IPCC报告第六次中文版,IPCC评估报告系列解读③人类活动对气候系统多圈层影响显著

2024-11-15 篮球阅读 10

IPCC 第六次气候变化评估报告概览

2023年3月20日，IPCC 正式发布第六次评估报告 (AR6) 综合报告《气候变化2023》。

综合报告在编写过程中经过了两轮政府评审，共收到近7千多条政府评审意见。在3月13-19日举行的 IPCC 第58次全会上，来自 195个成员国的政府代表和观察员组织代表对综合报告的《决策者摘要》进行了逐行审议，并逐页审议通过了综合报告的长报告。IPCC 第六次评估报告综合报告的发布将为国际社会和各国政府进一步推进气候行动、实现可持续发展提供重要的科学基础。

全球气候变暖及原因已明确：人类活动通过排放温室气体，导致全球变暖。2011-2020年全球地表温度比 1850-1900年升高了1.1℃。全球气候变化和影响广泛而快速，极端天气和气候事件频发。适应规划和实施在所有部门和区域都取得了进展，尽管存在适应差距。减缓气候变化的政策和法律不断增多，但与国家自主贡献 (NDCs) 相比，预计排放量仍然过高。

未来气候变化、风险和长期应对政策：持续的温室气体排放将导致全球温升进一步增加，全球温升的增加将导致危害多发并发。深度、快速和持续地减少温室气体排放可使全球变暖在大约 20年内明显减缓，并在几年内使大气成分出现明显变化。适应气候变化对于减轻气候变化风险至关重要，近期气候行动对于减少对人类和生态系统的预估损失与损害，并带来许多协同效益至关重要。

近期响应措施：紧急采取综合气候行动，大幅、快速和持续的减缓行动与加速实施的适应行动将减少对人类和生态系统的预估损失与损害，并带来许多协同效益，特别是对空气质量和健康。为实现气候韧性发展，需要所有行业与系统之间进行快速和深度转型，同时需要政治承诺、协调一致的多层治理、体制框架、法律、政策和战略以及资金和技术支持。

金融、科技与国际合作：资金、技术和国际合作是加速气候行动的关键推动因素。如果要实现气候目标，适应和减缓资金都需要成倍增加。加强技术创新系统是加速广泛采用技术和实践的关键。多种渠道可加强国际合作，资金和技术的转移有助于有效实施气候行动。

总之，全球气候变暖和影响的加剧是不争的事实，需要全球共同努力，采取紧急、综合、大规模和持续的行动，以限制全球温升，减少温室气体排放，并实施适应措施，以实现气候韧性发展和可持续发展目标。

专家解读 | IPCC 第六次评估综合报告：未来制定气候政策的指导文件

专家解读：IPCC 第六次评估综合报告——未来气候政策的行动指南

2023年3月20日，联合国政府间气候变化专门委员会 (IPCC) 在瑞士因特拉肯发布了备受瞩目的第六次评估报告 (AR6) 的综合报告《气候变化2023》(Synthesis Report)。作为 IPCC 评估周期的收官之作，该报告总结了过去一个多世纪全球气候变化的科学事实、影响、风险以及应对策略，对制定未来气候政策具有重要指导意义。

IPCC 作为全球权威的气候科学机构，其报告由 195个国家和地区代表严格审议，反映了政府的集体智慧和权威。自2021年起，IPCC 已陆续发布了多个工作组报告，包括自然科学基础、影响与适应、减缓气候变化等内容，还发布了《全球升温1.5°C》等特别报告，强调了全球气温上升的严峻形势和减排紧迫性。

IPCC报告第六次中文版,IPCC评估报告系列解读③人类活动对气候系统多圈层影响显著

报告指出，化石燃料的使用导致全球气温比工业化前水平上升 1.1摄氏度，加剧了极端天气和生态系统风险。为了将升温控制在 1.5摄氏度以内，全球需立即采取深度减排行动，转向气候韧性的发展路径，包括使用清洁能源、低碳交通和改善空气质量等。此外，报告呼吁增加气候投资，推动政策协调和国际合作，以确保气候行动的效益最大化和公平性。

综合报告强调了各国政府在减缓和适应气候变化方面的关键角色，提供了改进策略的清单。它发出明确信号，未来十年，不采取气候行动将不再被容忍。IPCC 主席李会晟表示，这份报告将是制定气候政策的重要文件，指导各国政府在关键时期采取决定性行动。

IPCC 作为联合国环境规划署和世界气象组织的合作机构，其工作分为三个核心工作组，分别负责自然科学、影响与适应、减缓气候变化的研究。每6至7年发布一次评估报告，对全球气候变化的科学理解与应对策略起到关键的推动作用。

IPCC 评估报告系列解读③人类活动对气候系统多圈层影响显著

专家：国家气候中心首席科学家孙颖

政府间气候变化专门委员会 (IPCC) 最新发布的第六次评估报告指出，毋庸置疑的是，自工业化以来，人类的影响已使大气、海洋和陆地持续变暖。

人类活动对气候系统的影响一直是 IPCC 历次评估报告的核心内容。从第一次评估报告到第六次评估报告，随着科学界对气候系统变化认识的不断加深，人类活动对气候系统影响程度的评估信度也逐渐提高。国家气候中心首席科学家孙颖表示，这种认识的加深得益于观测资料的增加、气候模式性能的改善以及归因方法学的改进。

评估变量更全面内容更系统

IPCC 第六次评估报告第一工作组报告基于全球最新的观测和模式结果，用一章的内容系统评估了人类活动对大气和地表、冰冻圈、海洋、生物圈以及气候变率模式的影响。

孙颖表示，与第五次评估报告相比，此次评估的变量更加全面、内容更为系统。评估的成员不仅包括传统的气候变量，如气温、降水等，还增加了生物圈等其他圈层变量，从而减少了对单一变量评估带来的不确定性。

在气候系统的不同圈层，包括大气、海洋、冰冻圈和地表气候变化等指标的变化中，均可检测到人类活动的影响，这些影响与模式模拟和基于物理机制预期的理解相一致。自工业化以来，人类活动的影响已经使全球气候系统变暖，这一评估结论的信度在第六次评估报告里面进一步提高。

全球变暖人类活动信号愈发清晰

近年来，全球气温持续上升，与工业化前 (1850-1900年) 相比，2011-2020年全球平均气温升高了 1.09℃。孙颖表示，由于对不确定性理解的改进以及持续的变暖，第六次评估报告可对自工业化革命以来的变暖进行归因，而第五次评估报告只能对 1951年以来的变暖归因。换句话说，人为信号的检测从第五次评估报告的 1951年提早到了 1850年，明确指出自工业革命以来的气候变化主要是由人类活动造成的。

由于全球气温持续升高，人类活动的信号愈发清晰。“但将观测到的变暖归因于某种特定的人为强迫仍然具有较大的不确定性。”孙颖补充道。

此外，与 1951-2012年相比，1998-2012年全球平均气温升温速率存在一个短暂变缓的现象。孙颖解释说，升温变缓是暂时的，究其原因可能是太平洋年代际变化以及太阳活动和火山爆发的变化部分抵消了人为活动导致的地表变暖趋势。在此期间，全球海洋热含量仍在持续增加，表明整个气候系统是持续变暖的。

2012年之后，全球平均气温急剧升高。数据显示，2016-2020年这五年至少是自 1850年有仪器观测记录以来最热的五年。

与此同时，人类活动对降水也产生了不小影响。孙颖表示，20世纪中期以来，人类活动很可能影响了大尺度的降水变化，在北半球中高纬陆地降水的增加中就检测到人类活动的影响。此外，人类活动还影响了湿润的热带和干燥的亚热带之间纬向平均降水差异的增加，1979年以来南半球夏季降水在高纬度地区增加和在中纬度地区减少都可能与人类活动有关。

随着全球变暖，近年来，极端天气气候事件也呈现多发频发的态势。第六次评估报告指出，全球和大多数大陆极端冷事件和极端暖事件变化的主要原因很可能是人类活动引起的温室气体强迫。近几十年全球陆地强降水加剧也可能是受人类活动的影响。

孙颖表示，由于最新的 CMIP6 检测归因模式比较计划资料的使用，第六次评估报告对于大尺度极端温度的变化归因更明确，指出温室气体强迫在极端气温变化中占主导作用，而在极端降水的变化中，温室气体的作用也可以在一些指标中检测到。

人类活动对气候系统影响进一步明晰

相较之前的评估报告，第六次评估报告进一步明晰了人类活动对气候系统的影响，这种影响可以在气候系统的多个圈层中检测到。

在冰冻圈，20世纪70年代后期以来北极海冰损耗的主要驱动因子很可能是人类活动。孙颖指出，尽管模式对北极海冰平均状态的模拟存在很大差异，但所有 CMIP5 和 CMIP6 模式都再现了近几十年来海冰范围和厚度的缩减。

IPCC报告第六次中文版,IPCC评估报告系列解读③人类活动对气候系统多圈层影响显著

此外，北半球 1950年以来春季积雪的减少也与人类活动有关，CMIP6 模式比 CMIP5 模式更好地再现了北半球积雪的季节变化周期。同时，人类活动也很可能是最近全球范围内几乎普遍发生的冰川退缩的主要驱动因子，比如过去 20年格陵兰冰盖表面融化很可能是受人类活动影响。

20世纪70年代以来全球海平面上升和海洋热含量增加的主要驱动因子极可能是人类活动。孙颖表示，综合考虑冰川、冰盖表面物质平衡和热膨胀的贡献，人类影响至少是观测到的 1970年以来全球平均海平面上升的主要驱动因子；而观测到的海洋热含量的增加已经延伸至深海，工业化以来 (1850-2014年) 海洋上层 (0-700米)、中层 (700–2000米)、深层 ( 2000米) 分别吸收了 58%、21% 和 22% 的热量。此外，人类活动也影响着海洋盐度，主要表现为 20世纪中期以来海洋表层和次表层盐分低的区域变得更淡，而盐分高的区域变得更咸。

在生物圈，同样检测到人类活动的影响。全球海洋酸化与人类活动排放的二氧化碳关系密切，而大气二氧化碳浓度增加可能导致植物生长施肥效应增强等。

IPCC 评估报告系列解读⑧减缓气候变化，除了减排还有哪些方法？

专家顾问：浙江大学地球科学学院教授曹龙

自工业革命以来，大气中温室气体浓度持续上升，减缓全球变暖需要大幅度、迅速和持续地减少二氧化碳等温室气体的排放。联合国政府间气候变化专门委员会 (IPCC) 第六次评估报告 (AR6) 指出，将人类活动造成的全球升温控制在一个特定的水平需要限制累积的二氧化碳排放，即至少实现净零二氧化碳排放，同时大力减少其他温室气体排放。

除了减少温室气体排放，还有哪些方法可以减缓全球变暖？据曹龙介绍，IPCC 还评估了其他两种方法：一种是二氧化碳移除，即通过人为的方式增加海洋或陆地碳汇，或直接从大气中捕捉二氧化碳并封存；另一种是太阳辐射干预，即通过人为的方法减少到达地-气系统的太阳辐射，或增加逃逸到太空的长波辐射。

IPCC 气候变化情景预测中的控温 1.5 或 2℃ 的低排放情景，将在 IPCC Sixth Assessment Report Overview

IPCC Sixth Assessment Report Overview

On March 20, 2023, the IPCC officially released its Sixth Assessment Report (AR6) Synthesis Report, "Climate Change 2023."

The synthesis report went through two rounds of government reviews, receiving nearly 7,000 government review comments. During the IPCC's 58th plenary session from March 13-19, government representatives and observer organization representatives from 195 member countries reviewed the report's "Summary for Policymakers" line by line and approved the full report page by page. The release of the IPCC's Sixth Assessment Report Synthesis Report provides a crucial scientific foundation for the international community and national governments to further advance climate action and achieve sustainable development.

Global warming and its causes are clear: human activities have caused global warming by emitting greenhouse gases. Global surface temperatures have risen by 1.1°C from 1850-1900 to 2011-2020. Global climate change and its impacts are widespread and rapid, and extreme weather and climate events are increasing. Adaptation planning and implementation are progressing across all sectors and regions, although adaptation gaps exist. Climate change mitigation policies and laws are becoming more prevalent, but emissions are still projected to be too high compared to national determined contributions (NDCs).

Future climate change, risks, and long-term response policies: Continued greenhouse gas emissions will lead to further increases in global warming. Increases in global warming will result in more frequent and severe hazards. Deep, rapid, and sustained reductions in greenhouse gas emissions would slow global warming considerably within about 20 years and lead to marked changes in atmospheric composition within a few years. Adapting to climate change is crucial for mitigating climate change risks, and immediate climate action is essential to reduce projected losses and damages to humans and ecosystems and generate many co-benefits.

Recent response measures: Comprehensive climate action is urgently needed. Significant, rapid, and sustained mitigation actions, together with accelerated adaptation actions, will reduce projected losses and damages to humans and ecosystems and bring many co-benefits, especially for air quality and health. For climate-resilient development, rapid and deep transformations across all sectors and systems are needed, along with political commitment, coordinated multi-level governance, institutional frameworks, laws, policies, and strategies, as well as financial and technological support.

Finance, technology, and international cooperation: Finance, technology, and international cooperation are key drivers for accelerating climate action. To achieve climate goals, both adaptation and mitigation financing need to increase manifold. Strengthening technology innovation systems is crucial to accelerate the widespread adoption of technologies and practices. Numerous channels can enhance international cooperation, and the transfer of finance and technology will contribute to the effective implementation of climate action.

In conclusion, the undeniable fact of global warming and the intensification of its impacts requires a collective global effort. Urgent, comprehensive, large-scale, and sustained actions are needed to limit global warming, reduce greenhouse gas emissions, and implement adaptation measures to achieve climate-resilient development and the Sustainable Development Goals.

Expert Interpretation | IPCC Sixth Assessment Synthesis Report: A Guiding Document for Future Climate Policy

Expert interpretation: IPCC Sixth Assessment Synthesis Report - An Action Guide for Future Climate Policies

On March 20, 2023, the Intergovernmental Panel on Climate Change (IPCC) released its highly anticipated Sixth Assessment Report (AR6) Synthesis Report, "Climate Change 2023," in Interlaken, Switzerland. As the culmination of the IPCC's assessment cycle, this report summarizes the scientific facts, impacts, risks, and mitigation strategies related to global climate change over the past century, providing crucial guidance for formulating future climate policies.

As a globally authoritative climate science body, the IPCC's reports are rigorously reviewed by representatives from 195 countries and regions, reflecting the collective wisdom and authority of governments. Since 2021, the IPCC has successively released several working group reports, covering topics such as the physical science basis, impacts and adaptation, and mitigation of climate change. It has also published special reports like "Global Warming of 1.5°C," emphasizing the urgent situation of global temperature rise and the pressing need for emission reductions.

The report highlights that fossil fuel use has led to a 1.1°C increase in global temperatures compared to pre-industrial levels, exacerbating extreme weather and ecosystem risks. To limit warming to 1.5°C, the world needs to immediately undertake deep emission reduction actions and transition to a climate-resilient development path, including the use of clean energy, low-carbon transportation, and improved air quality. Additionally, the report calls for increased climate investment, policy coordination, and international cooperation to maximize the benefits and fairness of climate action.

The synthesis report emphasizes the critical role of governments in mitigating and adapting to climate change, providing a checklist for improving strategies. It sends a clear signal that inaction on climate change will no longer be tolerated in the next decade. Hoesung Lee, IPCC Chair, stated that this report will be a crucial document for developing climate policies, guiding governments to take decisive actions at a critical juncture.

IPCC报告第六次中文版,IPCC评估报告系列解读③人类活动对气候系统多圈层影响显著

The IPCC, a collaborative body under the United Nations Environment Programme and the World Meteorological Organization, organizes its work into three core working groups, each responsible for research on physical science, impacts and adaptation, and mitigation of climate change. It releases an assessment report every 6 to 7 years, playing a key role in driving global understanding and response strategies for climate change.

IPCC Assessment Report Series Interpretation ③ Significant Impacts of Human Activities on Multi-Sphere Climate Systems

Expert: Sun Ying, Chief Scientist, National Climate Center

The latest Sixth Assessment Report (AR6) released by the Intergovernmental Panel on Climate Change (IPCC) states unequivocally that since industrialization, human influence has been the dominant cause of the observed warming of the atmosphere, ocean, and land.

The impact of human activities on the climate system has been a central theme in IPCC assessments across all cycles. From the First to the Sixth Assessment Report, as scientific understanding of climate system changes has deepened, the confidence in assessing the extent of human impact on the climate system has gradually increased. Sun Ying, Chief Scientist at the National Climate Center, noted that this deepening understanding is attributed to the increase in observational data, the improvement in climate model performance, and the advancement of attribution methodologies.

More comprehensive assessment variables and a more systematic approach

The IPCC's Sixth Assessment Report, Working Group I, systematically assessed the influence of human activities on the atmosphere and surface, cryosphere, oceans, biosphere, and climate variability patterns, drawing on the latest global observational and modeling results. The report devoted an entire chapter to this assessment.

Sun Ying pointed out that compared to the Fifth Assessment Report, this assessment incorporates more comprehensive variables and presents a more systematic approach. Its assessment members include not only traditional climate variables such as temperature and precipitation but also other sphere variables such as the biosphere, reducing uncertainties associated with single-variable assessments.

In the changes observed in various climate system spheres, including the atmosphere, ocean, cryosphere, and surface climate change indicators, human influence is detectable. These influences are consistent with modeling simulations and physical mechanism-based understanding. Since industrialization, human influence has warmed the global climate system, and the confidence in this assessment conclusion is further strengthened in the Sixth Assessment Report.

Global warming: Human activity signals become increasingly clear

Global temperatures have been steadily rising in recent years. Compared to pre-industrial levels (1850-1900), the global average temperature for 2011-2020 increased by 1.09°C. Sun Ying explained that due to improved understanding of uncertainties and continued warming, the Sixth Assessment Report can attribute warming since the Industrial Revolution, while the Fifth Assessment Report could only attribute warming since 1951. In other words, the detection of anthropogenic signals has been pushed back from 1951 in the Fifth Assessment Report to 1850 in the Sixth Assessment Report, clearly indicating that climate change since the Industrial Revolution is primarily caused by human activities.

As global temperatures continue to rise, human activity signals become increasingly clear. "However, attributing the observed warming to a specific anthropogenic forcing still carries significant uncertainty," Sun Ying added.

Additionally, there was a brief slowdown in the rate of global average temperature increase between 1998-2012 compared to 1951-2012. Sun Ying explained that this slowdown was temporary, likely due to the Pacific Decadal Oscillation, and variations in solar activity and volcanic eruptions partially offsetting anthropogenic warming trends. During this period, global ocean heat content continued to increase, indicating that the entire climate system was still warming.

Global average temperatures rose sharply after 2012. Data shows that the five years from 2016-2020 were among the warmest on record since instrumental observations began in 1850.

At the same time, human activities have also significantly impacted precipitation. Sun Ying noted that human activities have likely influenced large-scale precipitation changes since the mid-20th century, with human influence detected in the increase of precipitation in the mid-high latitude land areas of the Northern Hemisphere. Furthermore, human activities have affected the increase in the meridional average precipitation difference between humid tropics and arid subtropics, and the increase in precipitation in high latitudes and decrease in mid-latitudes in the Southern Hemisphere summer since 1979 are likely related to human activities.

With global warming, extreme weather and climate events have become more frequent and intense in recent years. The Sixth Assessment Report states that the primary driver of changes in extreme cold and warm events globally and on most continents is likely human-induced greenhouse gas forcing. The increase in heavy precipitation over global land areas in recent decades may also be influenced by human activities.

Sun Ying noted that due to the use of the latest CMIP6 detection attribution model comparison project data, the Sixth Assessment Report provides more clarity on the attribution of large-scale extreme temperature changes, stating that greenhouse gas forcing plays a dominant role in extreme temperature variations. While the role of greenhouse gases in extreme precipitation changes can also be detected in some indicators, further research is needed for a conclusive understanding.

Human impact on the climate system becomes clearer

Compared to previous assessment reports, the Sixth Assessment Report provides a clearer picture of human impact on the climate system, with this impact detectable in multiple climate system spheres.

In the cryosphere, the primary driver of Arctic sea ice loss since the late 1970s is very likely human activities. Sun Ying emphasized that although models show significant differences in simulating the average state of Arctic sea ice, all CMIP5 and CMIP6 models reproduce the shrinking of sea ice extent and thickness in recent decades.

Furthermore, the decrease in spring snow cover in the Northern Hemisphere since 1950 is also linked to human activities, and CMIP6 models better reproduce the seasonal cycle of snow cover in the Northern Hemisphere than CMIP5 models. Simultaneously, human activities are very likely the primary driver of widespread glacier retreat observed globally in recent years. For instance, the melting of the Greenland ice sheet surface in the past two decades is likely influenced by human activities.

The primary driver of global sea level rise and ocean heat content increases since the 1970s is very likely human activities. Sun Ying explained that considering the contributions of glaciers, ice sheet surface mass balance, and thermal expansion, human influence is at least the primary driver of global average sea level rise observed since 1970. The observed increase in ocean heat content has extended to deep waters, with the upper (0-700 meters), mid (700–2000 meters), and deep layers (2000 meters) of the ocean absorbing 58%, 21%, and 22%, respectively, of the heat since industrialization (1850-2014). Additionally, human activities have influenced ocean salinity, primarily evidenced by a decrease in salinity in areas with low surface and sub-surface salinity and an increase in salinity in areas with high salinity since the mid-20th century.

Human impact is also detectable in the biosphere. Global ocean acidification is closely linked to human emissions of carbon dioxide, and the increased atmospheric carbon dioxide concentration may enhance fertilization effects on plant growth.

IPCC Assessment Report Series Interpretation ⑧ Mitigation of Climate Change: Beyond Emission Reduction, What Other Methods Exist?

Expert Consultant: Cao Long, Professor, Department of Earth Sciences, Zhejiang University

Since the Industrial Revolution, atmospheric greenhouse gas concentrations have continuously risen, and mitigating global warming requires significant, rapid, and sustained reductions in emissions of carbon dioxide and other greenhouse gases. The Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) states that limiting the global temperature rise caused by human activities to a specific level requires limiting cumulative carbon dioxide emissions, achieving net-zero carbon dioxide emissions at a minimum, and drastically reducing other greenhouse gas emissions.

Beyond reducing greenhouse gas emissions, what other methods exist to mitigate global warming? According to Cao Long, the IPCC has evaluated two other methods: one is carbon dioxide removal, which involves artificially enhancing carbon sinks in the ocean or land or directly capturing carbon dioxide from the atmosphere and storing it; the other is solar radiation modification, which involves artificially reducing incoming solar radiation to the Earth-atmosphere system or increasing outgoing longwave radiation to space.

The low-emission scenarios in IPCC climate change scenario projections for limiting warming to 1.5°C or 2°C will heavily rely on carbon dioxide removal methods. Achieving low-emission scenarios for limiting warming to 1.5°C and 2°C requires net-negative carbon dioxide emissions after the middle of this century.

Changes in atmospheric carbon dioxide are determined by anthropogenic carbon dioxide emission rates, carbon dioxide removal rates, and the absorption rates of carbon dioxide by land and ocean. The difference between the carbon dioxide removal rate and the anthropogenic carbon dioxide emission rate is the net carbon dioxide emission rate. When the absorption rate of carbon dioxide by the ocean and land exceeds the net carbon dioxide emission rate, atmospheric carbon dioxide concentration begins to decline. If the carbon dioxide removal rate surpasses the carbon dioxide emission rate, net-negative carbon dioxide emissions occur, further decreasing atmospheric carbon dioxide concentration and reversing ocean acidification trends.

AR6, after evaluating different carbon dioxide removal methods, including afforestation, biochar, etc., considering their carbon removal potential and Earth system feedback, indicates that carbon dioxide removal methods have profound impacts on biogeochemical cycles and climate. These impacts could weaken or strengthen the potential of carbon dioxide removal to remove atmospheric carbon dioxide and cool the planet, affecting water resources, food production, and biodiversity.

From the perspective of Earth's climate system response, carbon dioxide removed from the atmosphere through carbon dioxide removal methods offsets some of the carbon dioxide released by land and ocean.

Multi-model simulation results show that after a one-time removal of 100 billion tons of carbon dioxide from the atmosphere, 49% and 29% of the removed carbon dioxide would be offset by carbon dioxide released by land and ocean, respectively, after 100 years, with only 23% being truly removed from the atmosphere. The reason is that after the atmospheric carbon dioxide concentration decreases, land and ocean may transition from carbon sinks to carbon sources, offsetting some of the carbon dioxide removed from the atmosphere.

If net-negative carbon dioxide emissions are achieved and sustained, the global temperature increase trend caused by carbon dioxide accumulation would gradually reverse. However, other climate changes would continue for decades or even centuries. Multi-model comparison project simulations of carbon dioxide removal indicate that after the atmospheric carbon dioxide concentration declines, the trends in surface temperature and Arctic sea ice would gradually reverse, and global average precipitation would briefly increase before decreasing. Global sea level rise would continue for at least several centuries after net-negative carbon dioxide emissions are achieved. This highlights the significant lag in the climate system's response to carbon dioxide removal.

Solar radiation modification has significant uncertainties regarding its impact on the climate system

Solar radiation modification cools the Earth by altering the radiative balance of the Earth's climate system. Solar radiation modification methods include stratospheric aerosol injection, marine cloud brightening, and high-altitude cirrus cloud thinning. AR6 assessment indicates that solar radiation modification has the potential to serve as a complementary measure to significant emissions reductions.

Due to large uncertainties in cloud-aerosol-radiation interactions and microphysical processes, the cooling potential and climate effects of solar radiation modification have significant uncertainties. Solar radiation modification can offset some climate changes caused by increased greenhouse gases on global and regional scales, such as reducing global temperatures, stabilizing ocean currents, decreasing hurricane frequency, mitigating extreme heat, slowing sea ice melting, and stabilizing regional precipitation variations. However, solar radiation modification cannot completely offset climate changes caused by increased greenhouse gases on global and regional scales.

"It is possible to design suitable solar radiation modification methods to achieve multiple climate change objectives simultaneously," said Cao Long. For instance, injecting sulfate aerosols into the stratosphere at different latitudes could potentially stabilize both global average temperature and the temperature gradients between the Northern and Southern hemispheres and the equator and poles at current climate levels. Combining stratospheric aerosol injection (shortwave) and high-altitude cirrus cloud thinning (longwave) solar radiation modification methods could potentially stabilize both global temperature and precipitation variations. It's essential to note that abruptly halting solar radiation modification after implementation could lead to rapid climate changes.

Simulation results indicate that if solar radiation modification were abruptly stopped, temperatures would suddenly rise. However, if solar radiation modification were gradually reduced to zero under emission reduction and carbon dioxide removal scenarios, it would significantly mitigate the risk of rapid climate change caused by the abrupt cessation of solar radiation modification.

Furthermore, compared to scenarios with sustained carbon dioxide emissions, the cooling effect of solar radiation modification could indirectly enhance carbon sinks in land and ocean, thus reducing atmospheric carbon dioxide concentration.

Cao Long emphasized that carbon dioxide removal and solar radiation modification, as potential auxiliary measures for significant emissions reductions, cannot replace greenhouse gas emissions reductions. Currently, no carbon dioxide removal or solar radiation modification methods have been proven effective for mitigating global warming on a large scale, and each method has different side effects. Significant, rapid, and sustained reductions in greenhouse gas emissions remain the safest measure for mitigating global warming.