Life cycle greenhouse gas emissions of blended cement concrete including carbonation and durability

Purpose

Blended cements use waste products to replace Portland cement, the main contributor to CO₂ emissions in concrete manufacture. Using blended cements reduces the embodied greenhouse gas emissions; however, little attention has been paid to the reduction in CO₂ capture (carbonation) and durability. The aim of this study is to determine if the reduction in production emissions of blended cements compensates for the reduced durability and CO₂ capture.

Methods

This study evaluates CO₂ emissions and CO₂ capture for a reinforced concrete column during its service life and after demolition and reuse as gravel filling material. Concrete depletion, due to carbonation and the unavoidable steel embedded corrosion, is studied, as this process consequently ends the concrete service life. Carbonation deepens progressively during service life and captures CO₂ even after demolition due to the greater exposed surface area. In this study, results are presented as a function of cement replaced by fly ash (FA) and blast furnace slag (BFS).

Results and discussion

Concrete made with Portland cement, FA (35 % FA), and BFS blended cements (80 % BFS) captures 47, 41, and 20 % of CO₂ emissions, respectively. The service life of blended cements with high amounts of cement replacement, like CEM III/A (50 % BFS), CEM III/B (80 % BFS), and CEM II/B-V (35 % FA), was about 10 % shorter, given the higher carbonation rate coefficient. Compared to Portland cement and despite the reduced CO₂ capture and service life, CEM III/B emitted 20 % less CO₂ per year.

Conclusions

To obtain reliable results in a life cycle assessment, it is crucial to consider carbonation during use and after demolition. Replacing Portland cement with FA, instead of BFS, leads to a lower material emission factor, since FA needs less processing after being collected, and transport distances are usually shorter. However, greater reductions were achieved using BFS, since a larger amount of cement can be replaced. Blended cements emit less CO₂ per year during the life cycle of a structure, although a high cement replacement reduces the service life notably. If the demolished concrete is crushed and recycled as gravel filling material, carbonation can cut CO₂ emissions by half. A case study is presented in this paper demonstrating how the results may be utilized.     

A comparative cradle-to-gate life cycle assessment of three concrete mix designs

Purpose

The concrete industry faces challenges to create concrete mix designs that reduce negative environmental impacts but also maintain high performance. This has led to ‘greener’ cementitious materials being developed which can decrease the use of traditional Portland cement (PC). This study intended to carry out a ‘cradle-to-gate’ life cycle assessment (LCA) on concrete mix designs containing different cementitious blends.

Methods

The aim of this study was to obtain the overall environmental impact, with a particular focus on carbon dioxide (CO₂) emissions of three concrete mix designs: CEM I (100 % PC content), CEM II/B-V (65 % PC content, 35 % Fly Ash (FA) content) and CEM III/B (30 % PC content, 70 % ground granulated blast furnace slag (GGBS) content). Evaluations of the three concrete mixes were performed using ‘SimaPro 8’ LCA software. A comparative cradle-to-gate LCA of these mixes has not currently been explored and could present a new insight into improving the environmental impact of concrete with the use of secondary materials. Recommendations from this work would help the industry make key decisions about concrete mix designs.

Results and discussion

Results show that Mix 2 (CEM II/B-V) and Mix 3 (CEM III/B) could potentially be taken forwards to improve their environmental impacts of concrete production. With respect to optimum mix design, it is strongly recommended that GGBS is selected as the addition of choice for reducing CO₂ emissions. FA does still considerably improve sustainability when compared to PC, but this work proved that inclusion of GGBS environmentally optimises the mix design even further. Advantages of using GGBS include lower CO₂ emissions, a substantial reduction of environmental impacts and an increased scope for sustainability due to the higher PC replacement levels that are permitted for GGBS. Due to mix designs enabling a higher contribution of GGBS additions, it would also indicate an increased positive effect regarding waste scenarios.

Conclusions and recommendations

The main contribution of this work demonstrated that concrete can be produced without loss of performance whilst significantly reducing the negative environmental impacts incurred in its production. The results obtained from this work would help to define the available options for optimising concrete mix design. The only material variations in each mix were the different cementitious blends. So, by determining the best option, a platform to make recommendations can be established based upon cementitious materials.

     

Dynamics of soil biogeochemical gas emissions shaped by remolded aggregate sizes and carbon configurations under hydration cycles

Changes in soil hydration status affect microbial community dynamics and shape key biogeochemical processes. Evidence suggests that local anoxic conditions may persist and support anaerobic microbial activity in soil aggregates (or in similar hot spots) long after the bulk soil becomes aerated. To facilitate systematic studies of interactions among environmental factors with biogeochemical emissions of CO₂, N₂O and CH₄ from soil aggregates, we remolded silt soil aggregates to different sizes and incorporated carbon at different configurations (core, mixed, no addition). Assemblies of remolded soil aggregates of three sizes (18, 12, and 6 mm) and equal volumetric proportions were embedded in sand columns at four distinct layers. The water table level in each column varied periodically while obtaining measurements of soil GHG emissions for the different aggregate carbon configurations. Experimental results illustrate that methane production required prolonged inundation and highly anoxic conditions for inducing measurable fluxes. The onset of unsaturated conditions (lowering water table) resulted in a decrease in CH₄ emissions while temporarily increasing N₂O fluxes. Interestingly, N₂O fluxes were about 80% higher form aggregates with carbon placement in center (anoxic) core compared to mixed carbon within aggregates. The fluxes of CO₂ were comparable for both scenarios of carbon sources. These experimental results highlight the importance of hydration dynamics in activating different GHG production and affecting various transport mechanisms about 80% of total methane emissions during lowering water table level are attributed to physical storage (rather than production), whereas CO₂ emissions (~80%) are attributed to biological activity. A biophysical model for microbial activity within soil aggregates and profiles provides a means for results interpretation and prediction of trends within natural soils under a wide range of conditions.     

Carbon Abatement and Emissions Associated with the Gasification of Walnut Shells for Bioenergy and Biochar Production

By converting biomass residue to biochar, we could generate power cleanly and sequester carbon resulting in overall greenhouse gas emissions (GHG) savings when compared to typical fossil fuel usage and waste disposal. We estimated the carbon dioxide (CO₂) abatements and emissions associated to the concurrent production of bioenergy and biochar through biomass gasification in an organic walnut farm and processing facility in California, USA. We accounted for (i) avoided-CO₂ emissions from displaced grid electricity by bioenergy; (ii) CO₂ emissions from farm machinery used for soil amendment of biochar; (iii) CO₂ sequestered in the soil through stable biochar-C; and (iv) direct CO₂ and nitrous oxide (N₂O) emissions from soil. The objective of these assessments was to pinpoint where the largest C offsets can be expected in the bioenergy-biochar chain. We found that energy production from gasification resulted in 91.8% of total C offsets, followed by stable biochar-C (8.2% of total C sinks), offsetting a total of 107.7 kg CO₂-C eq Mg^-1 feedstock. At the field scale, we monitored gas fluxes from soils for 29 months (180 individual observations) following field management and precipitation events in addition to weekly measurements within three growing seasons and two tree dormancy periods. We compared four treatments: control, biochar, compost, and biochar combined with compost. Biochar alone or in combination with compost did not alter total N₂O and CO₂ emissions from soils, indicating that under the conditions of this study, biochar-prompted C offsets may not be expected from the mitigation of direct soil GHG emissions. However, this study revealed a case where a large environmental benefit was given by the waste-to-bioenergy treatment, addressing farm level challenges such as waste management, renewable energy generation, and C sequestration.     

我国典型城市化石能源消费CO2排放及其影响因素比较研究

城市是化石能源消费和CO_2排放的主要区域。分析典型城市化石能源消费CO_2排放特征,明确不同城市CO_2排放动态及主要影响因素的差异,是开展城市减排行动的重要科学依据。采用IPCC推荐方法及中国的排放参数核算11个典型城市2006—2015年间化石能源消费产生的CO_2排放量。根据各城市经济发展和CO_2排放特征将之分为四类:经济高度发达城市(北京、上海、广州)、高碳排放城市(重庆、乌鲁木齐、唐山)、低排放低增长城市(哈尔滨、呼和浩特和大庆)和低排放高增长城市(贵阳、合肥),并运用对数平均迪氏指数法(Logarithmic Mean Divisia Index,即LMDI分解法)对比分析了四类城市CO_2排放量的影响因素。结果表明:(1)研究期内大部分城市CO_2排放总量有所增加,仅北京和广州呈下降趋势,工业部门CO_2排放在城市排放总量及其变化中占据主导地位;四类城市的人均CO_2排放量表现出与排放总量相似的变化趋势;CO_2排放强度整体上表现为经济高度发达城市(均值为0.88 t CO_2/万元)低排放低增长城市(均值为2.82 t CO_2/万元)低排放高增长城市(均值为3.05 t CO_2/万元)高碳排放城市(均值为6.62 t CO_2/万元)。(2)在城市CO_2排放的影响因素中,经济发展和人口规模均是4类城市CO_2排放增长的促进因素,但经济发展效应的累积贡献值大于人口规模效应;能源强度降低是4类城市CO_2排放最主要的抑制因素,且经济高度发达和高碳排放城市的抑制作用强于其他两类城市;对第三产业GDP年平均增速高于第二产业的6个城市来说,产业结构是CO_2排放的抑制因素;能源结构的变化仅对煤炭消费比重较低且降幅较大的北京和广州的CO_2排放是抑制作用,累积贡献值分别为-21.73Mt和-0.03Mt,而对其他城市,特别是高碳排放城市的CO_2排放具有明显的促进作用。     

城市能源利用碳足迹分析——以厦门市为例

城市能源利用碳足迹分析综合考虑直接与间接碳排放,对于深度分析碳排放的本质过程、制定科学全面的碳减排计划具有重要意义。以厦门市为研究案例,应用碳足迹的混合分析方法,对厦门市2009年能源利用碳足迹进行了分析,除了包括传统研究中的城市能源终端利用产生的直接碳排放,还计算了跨界交通和城市主要消耗物质的内含能引起的间接碳排放。研究结果表明:(1)城市边界内的工业、交通、商业等部门的能源消耗产生的直接碳排放(即层次1和层次2)只占到总碳足迹的64%,而一直被忽略的跨界交通和城市主要消耗物质的内含能引起的间接碳排放(层次3)占到36%;(2)在直接碳排放中,工业部门的碳排放贡献率最大,占到直接碳排放的55%,其中化工行业带来的碳排放占到工业部门的25%;(3)在间接碳排放中,跨界交通引起的碳排放占间接碳排放的27%,其中长途道路运输贡献率最大,占跨界交通碳排放的38%;主要材料内含能碳排放占间接碳排的73%,其中燃料的内含能碳排放占总内含能的份额最大,达51%。;(4)从人均碳足迹角度比较,厦门市人均碳足迹和丹佛市的人均直接碳排(层次1+层次2)分别为5.74 t CO2e/人、18.9 t CO2e/人,包含3个层次的人均碳足迹分别为9.01 tCO2e/人、25.3 t CO2e/人,其中跨界交通引起的碳排放均占总碳足迹的10%左右,主要材料的内含能引起的碳排放分别占到总碳足迹的26%、15%;通过国内外典型城市不同层次碳足迹比较可见厦门还是相对低碳的,但有个显著的特点是主要消耗物质的内含碳排放比例较高,这在一定程度上说明了发展中国家城市消耗更多的基础材料,进一步证明了传统核算中忽略的第3层次碳排放核算与管理的重要性。     

The role of forest residues in the accounting for the global warming potential of bioenergy

Bioenergy makes up a significant portion of the global primary energy pie, and its production from modernized technology is foreseen to substantially increase. The climate neutrality of biogenic CO₂ emissions from bioenergy grown from sustainably managed biomass resource pools has recently been questioned. The temporary change caused in atmospheric CO₂ concentration from biogenic carbon fluxes was found to be largely dependent on the length of biomass rotation period. In this work, we also show the importance of accounting for the unutilized biomass that is left to decompose in the resource pool and how the characterization factor for the climate impact of biogenic CO₂ emissions changes whether residues are removed for bioenergy or not. With the case of Norwegian Spruce biomass grown in Norway, we found that significantly more biogenic CO₂ emissions should be accounted towards contributing to global warming potential when residues are left in the forest. For a 100‐year time horizon, the global warming potential bio factors suggest that between 44 and 62% of carbon‐flux, neutral biogenic CO₂ emissions at the energy conversion plant should be attributed to causing equivalent climate change potential as fossil‐based CO₂ emissions. For a given forest residue extraction scenario, the same factor should be applied to the combustion of any combination of stem and forest residues. Life cycle analysis practitioners should take these impacts into account and similar region/species specific factors should be developed.     

Annual emissions of greenhouse gases from sheepfolds in Inner Mongolia

Sheepfolds represent significant hot spot sources of greenhouse gases (GHG) in semi-arid grassland regions, such as Inner Mongolia in China. However, the annual contribution of sheepfolds to regional GHG emissions is still unknown. In order to quantify its annual contribution, we conducted measurements of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes at two sheepfold sites in the Baiyinxile administrative region of Inner Mongolia for 1 year, using static opaque chamber and gas chromatography methods. Our data show that, at an annual scale, both sheepfolds functioned as net sources of CO₂, CH₄ and N₂O. Temperatures primarily determined the seasonal pattern of CO₂ emission; 60–84% of the CO₂ flux variation could be explained by temperature changes. High rates of net CH₄ emissions from sheepfold soils were only observed when animals (sheep and goats) were present. While nitrous oxide emissions were also stimulated by the presence of animals, pulses of N₂O emissions were also be related to rainfall and spring-thaw events. The total annual cumulative GHG emissions in CO₂ equivalents (CO₂: 1; CH₄: 25; and N₂O: 298) were quantified as 87.4?±?18.4 t ha^?1 for the sheepfold that was used during the non-grazing period (i.e., winter sheepfold) and 136.7?±?15.9 t ha^?1 used during the grazing period (i.e., summer sheepfold). Of the annual total GHG emissions, CH₄ release accounted for approximately 1% of emissions, while CO₂ and N₂O emissions contributed to approximately 59% and 40%, respectively. The total GHG emission factor (CO₂?+?CH₄?+?N₂O) per animal for the sheepfolds investigated in this study was 30.3 kg CO₂ eq yr^?1 head^?1, which translates to 0.3, 18.8 and 11.2 kg CO₂ eq yr^?1 head^?1 for CH₄, CO₂ and N₂O, respectively. Sheepfolds accounted for approximately 34% of overall N₂O emissions in the Baiyinxile administrative region, a typical steppe region within Inner Mongolia. The contribution of sheepfolds to the regional CO₂ or CH₄ exchange is marginal.     

Precipitation‐drainage cycles lead to hot moments in soil carbon dioxide dynamics in a Neotropical wet forest

Soil CO₂ concentrations and emissions from tropical forests are modulated seasonally by precipitation. However, subseasonal responses to meteorological events (e.g., storms, drought) are less well known. Here, we present the effects of meteorological variability on short‐term (hours to months) dynamics of soil CO₂ concentrations and emissions in a Neotropical wet forest. We continuously monitored soil temperature, moisture, and CO₂ for a three‐year period (2015–2017), encompassing normal conditions, floods, a dry El Niño period, and a hurricane. We used a coupled model (Hydrus‐1D) for soil water propagation, heat transfer, and diffusive gas transport to explain observed soil moisture, soil temperature, and soil CO₂ concentration responses to meteorology, and we estimated soil CO₂ efflux with a gradient‐flux model. Then, we predicted changes in soil CO₂ concentrations and emissions under different warming climate change scenarios. Observed short‐term (hourly to daily) soil CO₂ concentration responded more to precipitation than to other meteorological variables (including lower pressure during the hurricane). Observed soil CO₂ failed to exhibit diel patterns (associated with diel temperature fluctuations in drier climates), except during the drier El Niño period. Climate change scenarios showed enhanced soil CO₂ due to warmer conditions, while precipitation played a critical role in moderating the balance between concentrations and emissions. The scenario with increased precipitation (based on a regional model projection) led to increases of +11% in soil CO₂ concentrations and +4% in soil CO₂ emissions. The scenario with decreased precipitation (based on global circulation model projections) resulted in increases of +4% in soil CO₂ concentrations and +18% in soil CO₂ emissions, and presented more prominent hot moments in soil CO₂ outgassing. These findings suggest that soil CO₂ will increase under warmer climate in tropical wet forests, and precipitation patterns will define the intensity of CO₂ outgassing hot moments.     

Relationships among carbon emissions,economic growth,energy consumption and population growth: Testing Environmental Kuznets Curve hypothesis for Brazil,China, India and Indonesia

This study examines the impacts of income, energy consumption and population growth on CO₂ emissions by employing an annual time series data for the period 1970–2012 for India, Indonesia, China, and Brazil. The study used the Autoregressive Distributed Lag (ARDL) bounds test approach considering both the linear and non-linear assumptions for related time series data for the top CO₂ emitter emerging countries in both the short run and long run. The results show that CO₂ emissions have increased statistically significantly with increases in income and energy consumption in all four countries. While the relationship between CO₂ emissions and population growth was found to be statistically significant for India and Brazil, it has been statistically insignificant for China and Indonesia in both the short run and long run. Also, empirical observations from the testing of environmental Kuznets curve (EKC) hypothesis imply that in the cases of Brazil, China and Indonesia, CO₂ emissions will decrease over the time when income increases. So based on the EKC findings, it can be argued that these three countries should not take any actions or policies, which might have conservative impacts on income, in order to reduce their CO₂ emissions. But in the case of India, where CO₂ emissions and income were found to have a positive relationship, an increase in income over the time will not reduce CO₂ emissions in the country.     

Using a Rebound Matrix to Estimate Consumption Changes from Saving and its Environmental Impact in Japan

We investigate the extent to which Japanese people can change their consumption and the corresponding environmental impact. We propose a new analytical framework with a rebound matrix that captures the monetary flow from potential savings to their respending (referred to as rebound). A questionnaire is used to derive the matrix. On average, respondents spent 3.4 million Yen annually, resulting in 12.4 tons of carbon dioxide (CO₂) emissions in their daily lives. The survey results suggest that acceptable spending reductions would correspond to a CO₂ emissions reduction of nearly 6%. However, the CO₂ emissions would increase by nearly the same amount when the respondents respend their savable money (rebound CO₂ emissions). The annual CO₂ emissions and the annually reducible CO₂ emissions both increase with the increase in annual expenditure. Consequently, the net CO₂ emissions also increase with the increase in annual expenditure. The rebound spending is approximated using the rebound matrix. Finally, it is suggested that the net CO₂ emissions can be reduced through lifestyle changes whereby spending on energy items is reduced and the resulting savings are spent on telecommunication, clothes, shoes, education, and housing.     

Enhanced CH4 emission from a wetland soil exposed to Elevated CO2

Methane emissions from wetland soils are generally a positive function ofplant size and primary productivity, and may be expected to increase dueto enhanced rates of plant growth in a future atmosphere of elevatedCO₂. We performed two experiments with Orontium aquaticum, acommon emergent aquatic macrophyte in temperate and sub-tropical wetlands, todetermine if enhanced rates of photosynthesis in elevated CO₂atmospheres would increase CH₄ emissions from wetland soils.O. aquaticum was grown from seed in soil cores under ambient and elevated(ca. 2-times ambient) concentrations of CO₂ in an initialglasshouse study lasting 3 months and then a growth chamber study lasting 6months. Photosynthetic rates were 54 to 71% higher underelevated CO₂ than ambient CO₂, but plantbiomass was not significantly different at the end of the experiment. Ineach case, CH₄ emissions were higher under elevated thanambient CO₂ levels after 2 to 4 months of treatment, suggestinga close coupling between photosynthesis and methanogenesis in our plant-soilsystem. Methane emissions in the growth chamber study increased by 136%. We observed a significant decrease in transpirationrates under elevated CO₂ in the growth chamber study, andspeculate that elevated CO₂ may also stimulate CH₄ emissions by increasing the extent and duration offlooding in some wetland ecosystems. Elevated CO₂ maydramatically increase CH₄ emissions from wetlands, a sourcethat currently accounts for 40% of global emissions.     

Effects of an experimental drought on soil emissions of carbon dioxide, methane, nitrous oxide, and nitric oxide in a moist tropical forest

Changes in precipitation in the Amazon Basin resulting from regional deforestation, global warming, and El Niño events may affect emissions of carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and nitric oxide (NO) from soils. Changes in soil emissions of radiatively important gases could have feedback implications for regional and global climates. Here we report results of a large‐scale (1 ha) throughfall exclusion experiment conducted in a mature evergreen forest near Santarém, Brazil. The exclusion manipulation lowered annual N₂O emissions by >40% and increased rates of consumption of atmospheric CH₄ by a factor of >4. No treatment effect has yet been detected for NO and CO₂ fluxes. The responses of these microbial processes after three rainy seasons of the exclusion treatment are characteristic of a direct effect of soil aeration on denitrification, methanogenesis, and methanotrophy. An anticipated second phase response, in which drought‐induced plant mortality is followed by increased mineralization of C and N substrates from dead fine roots and by increased foraging of termites on dead coarse roots, has not yet been detected. Analyses of depth profiles of N₂O and CO₂ concentrations with a diffusivity model revealed that the top 25 cm soil is the site of most of the wet season production of N₂O, whereas significant CO₂ production occurs down to 100 cm in both seasons, and small production of CO₂ occurs to at least 1100 cm depth. The diffusivity‐based estimates of CO₂ production as a function of depth were strongly correlated with fine root biomass, indicating that trends in belowground C allocation may be inferred from monitoring and modeling profiles of H₂O and CO₂.     

Can biochar reduce soil greenhouse gas emissions from a Miscanthus bioenergy crop?

Energy production from bioenergy crops may significantly reduce greenhouse gas (GHG) emissions through substitution of fossil fuels. Biochar amendment to soil may further decrease the net climate forcing of bioenergy crop production, however, this has not yet been assessed under field conditions. Significant suppression of soil nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions following biochar amendment has been demonstrated in short‐term laboratory incubations by a number of authors, yet evidence from long‐term field trials has been contradictory. This study investigated whether biochar amendment could suppress soil GHG emissions under field and controlled conditions in a Miscanthus × Giganteus crop and whether suppression would be sustained during the first 2 years following amendment. In the field, biochar amendment suppressed soil CO₂ emissions by 33% and annual net soil CO₂ equivalent (eq.) emissions (CO₂, N₂O and methane, CH₄) by 37% over 2 years. In the laboratory, under controlled temperature and equalised gravimetric water content, biochar amendment suppressed soil CO₂ emissions by 53% and net soil CO₂ eq. emissions by 55%. Soil N₂O emissions were not significantly suppressed with biochar amendment, although they were generally low. Soil CH₄ fluxes were below minimum detectable limits in both experiments. These findings demonstrate that biochar amendment has the potential to suppress net soil CO₂ eq. emissions in bioenergy crop systems for up to 2 years after addition, primarily through reduced CO₂ emissions. Suppression of soil CO₂ emissions may be due to a combined effect of reduced enzymatic activity, the increased carbon‐use efficiency from the co‐location of soil microbes, soil organic matter and nutrients and the precipitation of CO₂ onto the biochar surface. We conclude that hardwood biochar has the potential to improve the GHG balance of bioenergy crops through reductions in net soil CO₂ eq. emissions.     

共享社会经济路径下中国2020—2100年碳排放预测研究

碳排放和减碳经济代价研究日益受到学术界和决策者的关注,中国政府做出的关于争取在2060年前实现碳中和的表态引起了国际社会的热议。在此背景下,开展中国未来长时间序列碳排放的情景预测具有切实意义。基于可拓展的随机性环境影响评估模型（STIRPAT）评估了人口、经济和受教育程度对碳排放的影响,对比历史数据并验证了碳排放预测模型的准确性,结合共享社会经济路径（SSPs）情景的设定和模型参数,预测了5种情景下中国2020年至2100年的碳排放轨迹及经济代价。结果表明：（1）考虑碳排放达峰目标的实现,SSP3情景是中国未来发展的最佳情景,在此情景下,中国有望提前三年实现碳排放达峰目标;（2） SSP3情景可使中国年度总碳排放量和人均碳排放量处于相对其他四种情景的最低值,但需要付出累积GDP下降5.49%至8.80%的代价;（3）为完成在2060年前实现碳中和的承诺,中国政府在未来的40年需面对409.36-467.42 Gt的碳中和量;（4）2020年中国的碳排放强度将会较2005年水平下降40.52%至41.39%,2030年碳排放强度将会较2005年水平下降59.64%至60.75%。5种情境中,SSP5情景是降低碳排放强度的最佳情景,可最大程度地超额实现碳排放强度目标。未来,受经济发展、人口增长等重要因素影响,中国政府减碳压力将进一步加大。后疫情时代,考虑到能源供应的减少和高科技产业的发展,碳排放社会成本的上升将为中国创造一个使能源系统脱碳的机遇。中国应在"十四五"期间继续提升能源利用效率、升级产业结构、提倡低碳消费、实施隐含碳战略,以尽快实现碳减排目标。     

Design of concrete : Setting a new basis for improving both durability and environmental performance

CO₂ emissions from cement production currently represent around 6% of global CO₂ emissions. However, cement concrete absorbs CO₂ from the atmosphere because of carbonation (i.e., penetration of atmospheric CO₂ inside bulk concrete). Carbonation has beneficial effects on the mechanical resistance of cement concrete. However, carbonation also has adverse effects because it provokes a decrease in pH that favors later corrosion of reinforcing bars and thus reduces service life. Current European standards provide recommendations concerning reinforcing concrete covers, but these are not based on actual service‐life durations. Thanks to a previously developed carbonation model combined with sensitivity analysis and LCA, we compare Climate Change indicators of 1 m² of reinforced concrete cover over a 100‐years service life exposed to XC4 conditions in Madrid, obtained on one hand by using current standards and on the other hand with concrete‐cover depths calculated with our carbonation model. Our results show that cement strength class is a key parameter to both increase durability and decrease climate‐change impacts. When the carbonation model is used to optimize both durability and climate‐change impacts, it drives to considerable and significant improvements. Finally, climate‐change indicators predicted from our carbonation model are not linearly linked to carbon intensity of cements, which is a current argument of so‐called “green cements.” The values of indicators presented in this article cannot be generalized: They mainly depend on the geographical location. However, the model and key action levers are general. Using high cement strength classes and low water‐to‐cement ratios allows use of lower concrete‐cover depths and thus save amounts of concrete compared to the standard. This generates an important benefit in terms of climate‐change impacts for identical service lives and improved mechanical resistance. Thus, considering the huge impact of cement and construction industry on climate change, we plead for a revision of standards which, instead of thresholds based on simplified models, should provide certified tools enabling the best design for every situation. This article met the requirements for a gold/gold JIE data openness badge described at http://jie.click/badges.     

A life cycle carbon dioxide inventory of the Million Trees Los Angeles program

Purpose

This study seeks to answer the question, “Will the Million Trees LA (Million Trees Los Angeles, MTLA) program be a carbon dioxide (CO₂) sink or source?” Because there has never been a full accounting of CO₂ emissions, it is unclear if urban tree planting initiatives (TPIs) are likely to be effective means for reaching local reduction targets.

Methods

Using surveys, interviews, field sampling, and computer simulation of tree growth and survival over a 40-year time period, we developed the first process-based life cycle inventory of CO₂ for a large TPI. CO₂ emissions and reductions from storage and avoided emissions from energy savings were simulated for 91,786 trees planted from 2006 to 2010, of which only 30,813 (33.6 %) were estimated to survive.

Results and discussion

The MTLA program was estimated to release 17,048 and 66,360 t of fossil and biogenic CO₂ over the 40-year period, respectively. The total amount emitted (83,408 t) was slightly more than the ?77,942 t CO₂ that trees were projected to store in their biomass. The MTLA program will be a CO₂ sink if projected 40-year-avoided fossil fuel CO₂ emissions from energy savings (?101,679 t) and biopower (?1,939 t) are realized. The largest sources of CO₂ emissions were mulch decomposition (65.1 %), wood combustion (14.5 %), and irrigation water (9.7 %).

Conclusions

Although trees planted by the MTLA program are likely to be a net CO₂ sink, there is ample opportunity to reduce emissions. Examples of these opportunities include selecting drought-tolerant trees and utilizing wood residue to generate electricity rather than producing mulch.     

旅游交通碳排放的空间结构与情景分析

旅游业作为全球第一大产业,是影响气候变化的重要因素之一,旅游碳排放的相关研究近年来已经引起学者们的关注.选择了九寨沟风景区、西安碑林博物馆、南京珍珠泉风景区3个旅游交通模式差异明显的案例地为例,根据实地问卷调查数据估算了九寨沟风景区、西安碑林博物馆、珍珠泉旅游风景区2010年的旅游交通碳排放总量分别为654.18,108.44和15.92 Gg.通过比较九寨沟、西安碑林和珍珠泉的碳排放累积曲线,得出不同旅游平均距离的景区的碳排放结构均衡度有所不同,旅游平均距离偏低景区的碳排放结构最不均衡.同时,旅游景区的交通碳排放在距离上具有分段性,不同旅游平均距离的景区交通碳排放的空间结构具有明显的差异性.通过4种不同的交通情景分析发现,旅游平均距离高和距离中等的景区对飞机的碳减排敏感度较高,旅游平均距离偏低的景区自驾车的碳减排效果最为明显.研究结果为旅游管理部门根据碳排放结构有针对性的制定差异化的旅游交通碳减排政策提供了参考和借鉴.     

Carbon emissions performance of commercial logging in East Kalimantan,Indonesia

Adoption of reduced‐impact logging (RIL) methods could reduce CO₂ emissions by 30–50% across at least 20% of remaining tropical forests. We developed two cost effective and robust indices for comparing the climate benefits (reduced CO₂ emissions) due to RIL. The indices correct for variability in the volume of commercial timber among concessions. We determined that a correction for variability in terrain slope was not needed. We found that concessions certified by the Forest Stewardship Council (FSC, N = 3), when compared with noncertified concessions (N = 6), did not have lower overall CO₂ emissions from logging activity (felling, skidding, and hauling). On the other hand, FSC certified concessions did have lower emissions from one type of logging impact (skidding), and we found evidence of a range of improved practices using other field metrics. One explanation of these results may be that FSC criteria and indicators, and associated RIL practices, were not designed to achieve overall emissions reductions. Also, commonly used field metrics are not reliable proxies for overall logging emissions performance. Furthermore, the simple distinction between certified and noncertified concessions does not fully represent the complex history of investments in improved logging practices. To clarify the relationship between RIL and emissions reductions, we propose the more explicit term ‘RIL‐C’ to refer to the subset of RIL practices that can be defined by quantified thresholds and that result in measurable emissions reductions. If tropical forest certification is to be linked with CO₂ emissions reductions, certification standards need to explicitly require RIL‐C practices.     

Alternatives for natural‐gas‐based heating systems: A quantitative GIS‐based analysis of climate impacts and financial feasibility

The heating of buildings currently produces 6% of global greenhouse gas emissions. Sustainable heating technologies can reduce heating‐related CO₂ emissions by up to 90%. We present a Python‐based GIS model to analyze the environmental and financial impact of strategies to reduce heating‐related CO₂ emissions of residential buildings. The city‐wide implementation of three alternatives to natural gas are evaluated: high‐temperature heating networks, low‐temperature heating networks, and heat pumps. We find that both lowering the demand for heat and providing more sustainable sources of heat will be necessary to achieve significant CO₂‐emission reductions. Of the studied alternatives, only low‐temperature heating networks and heat pumps have the potential to reduce CO₂ emissions by 90%. A CO₂ tax and an increase in tax on the use of natural gas are potent policy tools to accelerate the adoption of low‐carbon heating technologies.