inhalers and drugs from putting ecosystems at risk.

High-efficiency photocatalytic CO2 reduction enabled by interfacial Ov and isolated Ti3+ of g-C3N4/TiO2 Z-scheme heterojunction

Exploring the real force that drives the separation of Coulomb-bound electron-hole pairs in the interface of heterojunction photocatalysts can establish a clear mechanism for efficient solar energy conversion efficiency. Herein, the formation of oxygen vacancy (Ov) and isolated Ti3+ was precisely regulated at the interface of g-C3N4/TiO2 Z-scheme heterojunction (g-C3N4/Ov-Ti3+-TiO2) by optimizing the opening degree of the calcination system, showing excellent production rate of CO and CH4 from CO2 photoreduction under visible light. This photocatalytic system also exhibited prominent stability. Combining theoretical calculation and characterization, the introduction of Ov and isolated Ti3+ on the interface could construct a charge transfer channel to break the forbidden transition of n → π*, improving the separation process of photoexcited electron-hole pairs. The photoexcited electrons weakened the covalent interaction of CO bonds to promote the activation of adsorbed inert CO2 molecules, significantly reducing the energy barrier of the rate-limiting step during CO2 reduction. This work demonstrates the great application potential of reasonably regulating heterojunction interface for efficient photocatalytic CO2 reduction.

A Nanofiber-Based Gas Diffusion Layer for Improved Performance in Air Cathode Microbial Fuel Cells

This work investigates a new nanostructured gas diffusion layer (nano-GDL) to improve the performance of air cathode single-chamber microbial fuel cells (a-SCMFCs). The new nano-GDLs improve the direct oxygen reduction reaction by exploiting the best qualities of nanofibers from electrospinning in terms of high surface-area-to-volume ratio, high porosity, and laser-based processing to promote adhesion. By electrospinning, nano-GDLs were fabricated directly by collecting two nanofiber mats on the same carbon-based electrode, acting as the substrate. Each layer was designed with a specific function: water-resistant, oxygen-permeable polyvinylidene-difluoride (PVDF) nanofibers served as a barrier to prevent water-based electrolyte leakage, while an inner layer of cellulose nanofibers was added to promote oxygen diffusion towards the catalytic sites. The maximum current density obtained for a-SCMFCs with the new nano-GDLs is 132.2 ± 10.8 mA m-2, and it doubles the current density obtained with standard PTFE-based GDL (58.5 ± 2.4 mA m-2) used as reference material. The energy recovery (EF) factor, i.e., the ratio of the power output to the inner volume of the device, was then used to evaluate the overall performance of a-SCMFCs. a-SCMFCs with nano-GDL provided an EF value of 60.83 mJ m-3, one order of magnitude higher than the value of 3.92 mJ m-3 obtained with standard GDL.

Decoupling of land-use net carbon flux, economic growth, and population change in China

In the process of China's modernization, promoting the sustainable development of resource-based cities is a major strategic issue and it has now also become a worldwide issue. This study uses the coupling model to validate the coupling relationship between China's land-use net carbon flux and economic growth and population change during 2009-2017. The study for the first time draws the conclusion that the coupling degree among the three is getting lower, the correlation is gradually weaker, and the independent relationship is becoming more and more prominent. Utilizing the Tapio decoupling model, we obtained the weak decoupling conclusion that the economic growth rate is higher than the growth rate of the land-use net carbon flux, while negative decoupling of sprawl is where the rate of population growth is less than the rate of net land-use carbon flux growth.

Synthesis of Mesoporous Carbon Adsorbents Using Biowaste Crude Glycerol as a Carbon Source via a Hard Template Method for Efficient CO2 Capture

Biowaste utilization as a carbon source and its transformation into porous carbons have been of great interest to promote environmental remediation owing to biowaste's cost-effectiveness and useful physicochemical properties. In this work, crude glycerol (CG) residue from waste cooking oil transesterification was employed to fabricate mesoporous crude glycerol-based porous carbons (mCGPCs) using mesoporous silica (KIT-6) as a template. The obtained mCGPCs were characterized and compared to commercial activated carbon (AC) and CMK-8, a carbon material prepared using sucrose. The study aimed to evaluate the potential of mCGPC as a CO₂ adsorbent and demonstrated its superior adsorption capacity compared to AC and comparable to CMK-8. The X-ray diffraction (XRD) and Raman results clearly depicted the structure of carbon nature with (002) and (100) planes and defect (D) and graphitic (G) bands, respectively. The specific surface area, pore volume, and pore diameter values confirmed the mesoporosity of mCGPC materials. The transmission electron microscopy (TEM) images also clearly revealed the porous nature with the ordered mesopore structure. The mCGPCs, CMK-8, and AC materials were used as CO₂ adsorbents under optimized conditions. The mCGPC adsorption capacity (1.045 mmol/g) is superior to that of AC (0.689 mmol/g) and still comparable to that of CMK-8 (1.8 mmol/g). The thermodynamic analyses of the adsorption phenomena are also carried out. This work demonstrates the successful synthesis of a mesoporous carbon material using a biowaste (CG) and its application as a CO₂ adsorbent.

Assessment of solar energy potential in China using an ensemble of photovoltaic power models

Development of solar energy is one of the key solutions towards carbon neutrality in China. The output of solar energy is dependent on weather conditions and shows distinct spatiotemporal characteristics. Previous studies have explored the photovoltaic (PV) power potential in China but with single models and low-resolution radiation data. Here, we estimated the PV power potential in China for 2016-2019 using an ensemble of 11 PV models based on hourly solar radiation at the resolution of 5 km retrieved by the Himawari-8 geostationary satellite. On the national scale, the ensemble method revealed an annual average PV power potential of 242.79 kWh m^-2 with the maximum in the west (especially the Tibetan Plateau) and the minimum in the southeast (especially the Sichuan Basin). The multi-model approach shows inter-model spreads of 6 %-7 % distributed uniformly in China, suggesting a robust spatial pattern predicted by these models. The seasonal variation in general shows the largest PV power generation in summer months except for Tibetan Plateau, where the peak value appears in spring because the high cloud coverage dampens the regional solar radiation in summer. On the national scale, the deseasonalized PV power potential shows a high correlation with cloud coverage (R² = 0.71, p < 0.01) but a low correlation with aerosol optical depth (R² = 0.08, p < 0.05). Sensitivity experiments show that national PV power potential increases by 0.55 % per 1 W m^-2 increase of radiation and 0.79 % per 1 m s^-1 increase of wind speed, but decreases by 0.46 % per 1 °C increase of air temperature. These sensitivities provide a solid foundation for the future projection of PV power potential in China under climate change.

Eight Up-Coming Biotech Tools to Combat Climate Crisis

Biotechnology has a high potential to substantially contribute to a low-carbon society. Several green processes are already well established, utilizing the unique capacity of living cells or their instruments. Beyond that, the authors believe that there are new biotechnological procedures in the pipeline which have the momentum to add to this ongoing change in our economy. Eight promising biotechnology tools were selected by the authors as potentially impactful game changers: (i) the Wood-Ljungdahl pathway, (ii) carbonic anhydrase, (iii) cutinase, (iv) methanogens, (v) electro-microbiology, (vi) hydrogenase, (vii) cellulosome and, (viii) nitrogenase. Some of them are fairly new and are explored predominantly in science labs. Others have been around for decades, however, with new scientific groundwork that may rigorously expand their roles. In the current paper, the authors summarize the latest state of research on these eight selected tools and the status of their practical implementation. We bring forward our arguments on why we consider these processes real game changers.

3D Composite PDMS/MWCNTs Aerogel as High-Performing Anodes in Microbial Fuel Cells

Porous 3D composite materials are interesting anode electrodes for single chamber microbial fuel cells (SCMFCs) since they exploit a surface layer that is able to achieve the correct biocompatibility for the proliferation of electroactive bacteria and have an inner charge transfer element that favors electron transfer and improves the electrochemical activity of microorganisms. The crucial step is to fine-tune the continuous porosity inside the anode electrode, thus enhancing the bacterial growth, adhesion, and proliferation, and the substrate's transport and waste products removal, avoiding pore clogging. To this purpose, a novel approach to synthetize a 3D composite aerogel is proposed in the present work. A 3D composite aerogel, based on polydimethylsiloxane (PDMS) and multi-wall carbon nanotubes (MWCNTs) as a conductive filler, was obtained by pouring this mixture over the commercial sugar, used as removable template to induce and tune the hierarchical continuous porosity into final nanostructures. In this scenario, the granularity of the sugar directly affects the porosities distribution inside the 3D composite aerogel, as confirmed by the morphological characterizations implemented. We demonstrated the capability to realize a high-performance bioelectrode, which showed a 3D porous structure characterized by a high surface area typical of aerogel materials, the required biocompatibility for bacterial proliferations, and an improved electron pathway inside it. Indeed, SCMFCs with 3D composite aerogel achieved current densities of (691.7 ± 9.5) mA m^-2, three orders of magnitude higher than commercial carbon paper, (287.8 ± 16.1) mA m^-2.

Environmental management of industrial decarbonization with focus on chemical sectors: A review

A considerable portion of fossil CO₂ emissions comes from the energy sector for production of heat and electricity. The industrial sector has the second order in emission in which the main parts are released from energy-intensive industries, namely metallurgy, building materials, chemicals, and manufacturing. The decarbonization of industrial wastes contemplates the classic decarbonization through optimization of conventional processes as well as utilization of renewable energy and resources. The upgrading of existing processes and integration of the methodologies with a focus on efficiency improvement and reduction of energy consumption and the environment is the main focus of this review. The implementation of renewable energy and feedstocks, green electrification, energy conversion methodologies, carbon capture, and utilization, and storage are also covered. The main objectives of this review are towards chemical industries by introducing the potential technology enhancement at different subsectors. For this purpose, state-of-the-art roadmaps and pathways from the literature findings are presented. Both common and innovative renewable attempts are needed to reach out both short- and long-term deep decarbonization targets. Even though all of the innovative solutions are not economically viable at the industrial scale, they play a crucial role during and after the energy transition interval.

Living Bacteria Directly Embedded into Electrospun Nanofibers: Design of New Anode for Bio-Electrochemical Systems

The aim of this work is the optimization of electrospun polymeric nanofibers as an ideal reservoir of mixed electroactive consortia suitable to be used as anodes in Single Chamber Microbial Fuel Cells (SCMFCs). To reach this goal the microorganisms are directly embedded into properly designed nanofibers during the electrospinning process, obtaining so called nanofiber-based bio-composite (bio-NFs). This research approach allowed for the designing of an advanced nanostructured scaffold, able to block and store the living microorganisms inside the nanofibers and release them only after exposure to water-based solutions and electrolytes. To reach this goal, a water-based polymeric solution, containing 5 wt% of polyethylene oxide (PEO) and 10 wt% of environmental microorganisms, is used as the initial polymeric solution for the electrospinning process. PEO is selected as the water-soluble polymer to ensure the formation of nanofiber mats offering features of biocompatibility for bacteria proliferation, environment-friendliness and, high ionic conductivity. In the present work, bio-NFs, based on living microorganisms directly encapsulated into the PEO nanofiber mats, were analyzed and compared to PEO-NFs made of PEO only. Scanning electron microscopy allowed researchers to confirm the rise of a typical morphology for bio-NFs, evidencing the microorganisms’ distribution inside them, as confirmed by fluorescence optical microscopy. Moreover, the latter technique, combined with optical density measurements, allowed for demonstrating that after electrospinning, the processed microorganisms preserved their proliferation capability, and their metabolic activity after exposure to the water-based electrolyte. To demonstrate that the energy-production functionality of exo-electrogenic microorganisms was preserved after the electrospinning process, the novel designed nanomaterials, were directly deposited onto carbon paper (CP), and were applied as anode electrodes in Single Chamber Microbial Fuel Cells (SCMFCs). It was possible to appreciate that the maximum power density reached by bio-NFs, which resulted in being double of the ones achieved with PEO-NFs and bare CP. SCMFCs with bio-NFs applied as anodic electrodes reached a current density value, close to (250 ± 5.2) mA m⁻², which resulted in being stable over time and was comparable with the one obtained with carbon-based electrode, thus confirming the good performance of the whole device.

Effects of Rainfall Manipulation on Ecosystem Respiration and Soil Respiration in an Alpine Steppe in Northern Tibet Plateau

The response mechanism of ecosystem respiration (Re) and soil respiration (Rs) to different water conditions is of great significance for understanding the carbon cycle under future changes in the precipitation patterns. We used seven precipitation treatments to investigate the effects of precipitation on Re and Rs on a typical alpine steppe in Northern Tibet. Precipitation was captured and relocated to simulate the precipitation rates of −25, −50, −75, 0 (CK), +25, +50, and +75%. The soil moisture was influenced by all the precipitation treatments. There was a positive linear relationship between the soil moisture and Re, Rs in the study area during the experiment (July–October). Soil volumetric water content (VWC), absolute water content (AWC), soil temperature (ST), aboveground biomass (AGB), bulk density, soil total nitrogen (TN), and alkaline hydrolysis nitrogen (AHN) were the predictors of Re and Rs. The multiple linear regression analysis showed that ST and AWC could explain 90.6% of Rs, and ST, AWC, and AHN could explain 89.4% of Re. Ecosystem respiration was more sensitive to the increased precipitation (+29.5%) whereas Rs was more sensitive to the decreased precipitation (−23.8%). An appropriate increase in water (+25 and +50%) could improve the Re and Rs, but a greater increase (+75%) would not have a significant effect; it could have an effect even lower than those of the first two. Our study highlights the importance of increased precipitation and the disadvantage of decreased precipitation on Re and Rs in an arid region. The precipitation changes will lead to significant changes in the soil properties and AGB, and affect Re and Rs, to change the climate of the alpine steppe in Northern Tibet in the future. These findings contribute to our understanding of the regional patterns of environmental C exchange and soil C flux under the climate change scenarios and highlight the importance of water availability to the regulating ecosystem processes in semi-arid steppe ecosystems. In view of these findings, we urge future researchers to focus on manipulating the precipitation over longer time scales, seasonality, and incorporating more environmental factors to improve our ability to predict and model Re and Rs and feedback from climate change.

Recent Advances in Polymer-Inorganic Mixed Matrix Membranes for CO2 Separation

Since the second industrial revolution, the use of fossil fuels has been powering the advance of human society. However, the surge in carbon dioxide (CO2) emissions has raised unsettling concerns about global warming and its consequences. Membrane separation technologies have emerged as one of the major carbon reduction approaches because they are less energy-intensive and more environmentally friendly compared to other separation techniques. Compared to pure polymeric membranes, mixed matrix membranes (MMMs) that encompass both a polymeric matrix and molecular sieving fillers have received tremendous attention, as they have the potential to combine the advantages of both polymers and molecular sieves, while cancelling out each other's drawbacks. In this review, we will discuss recent advances in the development of MMMs for CO2 separation. We will discuss general mechanisms of CO2 separation in an MMM, and then compare the performances of MMMs that are based on zeolite, MOF, metal oxide nanoparticles and nanocarbons, with an emphasis on the materials' preparation methods and their chemistries. As the field is advancing fast, we will particularly focus on examples from the last 5 years, in order to provide the most up-to-date overview in this area.

Bio-conversion of CO2 into biofuels and other value-added chemicals via metabolic engineering

Carbon dioxide (CO₂) occurs naturally in the atmosphere as a trace gas, which is produced naturally as well as by anthropogenic activities. CO₂ is a readily available source of carbon that in principle can be used as a raw material for the synthesis of valuable products. The autotrophic organisms are naturally equipped to convert CO₂ into biomass by obtaining energy from sunlight or inorganic electron donors. This autotrophic CO₂ fixation has been exploited in biotechnology, and microbial cell factories have been metabolically engineered to convert CO₂ into biofuels and other value-added bio-based chemicals. A variety of metabolic engineering efforts for CO₂ fixation ranging from basic copy, paste, and fine-tuning approaches to engineering and testing of novel synthetic CO₂ fixing pathways have been demonstrated. In this paper, we review the current advances and innovations in metabolic engineering for bio-conversion of CO₂ into bio biofuels and other value-added bio-based chemicals.

The Microalga Chlorella vulgaris as a Natural Bioenergetic System for Effective CO2 Mitigation—New Perspectives against Global Warming

In the present contribution, the differentiation in the molecular structure and function of the photosynthetic apparatus of the unicellular green alga Chlorella vulgaris was studied at several light intensities (0–400 μmol m−2 s−1) and various CO2 concentrations (0.04–60% CO2), in completely autotrophic conditions. Asymmetries that occur by different light intensities and CO2 concentrations induce metabolic and functional changes. Using chlorophyll fluorescence induction techniques (OJIP test), we showed that Chlorella vulgaris tolerates extremely high CO2 levels and converts them photosynthetically into valuable products, including O2 and biomass rich in carbohydrates and lipids. Interestingly, the microalga Chlorella vulgaris under extremely high CO2 concentrations induces a new metabolic state intensifying its photosynthetic activity. This leads to a new functional symmetry. The results highlight a potent CO2 bio-fixation mechanism of Chlorella vulgaris that captures up to 288 L CO2 L PCV−1 day−1 under optimal conditions, therefore, this microalga can be used for direct biological CO2-reducing strategies and other green biotechnological applications. All of the above suggest that Chlorella vulgaris is one of the most prominent competitors for a closed algae-powered bioreactor that is able to consume huge amounts of CO2. Thus, it is a sustainable and natural bioenergetic system with perspectives in dealing with major environmental issues such as global warming. In addition, Chlorella vulgaris cultures could also be used as bioregeneration systems in extraterrestrial missions for continuous atmospheric recycling of the human settlements, paving the way for astrobiological applications.

Co-creating local socioeconomic pathways for achieving the sustainable development goals

The Sustainable Development Goals (SDGs) recognise the importance of action across all scales to achieve a sustainable future. To contribute to overall national- and global-scale SDG achievement, local communities need to focus on a locally-relevant subset of goals and understand potential future pathways for key drivers which influence local sustainability. We developed a participatory method to co-create local socioeconomic pathways by downscaling the SDGs and driving forces of the shared socioeconomic pathways (SSPs) via a local case study in southern Australia through contextual analysis and community engagement. We linked the SSPs and SDGs by identifying driving forces and describing how they affect the achievement of local SDGs. We co-created six local socioeconomic pathways with the local community which track towards futures with different levels of fulfilment of the SDGs and each encompasses a narrative storyline incorporating locally-specific ideas from the community. We tested and validated the local pathways with the community. This method extends the SSPs in two dimensions-into the broader field of sustainability via the SDGs, and by recontextualizing them at the local scale. The local socioeconomic pathways can contribute to achieving local sustainability goals from the bottom up in alignment with global initiatives.

Headwater stream ecosystem: an important source of greenhouse gases to the atmosphere

Although an increasing number of reports have revealed that rivers are important sources of greenhouse gases (GHGs), the magnitude and underlying mechanism of riverine GHG emissions are still poorly understood. The global extent of the headwater stream ecosystem may represent one of the important GHG emitters. A global database of GHG measurements from 595 rivers, indicated that the concentrations of riverine GHGs continually decrease as the stream order increases. Further analysis suggested that high GHG emissions from headwater streams (Strahler stream orders of 1 to 3) could be related to the low levels of dissolved oxygen, massive terrestrially derived carbon/nitrogen inputs and large gas exchange velocity. Through a combination of the predicted river surface areas and gas transfer velocities, we estimated that globally, the rivers emit approximately 6.6 (5.5-7.8) Pg CO₂, 29.5 (19.6-37.3) Tg CH₄, and 0.6 (0.2-0.9) Tg N₂O per year, and totally emit 7.6 (6.1-9.1) CO₂ equivalent into atmosphere per year. The headwater streams contribute 72.3%, 75.5%, and 77.2% of the global riverine CO₂, CH₄, and N₂O emissions, respectively. This study presents a systematic estimation of GHG emissions from river ecosystems worldwide and highlights the dominant role played by headwater streams in GHG evasions from global rivers.

Population growth and climate change: Addressing the overlooked threat multiplier

Demographic trends will play a role in determining the magnitude of climate disruption and the ability of societies to adapt to it. Yet policy makers largely ignore the potential of fertility changes and population growth when designing policies to limit climate disruption and lessen its impacts. Here we argue that rights-based policy interventions could decrease fertility rates to levels consistent with low population pathways. We review country and global level studies that explore the effects of low population pathways on climate change mitigation and adaptation. We then provide rights-based policy recommendations, such as the expansion of voluntary family planning programs that incorporate elements from successful past programs, and highlight current research gaps. In concert with policies that end fossil fuel use and incentivize sustainable consumption, humane policies that slow population growth should be part of a multifaceted climate response. These policies require attention from scientists, policy analysts and politicians.

Leaching of dissolved organic carbon from mineral soils plays a significant role in the terrestrial carbon balance

The leaching of dissolved organic carbon (DOC) from soils to the river network is an overlooked component of the terrestrial soil C budget. Measurements of DOC concentrations in soil, runoff and drainage are scarce and their spatial distribution highly skewed towards industrialized countries. The contribution of terrestrial DOC leaching to the global-scale C balance of terrestrial ecosystems thus remains poorly constrained. Here, using a process based, integrative, modelling approach to upscale from existing observations, we estimate a global terrestrial DOC leaching flux of 0.28 ± 0.07 Gt C year-1 which is conservative, as it only includes the contribution of mineral soils. Our results suggest that globally about 15% of the terrestrial Net Ecosystem Productivity (NEP, calculated as the difference between Net Primary Production and soil respiration) is exported to aquatic systems as leached DOC. In the tropical rainforest, the leached fraction of terrestrial NEP even reaches 22%. Furthermore, we simulated spatial-temporal trends in DOC leaching from soil to the river networks from 1860 to 2010. We estimated a global increase in terrestrial DOC inputs to river network of 35 Tg C year-1 (14%) from 1860 to 2010. Despite their low global contribution to the DOC leaching flux, boreal regions have the highest relative increase (28%) while tropics have the lowest relative increase (9%) over the historical period (1860s compared to 2000s). The results from our observationally constrained model approach demonstrate that DOC leaching is a significant flux in the terrestrial C budget at regional and global scales.

Toward Standardized Photocatalytic Oxygen Evolution Rates Using RuO2@TiO2 as a Benchmark

Quantitative comparison of photocatalytic performances across different photocatalysis setups is technically challenging. Here, we combine the concepts of relative and optimal photonic efficiencies to normalize activities with an internal benchmark material, RuO₂ photodeposited on a P25-TiO₂ photocatalyst, which was optimized for reproducibility of the oxygen evolution reaction (OER). Additionally, a general set of good practices was identified to ensure reliable quantification of photocatalytic OER, including photoreactor design, photocatalyst dispersion, and control of parasitic reactions caused by the sacrificial electron acceptor. Moreover, a method combining optical modeling and measurements was proposed to quantify the benchmark absorbed and scattered light (7.6% and 81.2%, respectively, of λ = 300-500 nm incident photons), rather than just incident light (≈AM 1.5G), to estimate its internal quantum efficiency (16%). We advocate the adoption of the instrumental and theoretical framework provided here to facilitate material standardization and comparison in the field of artificial photosynthesis.

Spatial Pattern of a Comprehensive fE Index for Provincial Carbon Emissions in China

China has committed to ambitious targets to reduce its carbon emissions in the next decades, in order to combat climate change and improve the environment. The realization of the targets depends on the fair and effective mitigation plans of all provinces. However, with varying ecological and environmental conditions and social-economic development, it is a critical issue to quantify the provinces’ efforts equally. This paper proposed a comprehensive fE index in coordinating ecology, equity and economy, by accounting for carbon emissions and sinks to characterize provincial carbon emission status in China, from 2000 to 2017, which shows a spatial pattern of “boundary high, central low”. The provinces with higher fE value (>1.5) in boundary areas can be seen as “relative equality” provinces with good ecology circulation, equity and economic efficiency. The provinces with lower fE value (<0.7) in central areas around Bohai Bay are regarded as “severe inequality” provinces, and are identified as the hot-spot provinces, which have emitted more CO2 than their equity share by occupying the carbon emission space of other provinces in recent decades. These results could provide a reference for a provincial guide for carbon reduction and sustainable development of the low-carbon economy.

Two-Dimensional Titanium and Molybdenum Carbide MXenes as Electrocatalysts for CO2 Reduction

Electrocatalytic CO₂ reduction reaction (CO₂RR) is an attractive way to produce renewable fuel and chemical feedstock, especially when coupled with efficient CO₂ capture and clean energy sources. On the fundamental side, research on improving CO₂RR activity still revolves around late transition metal-based catalysts, which are limited by unfavorable scaling relations despite intense investigation. Here, we report a combined experimental and theoretical investigation into electrocatalytic CO₂RR on Ti- and Mo-based MXene catalysts. Formic acid is found as the main product on Ti₂CT_x and Mo₂CT_x MXenes, with peak Faradaic efficiency of over 56% on Ti₂CT_x and partial current density of up to −2.5 mA cm⁻² on Mo₂CT_x. Furthermore, simulations reveal the critical role of the T_x group: a smaller overpotential is found to occur at lower amounts of –F termination. This work represents an important step toward experimental demonstration of MXenes for more complex electrocatalytic reactions in the future.

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Combined experimental and theoretical CO₂RR investigation on MXenes

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T_x group stabilizes ∗H-coordinated intermediates and breaks scaling relations

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Formic acid is the main CO₂RR product on Ti₂CT_x and Mo₂CT_x MXenes

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Lower degree of –F termination in T_x group results in smaller overpotential

Towards a more effective climate policy on international trade

In the literature on the attribution of responsibilities for greenhouse gas emissions, two accounting methods have been widely discussed: production-based accounting (PBA) and consumption-based accounting (CBA). It has been argued that an accounting framework for attributing responsibilities should credit actions contributing to reduce global emissions and should penalize actions increasing them. Neither PBA nor CBA satisfy this principle. Adapting classical Ricardian trade theory, we consider ex post measurement and propose a scheme for assigning credits and penalties. Their size is determined by how much CO₂ emissions are saved globally due to trade. This leads to the emission responsibility allotment (ERA) for assigning responsibilities. We illustrate the differences between ERA and PBA and CBA by comparing their results for 41 countries and regions between 1995-2009. The Paris Agreement (COP21) proposed new market mechanisms; we argue that ERA is well suited to measure and evaluate their overall mitigation impact.

Downscaling satellite soil moisture using geomorphometry and machine learning

Annual soil moisture estimates are useful to characterize trends in the climate system, in the capacity of soils to retain water and for predicting land and atmosphere interactions. The main source of soil moisture spatial information across large areas (e.g., continents) is satellite-based microwave remote sensing. However, satellite soil moisture datasets have coarse spatial resolution (e.g., 25-50 km grids); and large areas from regional-to-global scales have spatial information gaps. We provide an alternative approach to predict soil moisture spatial patterns (and associated uncertainty) with higher spatial resolution across areas where no information is otherwise available. This approach relies on geomorphometry derived terrain parameters and machine learning models to improve the statistical accuracy and the spatial resolution (from 27km to 1km grids) of satellite soil moisture information across the conterminous United States on an annual basis (1991-2016). We derived 15 primary and secondary terrain parameters from a digital elevation model. We trained a machine learning algorithm (i.e., kernel weighted nearest neighbors) for each year. Terrain parameters were used as predictors and annual satellite soil moisture estimates were used to train the models. The explained variance for all models-years was >70% (10-fold cross-validation). The 1km soil moisture grids (compared to the original satellite soil moisture estimates) had higher correlations (improving from r2 = 0.1 to r2 = 0.46) and lower bias (improving from 0.062 to 0.057 m3/m3) with field soil moisture observations from the North American Soil Moisture Database (n = 668 locations with available data between 1991-2013; 0-5cm depth). We conclude that the fusion of geomorphometry methods and satellite soil moisture estimates is useful to increase the spatial resolution and accuracy of satellite-derived soil moisture. This approach can be applied to other satellite-derived soil moisture estimates and regions across the world.

Negative emissions technologies: A complementary solution for climate change mitigation

Carbon dioxide (CO₂) is the main greenhouse gas (GHG) and its atmospheric concentration is currently 50% higher than pre-industrial levels. The continuous GHGs emissions may lead to severe and irreversible consequences in the climate system. The reduction of GHG emissions may be not enough to mitigate climate change. Consequently, besides carbon capture from large emission sources, atmospheric CO₂ capture may be also required. To meet the target defined for climate change mitigation, the removal of 10 Gt·yr^-1 of CO₂ globally by mid-century and 20 Gt·yr^-1 of CO₂ globally by the end of century. The technologies applied with this aim are known as negative emission technologies (NETs), as they lead to achieve a negative balance of carbon in atmosphere. This paper aims to present the recent research works regarding NETs, focusing the research findings achieved by academic groups and projects. Besides several advantages, NETs present high operational cost and its scale-up should be tested to know the real effect on climate change mitigation. With current knowledge, no single process should be seen as a solution. Research efforts should be performed to evaluate and reduce NETs costs and environmental impact.

Increase in severe and extreme soil moisture droughts for Europe under climate change

Droughts are among the costliest natural disasters. They affect wide regions and large numbers of people worldwide by tampering with water availability and agricultural production. In this research, soil moisture drought trends are assessed for Europe using the Soil Moisture Index (SMI) estimated on Joint UK Land Environment Simulator simulations under two Representative Concentration Pathways, the RCP 2.6 and RCP 6.0 scenarios. Results show that SMI drought conditions are expected to exacerbate in Europe with substantial differences among regions. Eastern Europe and Mediterranean regions are found to be the most affected. Spatially and temporally contiguous regions that exhibit SMI of Severe and Extreme index categories are identified as distinct drought events and are assessed for their characteristics. It is shown that even under strong emissions mitigation, these events are expected to increase in occurrence (22% to 123%), while their characteristics will become more unfavorable. Results indicate increase in their spatial extend (between 23% and 46%) and their duration (between 16% and 48%) depending on the period and the scenario. Additional analysis was performed for the exceptionally wide-area (over 10⁶ km²) severe and extreme soil moisture drought events that are expected to drastically increase comparing to the recent past. Projections show that those events are expected to happen between 11 and 28 times more frequently depending on the scenario and the period with a 59% to 246% larger duration. These findings indicate that even applying strong mitigation measures, agricultural drought risk in Europe is expected to become higher than our present experience.

Bridging the Gap between the Direct and Hydrocarbon Pool Mechanisms of the Methanol-to-Hydrocarbons Process

After a prolonged effort over many years, the route for the formation of a direct carbon-carbon (C-C) bond during the methanol-to-hydrocarbon (MTH) process has very recently been unveiled. However, the relevance of the "direct mechanism"-derived molecules (that is, methyl acetate) during MTH, and subsequent transformation routes to the conventional hydrocarbon pool (HCP) species, are yet to be established. This important piece of the MTH chemistry puzzle is not only essential from a fundamental perspective, but is also important to maximize catalytic performance. The MTH process was probed over a commercially relevant H-SAPO-34 catalyst, using a combination of advanced solid-state NMR spectroscopy and operando UV/Vis diffuse reflectance spectroscopy coupled to an on-line mass spectrometer. Spectroscopic evidence is provided for the formation of (olefinic and aromatic) HCP species, which are indeed derived exclusively from the direct C-C bond-containing acetyl group of methyl acetate. New mechanistic insights have been obtained from the MTH process, including the identification of hydrocarbon-based co-catalytic organic reaction centers.

Phloem function: a key to understanding and manipulating plant responses to rising atmospheric [CO2]?

Increasing atmospheric carbon dioxide concentration ([CO₂]) directly stimulates photosynthesis and reduces stomatal conductance in C₃ plants. Both of these physiological effects have the potential to alter phloem function at elevated [CO₂]. Recent research has clearly established that photosynthetic capacity is correlated to vascular traits associated with phloem loading and water transport, but the effects of elevated [CO₂] on these relationships are largely unexplored. Plants also employ different strategies for loading sucrose and other sugars into the phloem, and there is potential for species with different phloem loading strategies to respond differently to elevated [CO₂]. Recent research manipulating sucrose transporters and other key enzymes with roles in phloem loading show promise for maximizing crop performance in an elevated [CO₂] world.

Implications of possible interpretations of 'greenhouse gas balance' in the Paris Agreement

The main goal of the Paris Agreement as stated in Article 2 is 'holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C'. Article 4 points to this long-term goal and the need to achieve 'balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases'. This statement on 'greenhouse gas balance' is subject to interpretation, and clarifications are needed to make it operational for national and international climate policies. We study possible interpretations from a scientific perspective and analyse their climatic implications. We clarify how the implications for individual gases depend on the metrics used to relate them. We show that the way in which balance is interpreted, achieved and maintained influences temperature outcomes. Achieving and maintaining net-zero CO2-equivalent emissions conventionally calculated using GWP100 (100-year global warming potential) and including substantial positive contributions from short-lived climate-forcing agents such as methane would result in a sustained decline in global temperature. A modified approach to the use of GWP100 (that equates constant emissions of short-lived climate forcers with zero sustained emission of CO2) results in global temperatures remaining approximately constant once net-zero CO2-equivalent emissions are achieved and maintained. Our paper provides policymakers with an overview of issues and choices that are important to determine which approach is most appropriate in the context of the Paris Agreement.This article is part of the theme issue 'The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels'.