中国木质林产品碳储量及其减排潜力

伐后木质林产品作为森林生态系统碳循环的一个组成部分,对森林生态系统和大气之间的碳平衡起着至关重要的作用.为准确合理估算木质林产品碳储量和我国参加气候变化谈判提供依据,在达喀尔会议上确立的3种估算方法框架下,即:储量变化法、生产法和大气流动法,利用寿命分析法和逐步递归法以及国内参数计算我国木质林产品的碳储量,且对生产法的应用提出了一种新假设思路,并分析我国木质林产品在替代建筑材料方面的减排潜力,结果显示:①分别利用储量变化法、生产法和大气流动法估算我国1961～2004年木质林产品的碳储量变化,证明我国的木质林产品是一个碳库,并且这个碳库的碳储量不断在增长;② 3种方法估算1961～2004年木质林产品碳储量的年平均增长量分别是11.73Mt · a-1(Mt=106t)、9.31Mt · a-1和7 90Mt · a-1;③木质林产品在建筑材料领域的替代作用以及延长其使用寿命使其减排潜力增大;④从碳储量计量和估算的难易程度来看,储量变化法的应用可能对我国较为有利;⑤针对生产法假设的实践应用,还需要进一步的研究.     

中国木质林产品碳贮量

一直以来,在国家温室气体清单的计量中,均假定森林采伐后其贮存的生物量碳在采伐年全部释放进入大气。实际上,森林采伐后形成的木质林产品中的碳并没有立即排放,而是在随后的数年或数十年间逐渐排放,部分以垃圾形式填埋的废旧木质林产品中的碳还可能得到长期保存。但是,由于不同的计量方法导致计量结果的不确定性,木质林产品碳贮量变化是否纳入国家温室气体清单土地利用、土地利用变化和林业(LULUCF)部门的计量和报告,还存在很大争议,这也是缔约方会议争论的焦点议题之一。许多国家在国家温室气体清单中报告了木质林产品碳贮量。采用IPCC好的做法指南提出的碳贮量变化法、大气通量法、生产国法和一阶衰变法计算了我国木质林产品碳贮量及其变化,比较了不同方法和不同数据源(我国统计数据和联合国粮食和农业组织(FAO)统计数据)计算得到的碳贮量的差异,并分析了产生差异的原因。结果表明:(1)我国木质林产品碳贮量一直处于增长趋势,1900~2003年年均增长量为2.25×106MgC.a-1,随着木质林产品消费量的增加,2020年碳贮量将达到6.14×108MgC。(2)3种计量方法计算结果差异显著,用碳贮量变化法计算2003年的碳贮量为2.35×108MgC,大气通量法为0.47×108MgC,生产国法为1.79×108MgC。由于我国是木质林产品生产及进口大国,运用碳贮量变化法,1900~2020年间碳贮量年变化的平均值比生产国法多1.17×106MgC,比大气通量法多5.66×106MgC。(3)用FAO数据计算我国2003年碳贮量,其结果是我国统计数据的2~7倍。     

基于CO2FIX模型的华北落叶松人工林碳循环过程

为了探明华北落叶松(Larix gmelinii var. principis-rupprechtii)人工林的碳循环过程, 该研究以河北围场地区的华北落叶松人工林为例, 基于CO2FIX模型, 以在当地的实际调查数据、文献数据作为输入数据, 从生物量、土壤和木质林产品碳库3个方面探讨了华北落叶松人工林的碳循环过程和碳汇能力。结果表明: 华北落叶松人工林土壤碳库最大, 生物量碳库次之, 林产品碳库最小, 但是林产品碳库随时间呈逐渐增加的趋势; 在一个轮伐期内(50年), 每公顷华北落叶松人工林约固定了250 t碳, 其中约70%通过凋落物和采伐剩余物的方式进入土壤碳库, 约30%进入木质林产品碳库; 华北落叶松人工林在生长的大部分时间是一个碳的吸收汇, 而在森林采伐时成为暂时的排放源, 从长时间尺度上看, 每公顷华北落叶松人工林每年大约固定0.3 t左右的碳。该研究结果表明了木质林产品碳库在人工林碳循环中的重要作用, 这将有助于更加全面地认识人工林的碳循环过程和碳汇能力。     

江西省森林碳蓄积过程及碳源/汇的时空格局

森林碳蓄积是研究森林与大气碳交换以及估算森林吸收或排放含碳气体的关键参数,不同年龄森林的碳源/汇功能差异则体现出森林生态系统碳蓄积过程的时间特征。以森林资源清查的样方数据作为数据源,通过刻画主要树种的林分蓄积生长曲线、林龄与净初级生产力(NPP)之间的关系,驱动区域碳收支模型(InTEC)模拟江西省1950—2008年的森林碳蓄积过程,了解山江湖工程实施以来的森林碳源/汇状况。结果表明,20世纪80年代以前,江西省森林年平均NPP波动于450—813 gCm-2a-1之间,年净增生物量碳26.55—36.23 TgC/a,年净增木质林产品碳0.01—0.3 TgC/a;80年代初,NPP和年净增生物量碳分别降至307.39 gC m-2a-1和17.31 TgC/a,而年净增木质林产品碳却高达0.6 TgC/a,说明森林被大量砍伐进入林产品碳库;1985年山江湖工程实施后,大面积造林使得年净增碳蓄积呈现急剧上升趋势,生物量和木质林产品碳蓄积分别上升至目前的42.37 TgC/a和0.79 TgC/a,而平均NPP值增加缓慢、碳汇功能降低,说明林分质量有待提高;90年代后碳汇功能开始稳步增强,说明造林面积的迅速增加是引起江西省森林碳增汇的主要驱动因素,但未来森林增汇潜力应源于森林生长和有效的经营管理。     

近20年我国森林碳汇政策演变和评价

巩固提升生态系统碳汇能力是碳达峰十大行动计划之一，是助力碳中和目标实现、应对气候变化的重要举措。森林作为陆地生态系统中最大的碳库，是我国当前碳汇政策的主体。研究梳理了2000年以来我国森林碳汇有关政策的发展演变历程，并从生态政策、经济政策和保障体系建设三个维度分析和评价了政策成效与存在问题，以期为构建适应“双碳”目标的碳汇政策体系提供决策依据。研究结果表明：(1)从生态政策看：天然林保护、退耕还林还草和“三北”防护林三大林业工程增加了我国森林面积和蓄积量，显著提升了森林碳汇增量，但森林可持续经营管理体系尚未健全，需进一步精准提升森林质量，健全成果长效巩固机制，增强森林固碳能力；(2)就经济政策而言：我国已形成多层级林业碳汇交易市场，有效推动林业碳汇项目建设，同时各类金融产品的开发和补贴政策的实施为碳汇项目提供了多元化资金支持体系，但整体融资规模和补贴范围有限，需拓宽融资渠道，强化资金支持；(3)在保障体系建设方面：我国森林碳汇保障体系处于重点建设阶段，需完善森林碳汇有关法律法规、加快各类森林技术研发与标准制定，保障我国森林碳汇政策平稳运行。     

火干扰对大兴安岭呼中林区地上死木质残体碳储量的影响

基于野外样点实测数据,分析了不同烈度火烧对大兴安岭呼中林区地上死木质残体碳储量的影响.结果表明:不同烈度的火烧会对地上死木质残体碳储量产生显著影响.兴安落叶松林和针阔混交林(落叶松与白桦)中死木质残体碳储量大小为重度火烧>轻度火烧>未火烧,而白桦林中死木质残体碳储量大小为重度火烧>未火烧>轻度火烧.火干扰能显著改变森林中死木质残体碳储量的组成百分比.随着火烧烈度的增加,枯立木比重显著增大,枯落物比重显著减小,而不同火烧烈度下倒木和树桩的碳储量比重变化不显著.不同烈度的火烧对死木质残体碳储量特征的影响不同,轻度火烧下死木质残体碳储量的空间变异性最高,重度火烧下空间变异性最弱.不同火烧烈度下大兴安岭森林死木质残体碳储量差异显著,在进行森林死木质残体碳储量估算时,需要充分考虑这种差异性.     

中国森林生态系统碳汇现状与潜力

巩固和提升森林碳汇，是实现中国“碳中和”目标的重要路径之一。研究总结梳理了近10年来有关中国森林碳储量及其变化的研究文献，一方面在于探明中国森林碳汇现状和潜力以及对实现“碳中和”的贡献，同时分析当前森林碳汇计量与模拟预测研究的差距与不足，更好地支撑国家碳中和实施路径与行动方案。通过整合分析，1999—2018年间中国森林生态系统碳储量年均增长量约(208.0±44.5)TgC/a或(762.0±163.2)TgCO₂-eq/a,其中生物质、死有机质和土壤有机碳库的年均增长量分别约为(168.8±42.4)TgC/a、(12.5±8.1)TgC/a和(26.7±10.9)TgC/a。此外，木质林产品和森林之外的其它林木碳储量分别增长(49.0±15.1)TgC/a和(12.0±11.1)TgC/a。预计中国乔木林生物质碳储量年变化量将从1999—2018年间的(145.9±38.3)TgC/a增长至2030—2039年间的(171.9±60.5)TgC/a,到2050—2059年间逐渐下降至(146.9±57.7)TgC/a。2050—2059年间中国森林生态系统碳...     

四川省森林植被碳储量的空间分异特征

森林植被碳储量的空间分异特征研究可为以减排增汇为目标的森林生态系统碳库管理提供重要的基础数据.根据实测的林分含碳量和区域生物量-蓄积量回归模型计算了四川省森林植被碳储量,使用ArcGIS软件绘制和分析了四川森林植被碳储量的空间分异特征.结果表明,四川省森林植被的平均碳密度为38.04 MgC·hm-2(12.15～59.51 MgC·hm-2).受青藏高原隆升和人类活动干扰及其叠加效应的影响,四川森林植被碳密度空间分异明显,总体上表现出随纬度、海拔高度和坡度的增加而增加,随经度的增加而减小,高海拔地区和陡坡地带具有较高的碳密度.减少人类活动对森林的破坏及采取森林分区经营管理是稳定和增强四川森林碳汇功能的有效途径.     

基于MRICES模型的气候融资模拟分析

气候融资是促进发展中国家获得减排资金的重要途径之一.基于发达国家成立专项资金用于国际气候融资且发展中国家将其所获得的转移资金完全用于碳减排的经济机制下,在MRICES(Multi-regional integrated model of climate and economy with GDP spillovers,GDP溢出作用下的多区域气候经济综合模型)模型的基础上,扩展了模拟国际气候融资的模块.实验分析了气候融资在全球减排中的气候保护效益和对全球各国产生的经济效益.研究发现,持续的气候融资能有效抑制全球升温,但《坎昆协议》中提出的资金额度仅能使2100年全球升温比无资金转移时降低0.01℃,全球气候保护需要制定更长期的转移计划;气候融资能使发展中国家的经济受益,而发达国家虽然在转移初期会遭受轻微的GDP损失,但从长期看资金转移将促进发达国家的经济增长,最终出现资金转移输出方和输入方双赢的局面;同时,国际气候资金适量转移至中国有助于实现资金的优化高效使用,中国在全球减排中的贡献不容小觑.气候融资是一项气候保护有效、经济效益显著的减排机制.     

林业活动对区域森林生物量碳源汇格局的影响——以南平市为例

林业活动在一定程度上影响着区域森林的时空分布格局和碳汇/源功能。明确并量化林业活动对区域森林碳汇功能的影响与空间分布，对于区域森林碳汇提升和实现区域"碳中和"具有重要意义。以国家级生态示范区福建省南平市为例，以多期森林资源规划调查数据为基础，采用IPCC材积源-生物量法，基于土地利用类型的时空变化和林业活动类型划分，分类分析了南平市森林碳源和碳汇的空间分布特征，并量化了不同林业活动（一直保持为森林、人工造林、自然恢复、毁林和森林退化）对森林碳汇和碳源的影响。研究结果表明，2013年南平市森林碳储量总量为80.84Tg C，2020年森林碳储量总量增加至89.87Tg C，年均变化量为1.29Tg C/a （或4.73Tg CO₂/a）。平均胸径、公顷蓄积等林分因子是当前主要影响森林碳储量的因素。在其他影响因素中，暗红壤分布区的森林生物质碳密度较高而在水稻土分布区则较低；此外，高海拔、中等立地质量土地上的森林碳密度较高。对于不同林业活动，2013-2020年南平市一直保持为森林（森林经营）、自然恢复增加的天然林和人工造林分别使森林生物质碳储量增加了0.34Tg C/a、0.85Tg C/a和1.05Tg C/a，同期因毁林和森林退化导致森林生物质碳储量分别减少0.75Tg C/a和0.42Tg C/a，森林生物质碳储量净增加1.09Tg C/a （或3.98Tg CO₂/a），明显低于2013-2020森林碳储量净增量。对于土地利用变化较剧烈的区域，本文基于土地利用变化且区分林业活动路径的方法，能更准确地反映森林的碳汇和碳源及时空格局。2013-2020年间南平市一直保持为森林的生物质碳密度仅增长0.22Mg C hm^-2 a^-1，成熟林、过熟林面积占比增加使森林平均生长速率下降可能是主要原因。而同期通过自然恢复和人工造林使森林生物质碳密度分别增长4.00Mg C hm^-2 a^-1和4.10Mg C hm^-2 a^-1。优化龄组结构提升森林生长量、减少毁林和防止森林退化可以作为该区域未来森林增汇减排的有效举措。     

Environmental Impact Assessment of Wood Use in Japan through 2050 Using Material Flow Analysis and Life Cycle Assessment

In this study, we used material flow analysis and life cycle assessment to quantify the environmental impacts and impact reductions related to wood consumption in Japan from 1970 to 2013. We then conducted future projections of the impacts and reductions until 2050 based on multiple future scenarios of domestic forestry, wood, and energy use. An impact assessment method involving characterization, damage assessment, and integration with a monetary unit was used, and the results were expressed in Japanese yen (JPY). We found that environmental impacts from paper consumption, such as climate change and urban air pollution, were significant and accounted for 56% to 83% of the total environmental impacts between 1970 and 2013. Therefore, reductions of greenhouse gas, nitrogen oxide, and sulfur oxide emissions from paper production would be an effective measure to reduce the overall environmental impacts. An increase in wood use for building construction, civil engineering, furniture materials, and energy production could lead to reductions of environmental impacts (via carbon storage, material substitution, and fuel substitution) amounting to 357 billion JPY in 2050, which is equivalent to 168% of the 2013 levels. Particularly, substitution of nonwooden materials, such as cement, concrete, and steel, with wood products in building construction could significantly contribute to impact reductions. Although an increase of wood consumption could reduce environmental impacts, such as climate change, resource consumption, and urban air pollution, increased wood consumption would also be associated with land‐use impacts. Therefore, minimizing land transformations from forest to barren land will be important.     

Impact of structural changes in wood‐using industries on net carbon emissions in Finland

Forests and forest industries can contribute to climate change mitigation by sequestering carbon from the atmosphere, by storing it in biomass, and by fabricating products that substitute more greenhouse gas emission intensive materials and energy. The objectives of the study are to specify alternative scenarios for the diversification of wood product markets and to determine how an increasingly diversified market structure could impact the net carbon emissions (NCEs) of forestry in Finland. The NCEs of the Finnish forest sector were modelled for the period 2016–2056 by using a forest management simulation and optimization model for the standing forests and soil and separate models for product carbon storage and substitution impacts. The annual harvest was fixed at approximately 70 Mm³, which was close to the level of roundwood removals for industry and energy in 2016. The results show that the substitution benefits for a reference scenario with the 2016 market structure account for 9.6 Mt C (35.2 Mt CO₂ equivalent [CO₂ eq]) in 2056, which could be further increased by 7.1 Mt C (26 Mt CO₂ eq) by altering the market structure. As a key outcome, increasing the use of by‐products for textiles and wood–plastic composites in place of kraft pulp and biofuel implies greater overall substitution credits compared to increasing the level of log harvest for construction.     

Under what circumstances can the forest sector contribute to 2050 climate change mitigation targets? A study from forest ecosystems to landfill methane emissions for the province of Quebec,Canada

Meeting climate change mitigation targets by 2050, as outlined in international pledges, involves determining optimal strategies for forest management, wood supply, the substitution of greenhouse gas-intensive materials and energy sources, and wood product disposal. Our study quantified the cumulative mitigation potential by 2050 of the forest sector in the province of Quebec, Canada, using several alternative strategies and assessed under what circumstances the sector could contribute to the targets. We used the Carbon Budget Model of the Canadian Forest Sector to project ecosystems emissions and sequestration of seven alternative and one baseline (business-as-usual [BaU]) forest management scenarios over the 2018–2050 period. Three baskets of wood products were used in a Harvested Wood Products model to predict wood product emissions. The mitigation potential was determined by comparing the cumulative CO₂e budget of each alternative scenario to the BaU. The proportion of methane emissions from landfills (RCH₄%) and the required displacement factor (RDF) to achieve mitigation benefits were assessed both independently and jointly. The fastest and most efficient way to improve mitigation outcomes of the forest sector of Quebec is to reduce end-of-life methane emissions from wood products. By reducing methane emissions, the RDF for achieving mitigation benefits through intensification strategies can be reduced from 1.2–2.3 to 0–0.9 tC/tC, thus reaching the current provincial mean DF threshold (0.9). Both a reduction and an increase in the harvested volume have the potential to provide mitigation benefits with adequate RCH₄% and RDF. Increased carbon sequestration in ecosystems, innovations in long-lived wood products, and optimal substitution in markets offer potential avenues for the forest sector to contribute to mitigation benefits but are subject to significant uncertainties. Methane emission reduction at the end of wood product service life is emerging as a valuable approach to enhance mitigation benefits of the forest sector.     

Recycle,Bury, or Burn Wood Waste Biomass?: LCA Answer Depends on Carbon Accounting,Emissions Controls,Displaced Fuels,and Impact Costs

This study extends existing life cycle assessment (LCA) literature by assessing seven environmental burdens and an overall monetized environmental score for eight recycle, bury, or burn options to manage clean wood wastes generated at construction and demolition activity sites. The study assesses direct environmental impacts along with substitution effects from displacing fossil fuels and managed forest wood sourcing activities. Follow‐on effects on forest carbon stocks, land use, and fuel markets are not assessed. Sensitivity analysis addresses landfill carbon storage and biodegradation rates, atmospheric emissions controls, displaced fuel types, and two alternative carbon accounting methods commonly used for waste management LCAs. Base‐case carbon accounting considers emissions and uptakes of all biogenic and fossil carbon compounds, including biogenic carbon dioxide. Base‐case results show that recycling options (recycling into reconstituted wood products or into wood pulp for papermaking) rank better than all burning or burying options for overall monetized score as well as for climate impacts, except that wood substitution for coal in industrial boilers is slightly better than recycling for the climate. Wood substitution for natural gas boiler fuel has the highest environmental impacts. Sensitivity analysis shows the overall monetized score rankings for recycling options to be robust except for the carbon accounting method, for which all options are highly sensitive. Under one of the alternative methods, wood substitution for coal boiler fuel and landfill options with high methane capture efficiency are the best for the overall score; recycling options are next to the worst. Under the other accounting alternative, wood substitution for coal and waste‐to‐energy are the best, followed by recycling options.     

Under What Circumstances Do Wood Products from Native Forests Benefit Climate Change Mitigation?

Climate change mitigation benefits from the land sector are not being fully realised because of uncertainty and controversy about the role of native forest management. The dominant policy view, as stated in the IPCC’s Fifth Assessment Report, is that sustainable forest harvesting yielding wood products, generates the largest mitigation benefit. We demonstrate that changing native forest management from commercial harvesting to conservation can make an important contribution to mitigation. Conservation of native forests results in an immediate and substantial reduction in net emissions relative to a reference case of commercial harvesting. We calibrated models to simulate scenarios of native forest management for two Australian case studies: mixed-eucalypt in New South Wales and Mountain Ash in Victoria. Carbon stocks in the harvested forest included forest biomass, wood and paper products, waste in landfill, and bioenergy that substituted for fossil fuel energy. The conservation forest included forest biomass, and subtracted stocks for the foregone products that were substituted by non-wood products or plantation products. Total carbon stocks were lower in harvested forest than in conservation forest in both case studies over the 100-year simulation period. We tested a range of potential parameter values reported in the literature: none could increase the combined carbon stock in products, slash, landfill and substitution sufficiently to exceed the increase in carbon stock due to changing management of native forest to conservation. The key parameters determining carbon stock change under different forest management scenarios are those affecting accumulation of carbon in forest biomass, rather than parameters affecting transfers among wood products. This analysis helps prioritise mitigation activities to focus on maximising forest biomass. International forest-related policies, including negotiations under the UNFCCC, have failed to recognize fully the mitigation value of native forest conservation. Our analyses provide evidence for decision-making about the circumstances under which forest management provides mitigation benefits.     

Life Cycle Impacts and Benefits of Wood along the Value Chain: The Case of Switzerland

Sustainable use of wood may contribute to coping with energy and material resource challenges. The goal of this study is to increase knowledge of the environmental effects of wood use by analyzing the complete value chain of all wooden goods produced or consumed in Switzerland. We start from a material flow analysis of current wood use in Switzerland. Environmental impacts related to the material flows are evaluated using life cycle assessment–based environmental indicators. Regarding climate change, we find an overall average benefit of 0.5 tonnes carbon dioxide equivalent per cubic meter of wood used. High environmental benefits are often achieved when replacing conventional heat production and energy‐consuming materials in construction and furniture. The environmental performance of wood is, however, highly dependent on its use and environmental indicators. To exploit the mitigation potential of wood, we recommend to (1) apply its use where there are high substitution benefits like the replacement of fossil fuels for energy or energy‐intensive building materials, (2) take appropriate measures to minimize negative effects like particulate matter emissions, and (3) keep a systems perspective to weigh effects like substitution and cascading against each other in a comprehensive manner. The results can provide guidance for further in‐depth studies and prospective analyses of wood‐use scenarios.     

Using the Lashof Accounting Methodology to Assess Carbon Mitigation Projects With Life Cycle Assessment

As governments elaborate strategies to counter climate change, there is a need to compare the different options available on an environmental basis. This study proposes a life cycle assessment framework integrating the Lashof accounting methodology, which enables the assessment and comparison of different carbon mitigation projects (e.g., biofuel use, a sequestering plant, an afforestation project). The Lashof accounting methodology is chosen amid other methods of greenhouse gas (GHG) emission characterization for its relative simplicity and capability to characterize all types of carbon mitigation projects. Using the unit of megagram‐year (Mg‐year), which accounts for the mass of GHGs in the atmosphere multiplied by the time it stays there, the methodology calculates the cumulative radiative forcing caused by GHG emission within a predetermined time frame. Basically, the developed framework uses the Mg‐year as a functional unit and isolates impacts related to the climate mitigation function with system expansion. The proposed framework is demonstrated with a case study of tree ethanol pathways (maize, sugarcane, and willow). The study shows that carbon mitigation assessment through life cycle assessment is possible and that it could be a useful tool for decision makers, as it can compare different projects regardless of their original context. The case study reveals that system expansion, as well as each carbon mitigation project's efficiency at reducing carbon emissions, are critical factors that have a significant impact on the results. Also, the framework proves to be useful for treating land‐use change emissions, as they are considered through the functional unit.     

The incentive gap: LULUCF and the Kyoto mechanism before and after Durban

To‐date, forest resource‐based carbon accounting in land use, land use change and forestry (LULUCF) under the United Nations Framework Convention on Climate Change (UNFCCC), Kyoto Protocol (KP), European Union (EU) and national level emission reduction schemes considers only a fraction of its potential and fails to adequately mobilize the LULUCF sector for the successful stabilization of atmospheric greenhouse gas (GHG) concentrations. Recent modifications at the 2011 COP17 meetings in Durban have partially addressed this basic problem, but leave room for improvement. The presence of an Incentive Gap (IG) continues to justify reform of the LULUCF carbon accounting framework. Frequently neglected in the climate change mitigation and adaptation literature, carbon accounting practices ultimately define the nuts and bolts of what counts and which resources (forest, forest‐based or other) are favored and utilized. For Annex I countries in the Kyoto Mechanism, the Incentive Gap under forest management (FM) is significantly large: some 75% or more of potential forestry‐based carbon sequestration is not effectively incentivized or mobilized for climate change mitigation and adaptation (Ellison et al. 2011a). In this paper, we expand our analysis of the Incentive Gap to incorporate the changes agreed in Durban and encompass both a wider set of countries and a larger set of omitted carbon pools. For Annex I countries, based on the first 2 years of experience in the first Commitment Period (CP1) we estimate the IG in FM at approximately 88%. Though significantly reduced in CP2, the IG remains a problem. Thus our measure of missed opportunities under the Kyoto and UNFCCC framework – despite the changes in Durban – remains important. With the exception perhaps of increased energy efficiency, few sinks or sources of reduced emissions can be mobilized as effectively and efficiently as forests. Thus, we wonder at the sheer magnitude of this underutilized resource.     

Carbon sequestration and climate change mitigation using macroalgae: a state of knowledge review

The conservation, restoration, and improved management of terrestrial forests significantly contributes to mitigate climate change and its impacts, as well as providing numerous co-benefits. The pressing need to reduce emissions and increase carbon removal from the atmosphere is now also leading to the development of natural climate solutions in the ocean. Interest in the carbon sequestration potential of underwater macroalgal forests is growing rapidly among policy, conservation, and corporate sectors. Yet, our understanding of whether carbon sequestration from macroalgal forests can lead to tangible climate change mitigation remains severely limited, hampering their inclusion in international policy or carbon finance frameworks. Here, we examine the results of over 180 publications to synthesise evidence regarding macroalgal forest carbon sequestration potential. We show that research efforts on macroalgae carbon sequestration are heavily skewed towards particulate organic carbon (POC) pathways (77% of data publications), and that carbon fixation is the most studied flux (55%). Fluxes leading directly to carbon sequestration (e.g. carbon export or burial in marine sediments) remain poorly resolved, likely hindering regional or country-level assessments of carbon sequestration potential, which are only available from 17 of the 150 countries where macroalgal forests occur. To solve this issue, we present a framework to categorize coastlines according to their carbon sequestration potential. Finally, we review the multiple avenues through which this sequestration can translate into climate change mitigation capacity, which largely depends on whether management interventions can increase carbon removal above a natural baseline or avoid further carbon emissions. We find that conservation, restoration and afforestation interventions on macroalgal forests can potentially lead to carbon removal in the order of 10's of Tg C globally. Although this is lower than current estimates of natural sequestration value of all macroalgal habitats (61–268 Tg C year⁻¹), it suggests that macroalgal forests could add to the total mitigation potential of coastal blue carbon ecosystems, and offer valuable mitigation opportunities in polar and temperate areas where blue carbon mitigation is currently low. Operationalizing that potential will necessitate the development of models that reliably estimate the proportion of production sequestered, improvements in macroalgae carbon fingerprinting techniques, and a rethinking of carbon accounting methodologies. The ocean provides major opportunities to mitigate and adapt to climate change, and the largest coastal vegetated habitat on Earth should not be ignored simply because it does not fit into existing frameworks.