冰川冻土, 2023, 45(2): 318-326 DOI: 10.7522/j.issn.1000-0240.2023.0024

第二次青藏高原综合科学考察研究

IPCC第六次评估报告解读: 北半球多年冻土碳的观测结果和预估

刘和斌,1,3, 母梅1,3, 牟翠翠,1,2,3, 吴晓东4

1.兰州大学 资源环境学院 西部环境教育部重点实验室,甘肃 兰州 730000

2.青海师范大学 高原科学与可持续发展研究院,青海 西宁 810016

3.兰州大学 祁连山冻土生态环境野外科学观测研究站,甘肃 兰州 730000

4.中国科学院 西北生态环境资源研究院 冰冻圈科学国家重点实验室 藏北高原冰冻圈特殊环境与灾害国家野外科学观测研究站,甘肃 兰州 730000

Interpretation of IPCC Sixth Assessment Report: observations and projections of permafrost carbon in the Northern Hemisphere

Hebin LIU,1,3, Mei MU1,3, Cuicui MU,1,2,3, Xiaodong WU4

1.Key Laboratory of Western China’s Environmental Systems (Ministry of Education),College of Earth and Environmental Sciences,Lanzhou University,Lanzhou 730000,China

2.Academy of Plateau Science and Sustainability,Qinghai Normal University,Xining 810016,China

3.Observation and Research Station on Eco-Environment of Frozen Ground in the Qilian Mountains,Lanzhou University,Lanzhou 730000,China

4.Cryosphere Research Station on the;Qinghai-Tibet Plateau,State Key Laboratory of Cryospheric Science,Northwest Institute of Eco-Environment and Resources,Chinese Academy of Sciences,Lanzhou 730000,China

通讯作者: 牟翠翠,教授,主要从事冻土碳循环与气候变化研究. E-mail:mucc@lzu.edu.cn

收稿日期: 2022-05-01   修回日期: 2022-09-12  

基金资助: 国家重点研发计划“北极快速变化的机理、影响及其气候效应研究”项目.  2019YFA0607003
第二次青藏高原综合科学考察研究项目.  2019QZKK0605
兰州大学中央高校基本科研业务费专项.  lzujbky-2021-ct13.  lzujbky-2023-eyt01
甘肃省科技计划项目基础研究创新群体.  23JRRA1171

Received: 2022-05-01   Revised: 2022-09-12  

作者简介 About authors

刘和斌,博士研究生,主要从事多年冻土研究.E-mail:liuhb18@lzu.edu.cn

摘要

政府间气候变化专门委员会(IPCC)第六次评估报告(AR6)第一工作组报告对多年冻土区土壤碳储量、碳汇效应及未来气候情景下温室气体排放进行了归纳和总结。报告明确指出,北半球多年冻土区表层土壤和深层沉积物的有机碳储量为1 460~1 600 PgC(1 Pg=10亿吨)(中等信度)。随着气候持续变暖,多年冻土显著退化,土壤有机质迅速分解并以二氧化碳(CO2)或甲烷(CH4)的形式释放到大气中,加速了气候变暖。在未来全球变暖情景下,近地表多年冻土面积将显著减少,并向大气释放CO2和CH4,造成多年冻土碳与气候的正反馈作用。报告还指出,预计到2100年,气温每升高1 ℃,多年冻土区CO2和CH4的排放量分别相当于18(3.1~41) PgC和2.8(0.7~7.3) PgC(低信度)。但由于所使用的估算数据异质性较大及模型之间的一致性有限,并且对多年冻土环境驱动因素及过程模型的认知尚不完整,故多年冻土对气候变化反馈的时间及幅度的可信度还处于较低水平。

关键词: IPCC第六次评估报告 ; 多年冻土 ; 甲烷 ; 二氧化碳

Abstract

The first working group report of the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) summarized soil carbon storage, carbon source and sink effects, and greenhouse gas emissions under future climate scenarios in permafrost regions. The report identifies that 1 460~1 600 Pg of organic carbon is stored in surface soils and deep sediments in permafrost regions of the Northern Hemisphere (medium confidence). As the climate continues to warm, permafrost degrades significantly, allowing soil organic matter to decompose rapidly and release into the atmosphere as carbon dioxide (CO2) or methane (CH4), accelerating climate warming. Under future global warming scenarios, near-surface permafrost will decrease significantly, which will in turn release CO2 and CH4 into the atmosphere, resulting in a positive feedback effect of carbon-climate. The report also points out that by 2100, the CO2 and CH4 releases in permafrost regions are expected to be 18 (3.1~41) PgC and 2.8 (0.7~7.3) PgC for every 1 ℃ increase in temperature (low confidence). However, due to the wide range of estimated data and limited agreement among models, as well as the incomplete understanding of driving factors and carbon models in the permafrost environment, there is low confidence in the timing and magnitude of permafrost climate feedback.

Keywords: IPCC Sixth Assessment Report ; permafrost ; methane ; carbon dioxide

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本文引用格式

刘和斌, 母梅, 牟翠翠, 吴晓东. IPCC第六次评估报告解读: 北半球多年冻土碳的观测结果和预估. 冰川冻土[J], 2023, 45(2): 318-326 DOI:10.7522/j.issn.1000-0240.2023.0024

Hebin LIU, Mei MU, Cuicui MU, Xiaodong WU. Interpretation of IPCC Sixth Assessment Report: observations and projections of permafrost carbon in the Northern Hemisphere. Journal of Glaciology and Geocryology[J], 2023, 45(2): 318-326 DOI:10.7522/j.issn.1000-0240.2023.0024

0 引言

多年冻土定义为温度在0 ℃或低于0 ℃至少连续存在两年的岩土层,其主要分布在高纬度和高海拔地区,约占北半球陆地面积的22%1。多年冻土区土壤有机碳储量约占全球碳储量的一半以上,相当于植被和大气碳储量的总和2。在全球变暖的背景下,多年冻土退化导致长期封存的有机碳被微生物分解,以温室气体的形式释放到大气中,对气候变化造成正反馈效应。因此,多年冻土碳对气候变化的反馈是国际前沿科学问题,明确其释放速率也是降低未来气候变化预估不确定性的关键内容之一。

IPCC第四次评估报告(AR4)指出全球气候变暖导致多年冻土面积在减少,冻土融化深度也在不断增加。IPCC第五次评估报告(AR5)指出,自1980年代初以来,由于气温升高和积雪变化,大多数地区的多年冻土温度都有所上升(高信度)。此外,根据多模式预估的结果表明,随着全球地表平均温度升高,近地表多年冻土范围(指表层3.5 m深度有多年冻土发育的地区)的平均值可能减少37%(Representative Concentration Pathway(RCP)2.6情景)到81%(RCP8.5情景)(中等信度1)。AR5中还给出了多年冻土区土壤有机碳库的储量变化3-4,但其存在很大的不确定性。随着全球气候变暖,多年冻土退化很可能会造成CO2和CH4的大量排放,导致现有的多年冻土区成为含碳温室气体的净排放源。预计在21世纪,在RCP8.5情景下,多年冻土退化将排放180~920 Pg CO2(根据CO2和C的分子量关系,180~920 Pg的CO2相当于50~250 PgC)(低信度)。IPCC第六次评估报告(AR6)第一工作组报告在AR5、《全球1.5 ℃增温特别报告》(SR1.5)和《气候变化中的海洋和冰冻圈特别报告》(SROCC)的基础上,结合最新的观测和模拟结果,对多年冻土区土壤有机碳储量、碳源汇效应及温室气体的未来排放情况进行了全面总结和评估。本文根据IPCC(AR6)报告中对北半球多年冻土碳的相关内容和讨论进行解读。

1 多年冻土碳库及气候效应

数千万年以来,多年冻土区的低温环境限制了土壤有机质的分解,使埋藏在冻土层中的动植物残体以有机碳的形式逐渐累积。据估计,北半球多年冻土区(图1)表层土壤和深层沉积物的有机碳储量为1 460~1 600 Pg5-7(中等信度),其中多年冻土区表层0~3 m土壤和沉积物的碳储量约为1 070~1 360 Pg,表层0~1 m土壤的有机碳储量约为300~400 Pg,其余的有机碳存储在深层沉积物中。此外,280~340 PgC储存在北半球多年冻土区内的无多年冻土的地区(由于多年冻土埋藏在地表以下,精确的多年冻土分布范围资料不可获得,因此按大致的范围划分多年冻土区,其中包括一些实际上没有多年冻土的地区)。除北极地区以外,在其他寒冷的地区(如高海拔地区)也存在多年冻土,但这些地区的有机碳含量比北极要低。例如,青藏高原多年冻土区表层2 m的土壤碳储量估算值约为(27.9±6.2) Pg8,其约占北极表层2 m土壤碳储量的3%左右。与极地地区相比,高寒多年冻土区有机碳含量存在空间异质性和分析方法的差异,因此青藏高原多年冻土碳储量的估算存在较大的差异9-13。多年冻土区作为全球碳库的重要组成部分,在全球变暖的气候背景下,其升温速率是全球平均水平的两倍多,这可能会导致多年冻土加速退化并促进微生物分解有机质释放温室气体的过程,从而使气候进一步变暖613

图1

图1   北半球多年冻土分布14-15

Fig. 1   The distribution of permafrost in the Northern Hemisphere14-15


此外,地下冰的大量融化、湖泊扩张、排干和火灾等过程会导致多年冻土的快速崩塌。在山地多年冻土区(如青藏高原),冻土快速退化会形成热融滑塌,热侵蚀沟以及活动层边坡坍塌,并且多年冻土的退化将会降低边坡的稳定性,并增加冰川湖的数量和面积(高信度);而在排水不良的地区,冻土快速退化会形成热喀斯特湿地和热融湖塘1316。在这一些地形上,多年冻土的融化通常会因滑坡运输、原地淹没或排水等原因,从而间接改变多年冻土层的水文状态17。若融化的多年冻土层物质暴露在还原条件下,土壤碳的矿化速率会因缺氧而受限,但CH4的释放量会增加。随着冻融特征的稳定和生态演替,多年冻土退化区的碳平衡也会发生变化(例如,热喀斯特湖泊在千年时间尺度上,会从最初形成时的碳源转变为碳汇)18-20

目前,多年冻土退化现象广泛存在,利用气候模型预测发现,在中到高强度的全球变暖(升温2~4 ℃)情景下,北极地区许多浅层(深度小于3 m)的多年冻土将会融化,从而导致多年冻土厚度和范围的减少(图2)。近几十年来,研究发现青藏高原多年冻土区的年平均地温和活动层厚度均有所增加21,表明多年冻土正在退化。此外,预计到2100年,在RCP2.6和RCP8.5情景下,青藏高原的多年冻土面积也将大幅减少(减少22%~64%)22。AR6指出,21世纪多年冻土区温室气体的释放量存在很大的不确定性,其中以CO2形式可以排放数百至数千亿吨的碳,而CH4排放量可高达约100 TgCH4·a-1(中等信度)。这些温室气体排放将增加大气中CO2和CH4的含量,然而根据多年冻土生态系统模型的预测,虽然在未来气候情景下多年冻土退化会促进气候变暖,但这还并不足以使全球气温达到“失控变暖”的情况。

图2

图2   北极多年冻土区0~3 m碳储量(NCSCDv2数据集)(a)及其易受气候影响快速退化的多年冻土范围(环北极热喀斯特地貌数据集)(b)(修改自IPCC AR6)

Fig. 2   Quantity of carbon stored in the permafrost, to 0~3 m depth (NCSCDv2 dataset) (a) and area of permafrost vulnerable to abrupt thaw (Circumpolar Thermokarst Landscapes dataset) (b) (Modified from IPCC AR6)


2 多年冻土碳观测

2.1 二氧化碳

从过去几百年至上千年以来,多年冻土区一直是一个重要的碳汇(高信度)23-24,但目前部分多年冻土区已成为CO2源(高信度)。AR6中的研究通过对多年冻土快速退化区的生态系统呼吸25、多年冻土融化影响的河流中溶解性有机碳26-27以及热融湖塘产生的CH428进行14C年代测定,发现人为活动造成的气候变暖导致了多年冻土中老碳的分解,从而增加了多年冻土区中的碳排放。

众多的证据表明多年冻土碳损失呈增加趋势,但由于空间异质性高、季节性周期强及难以进行全年持续监测等因素,很难通过增加站点和生态系统级别的监测来整体评估多年冻土区的净碳收支。SROCC的评估表明北半球多年冻土区生态系统在夏季表现为碳汇,而冬季碳损失显著(高信度)。例如,从单个站点的长期观测来看,阿拉斯加巴罗地区初冬期间CO2排放量在数十年来呈现小幅增加29-31;而且,Natali等32通过对目前已有冬季通量的实地观测资料的综合分析表明,环北极多年冻土区在非生长季有大量的碳损失。秋冬季节生态系统呼吸速率增加是生态系统模型预测和自上而下(大气反演)多年冻土退化在大尺度上的一个关键特征33。Commane等34研究通过推测阿拉斯加苔原带(1.6×106 km2)的多年的净二氧化碳源,再将其按比例扩展到北半球多年冻土区(17.8×106 km2),发现该区域的净碳损失相当于0.3 PgC·a-1(低信度)31。然而,有关冬季生态系统碳排放的观测站点较少、记录时间尺度太短,对其未来变化趋势还具有很大的不确定性。

自AR5以来,通过增加CO2季节周期循环的监测,AR6中发现了北半球高纬度地区碳循环更为活跃的证据。然而,碳循环在当地来源的相对作用与中纬度地区的影响相比,很难从个别站点的观测结果扩展到整个生态系统,使得难以通过这些观测从而推断北极生态系统碳循环的变化35-38。例如,大气反演模型对北半球高纬度地区CO2通量的估计结果显示,该区域生态系统夏季对CO2的吸收量较高,导致了CO2浓度的季节性幅度增加,但年CO2总通量没有显著变化趋势,这与过去10年(2004—2013年)西伯利亚雅库茨克通量塔对CO2通量的估算结果相一致3739

2.2 甲烷

AR6指出,北半球多年冻土区CH4排放对全球甲烷收支也有贡献,但关于这些排放是否因多年冻土融化而增加的证据是“混杂”的,还未得出统一的结论。SROCC报告也表明在北半球多年冻土区,不同来源(非多年冻土融化过程,包括湖泊面积变化、冰盖覆盖的地质源及北冰洋浅层大陆架)的CH4排放数据的可信度较低2840-41。例如,观测结果表明,过去50年北极区域湖泊面积变化导致CH4的增加速率为1.6~5 Tg CH4·a-1[28。浅层海底CH4的排放量也具有很大的不确定性,其估算范围较大(3~17 Tg CH4·a-1[42-43),而且缺乏可以推断任何变化的基线。

全球和区域尺度上开展的大气测量和反演模型结果显示,过去30年北半球多年冻土区年平均CH4排放量没有呈现明显的变化趋势3844-45。例如,尽管气温显著升高,但阿拉斯加地区通过大气测量的结果表明,该区域的CH4排放量没有明显的年际变化趋势29。此外,大气反演和生物圈模型结果显示,2000—2016年期间高纬度湿地地区CH4排放无明显趋势44-47,但由于湿地范围存在较大的不确定性,且监测数据有限,因此这些模型的结果可信度较低。SROCC报告也指出由于在空间和时间尺度上CH4通量的变化范围较大,一直未得到充分监测,并且利用自上而下(大气反演)和自下而上(通过通量观测和基于过程的模型)方法计算的CH4收支状况一直存在不一致性。例如,通过地面观测资料扩展计算的CH4排放量通常高于大规模大气反演推断出的排放量48。强有力的证据表明,过去几十年来一些地区的CH4排放源有所增加(中等信度),但在北半球多年冻土区,几十年来未发现CH4通量明显的变化趋势,鉴于目前监测和模拟系统的分辨率较低且观测数据有限,该结果为低信度。

除了陆地部分以外,北极大陆架浅海沉积物中储存着部分碳,其在海底多年冻土层中以甲烷水合物的形式储存。若这些物质分解,就会释放出CH4,并随着气泡上升排放到地表。据估计,全球甲烷水合物储量为1 500~2 000 PgC49-50,其主要储存在海洋沉积物,北极(包括海底和陆架)多年冻土层及其下方的甲烷水合物中只储存了约20 PgC51。目前,陆架甲烷水合物中的CH4排放量小于10 Tg CH4·a-1[4552。尽管存在极地放大效应,但研究表明53-55,末次冰期冰消期期间北极陆架甲烷水合物中整体CH4的排放量较少,因此北极海底多年冻土中甲烷水合物大量释放CH4的可能性很小56-58。此外,甲烷水合物的不稳定性与较长的时间尺度相关59,在未来的一个世纪中,海底多年冻土中只有小部分现有的甲烷水合物可能会被破坏。即使CH4从海底水合物中释放出来,预计其大部分在到达大气之前会在海洋中被消耗和氧化成CO2。此外,模型结果也表明,海底多年冻土中甲烷水合物融化释放到大气中的CH4不足目前人类活动引起CH4排放的2%。综上所述,在未来的几个世纪里,全球甲烷水合物中的CH4排放不太可能使气候显著变暖。

3 多年冻土碳释放的未来预估

多年冻土的变化过程非常复杂,由于其强烈的气候反馈作用已被纳入到反映气候和碳循环相互作用的模型当中。多年冻土碳模型的模拟结果显示,多年冻土中含碳温室气体的释放将会造成碳与气候的正反馈作用,而且影响范围广泛,其相当于全球气温每上升1 ℃所释放的CO2和CH4的含量,经换算相当于14~175 Pg的CO2。相比之下,2019年人类活动向大气中排放了约40 Pg的CO2,这表明多年冻土退化对气候变暖的影响非常大。因此,在计算将气候稳定在全球变暖给定水平上所需的剩余碳排放总量时,必须考虑多年冻土碳排放过程。多年冻土碳释放预估的方法很多,在不同的排放情景下,对净土壤碳库变化预估正负(源汇)和大小上存在较大的差别(图3)。

图3

图3   中、高排放情景下到2100年环北极多年冻土区累积净土壤碳库变化的预估(修改自SROCC)

Fig. 3   Estimates of cumulative net soil carbon pool change for the northern circumpolar permafrost region by 2100 following medium and high emission scenarios (Modified from SROCC)


多年冻土碳损失的过程可分为渐进式和突变式。渐进式过程包括活动层厚度的逐渐加深和活动层融化季的延长,其增加了溶解性有机碳的含量和滞留时间。快速崩塌(突变式)过程(热喀斯特)包括多边形景观中冰楔的融化、坡面崩塌、热融湖塘的扩张和排水等,这些过程主要发生在土壤碳含量非常高的地区1660。快速崩塌过程可贡献多年冻土退化过程中总净碳排放量的一半,其余的归因于逐渐融化过程2061。火灾发生的频率和严重程度的增加62也会导致地表植被的移除和温室气体的突然排放,从而加速多年冻土的退化63。生态反馈可以减轻和放大碳损失:即土壤有机质在分解过程中会增加营养物质的释放,促进植被的生长,从而部分抵消土壤碳损失64,但这一过程也会进一步加剧变暖的生物物理反馈65

在CMIP5中,地球系统模型(ESMs)均不包括多年冻土碳动力学。而在CMIP6中,大多数模型也未考虑多年冻土碳过程,只有少数模型加入了活动层厚度逐渐增加对碳分解的影响,此外,ESMs也没有考虑热喀斯特及火灾与多年冻土碳的相互作用等过程。CMIP6中大多数不同模式预测的平均值表明多年冻土碳与气候之间是负反馈效应,仅少数包含多年冻土碳分解的模型显示出碳-气候正反馈效应。鉴于目前ESMs评估多年冻土碳反馈的能力有限,在现有的多年冻土碳模型中没有明确指出多年冻土退化会导致一个特定气候变暖的量,从而使其成为气候系统中的一个“临界点”或阈值,导致全球变暖出现失控的现象。然而,模型预测结果表明多年冻土区CO2和CH4的排放量会随着变暖持续增加,并且这种趋势可能会持续数百年。

SROCC评估指出,在高排放情景(RCP8.5或类似情况)下,预计到2100年,气候变暖将导致北半球多年冻土碳损失较大,其最大值为240 PgC,平均值为(92±17) PgC31。到2100年,在RCP2.6和RCP4.5情景下预估的多年冻土碳反馈分别为20~58 PgC-CO2和28~92 PgC-CO261。AR6基于SROCC中发表的研究6166-70,估计全球气温每上升1 ℃,多年冻土区CO2平均排放量为18(3.1~41) PgC。到2100年,在不同气候情景下评估CH4的排放速率,预计全球气温每上升1 ℃北半球多年冻土区CH4的平均排放量所产生的温室气体效应相当于2.8(0.7~7.3) PgC。报告指出在2100年以后,模型结果(表1)表明,在高排放情景下,2100—2300年多年冻土碳反馈幅度显著增强6171。然而,多年冻土碳反馈与气候变暖是否大致呈线性关系67,还是以更大71-72或更小20的速率变化还存在不确定性。目前,在给定的全球变暖水平下,多年冻土碳库是否具有单一的突变阈值尚不清楚73

表1   北极多年冻土区CO2和CH4排放量的模型预估结果

Table 1  Model estimates of CO2 and CH4 emissions in the Arctic permafrost region

项目2100年2100—2300年参考文献
RCP4.5RCP8.5RCP2.6RCP4.5RCP8.5
CO2排放量/PgC20~40115~17261
50~25081~64214~547174
CH4排放量/Tg CH4538~2 356836~2 6142 000~6 1002 800~7 40061
4 1005 30010 00016 00020

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综上所述,在全球变暖的背景下,陆地多年冻土退化将导致碳排放(高信度)。然而,由于已发表的多年冻土相关数据范围较大,其驱动因素和关系模型的知识和表述不完整,因此对多年冻土气候反馈的时间、幅度和线性度的可信度较低。预计到2100年,气温每升高1 ℃北半球多年冻土区CO2和CH4的排放量分别为18(3.1~41) PgC和2.8(0.7~7.3) PgC。

4 结论

IPCC AR6第一工作组报告对多年冻土碳的变化和预估得出了一些新认识,有助于我们理解在全球变暖背景下多年冻土碳排放对气候的影响。北极多年冻土退化过程中释放的气体是大气中一个重要的温室气体来源,其比海底甲烷水合物的影响更大。目前,部分气候和碳循环模型已开始考虑多年冻土过程,虽然这些模型在估算多年冻土排放到大气中温室气体的量还存在分歧,但报告一致认为:

(1)从多年冻土中释放的温室气体的量将随着全球变暖的程度而增加(高信度);

(2)多年冻土退化所引起的变暖效应非常显著,在预估未来气候变暖的剩余碳收支时需要加以考虑。

本文中使用AR6中的“很低”、“低”、“中等”、“高”和“很高”五个限定词对研究结论的有效信度进行定性评估。

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