The IPCC Working Group I contribution to the Sixth Assessment Report (AR6) improves our understandings of the changes in climate system, the causes of the climate change, and the projected future changes and gives us a clearer and more reliable relationship between human activities and climate change, from the following progress. Comprehensive assessments based on multiple lines of evidence point out that climate across the globe is undergoing unprecedented changes; progress in detection and attribution including event attribution studies has expanded the understandings of human influence on the climate system from the atmosphere to the hydrosphere, cryosphere, and biosphere, further strengthening the understandings of human influence on global and regional climate; the content of relevant regional climate change information is richer, and it is more closely related to the impact of climate change in various industries and sensitive regions, providing strong support for better risk assessment and adaptation planning at regional scale; the improvement of climate models and constraint for projection and the deepened understandings of climate sensitivity have reduced the uncertainty of the projected changes in global surface temperature, ocean warming and sea level under different emission scenarios. This latest report shows great significance in providing guidance for China to improve the level of climate change research and the capability of disaster prevention and mitigation.

The IPCC Working Group I (WGI) contributes to the Sixth Assessment Report (AR6) with the most up-to-date physical understanding of the climate system and climate change, integrating multiple lines of evidence from paleoclimate, observations, process understanding, and global and regional climate simulations, and documenting the latest advances in climate science. It aims to deliver relevant content and knowledge about how climate has changed in the past, and what role human activity has played in these changes, and what is to be expected in the future given a set of emission scenarios based on different socioeconomic paths we will choose. These are important and relevant for policymakers, including climate change mitigation, regional adaptation planning based on a risk management framework, and the global stocktake coming up in 2023. In this interpretation of the report, we intend to introduce the main features of the AR6 WGI report in terms of its context, structure, and methods. Compared with previous reports, AR6 offers more integrated and actionable information and understanding, greater emphasis on regional climate change, and better-constrained climate sensitivity estimates. One of the most important conclusions of this assessment is that the influence of human activity on the warming of the climate system has evolved from theory to established fact.

The Working Group I contribution to the IPCC Sixth Assessment Report (AR6) has been released on 9 August 2021. Chapter 3 of the report, entitled “human influence on the climate system”, has quantitatively assessed the human influence on climate system and the climate model representation of observed mean climate, changes and variability. The combined evidence from across the climate system clearly indicated that it is unequivocal that human influence has warmed the atmosphere, ocean and land, and that for most large-scale indicators of climate change, the simulated recent mean climate from the latest generation Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models underpinning this assessment has improved compared to the CMIP5 models assessed in the AR5. With updated observation datasets and paleoclimate evidence, new modelling evidence, improved analysis methods, and deeper process understanding, the report has obtained more reliable and strong evidence of human influence on the climate system. However, uncertainties remain in quantification of the human influence on large-scale indicators of climate change in the atmosphere, ocean, cryosphere and at the land surface. And there is still a significant gap, after decades of works in this field. The limitation includes brevity of the observational records, poor model performance and limited process understanding.

Based on the content of Chapter 4 from the Sixth Assessment Report (AR6) contributed by the IPCC Working Group I, we interpret the future projections of global climate change. The AR6 systematically assessed possible changes of global surface air temperature, precipitation, large-scale circulation and modes of variability, and changes in ocean and cryosphere, and further reasonably estimated the climate change beyond 2100. The assessments show that global mean surface air temperature would reach 1.5℃ or even beyond it. Mean-state and variability of precipitation would increase as well, but varying with seasons and regions. Large-scale circulation and modes of variability are more affected by internal variability rather than external forcing. By the end of the 21st century, ice-free period would be seen in the Arctic. Ocean acidification and increase of global mean sea level (GMSL) would continue at the century time scale with uncertain magnitudes depending on emission scenarios. The projected GMSL would go higher beyond 2100 under all the scenarios. Multiple constraining methods are introduced in this latest assessment, reducing the uncertainty range of future projection. By paying an additional attention to the low emission scenarios and low-likelihood high-impact storylines, the AR6 provided richer and more comprehensive information for addressing climate change. Integrating the assessment conclusions, we suggest that future studies need to reduce the projected uncertainties in regional climate change, especially in the monsoon regions, and that capability construction of climate projection in China need to be strengthened in both scientific research and model development.

The IPCC recently released the Six Assessment Report (AR6), addressing the most up-to-date physical understanding of the climate system and climate change. Climate system and the carbon cycle response to carbon dioxide removal is assessed in this report. Carbon dioxide removal (CDR) is a necessary component of the IPCC low emissions scenarios of SSP1-1.9 and SSP1-2.6 that keeps global warming from preventing 1.5℃ or 2.0℃. AR6 key assessment relevant to CDR are: CDR has the potential to removal CO₂ from the atmosphere and durably store it in reservoirs (high confidence); If the amount of CO₂ removed from the atmosphere exceeds anthropogenic CO₂ emission, CDR would lead to net negative emissions, lowering atmospheric CO₂ concentration and reversing surface ocean acidification (high confidence); CO₂ sequestered from the atmosphere by CDR would be partially compensated by CO₂ release from land and ocean carbon stores (very high confidence); If global net negative CO₂ emissions were to be achieved and be sustained, the global CO₂-induced surface temperature increase would be gradually reversed but other climate changes (such as sea level rise) would continue in their current direction for decades to millennia (high confidence). CDR methods can have potentially wide-ranging effects on biogeochemical cycles and climate, which can either weaken or strengthen the potential of these methods to remove CO₂ and reduce warming, and can also influence water availability and quality, food production and biodiversity (high confidence).

The IPCC recently released the Sixth Assessment Report (AR6). The Working Group I contribution to the AR6 “Climate change 2021: the physical science basis” addresses the most up-to-date physical understanding of the climate system and climate change. Climate system and the carbon cycle response to solar radiation modification (SRM) is assessed in this report. SRM can be considered as a potential supplement to deep emission reduction to counteract anthropogenic climate change. All assessment of climate effect from SRM are from modeling work. Key AR6 assessment relevant to SRM are: SRM could offset some of the effects from increasing greenhouse gases on global and regional climate (high confidence), but there would be substantial residual or overcompensating climate change at the regional scales and seasonal time scales (virtually certain). It is possible to stabilize multiple large-scale temperature indicators simultaneously by tailoring the deployment strategy of SRM options (medium confidence). A sudden and sustained termination of SRM in a high greenhouse gas emissions scenario would cause rapid climate change (high confidence), but a gradual phase out of SRM combined with emissions reductions and carbon dioxide removal would avoid large rates of changes (medium confidence). The cooling caused by SRM would increase the global land and ocean CO₂ sinks (medium confidence), but SRM would not mitigate ocean acidification (high confidence). Our understanding of climate response to aerosol-based SRM options including stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning is limited due to large uncertainties associated with aerosol-cloud-radiation interactions.

Atmospheric components that can influence climate change can be classified as long-lived greenhouse gases and short-lived climate forcers (SLCFs), according to their lifetimes in the atmosphere. Considering the important roles of SLCFs in climate change and air quality, IPCC AR6 has for the first time the dedicated chapter for the assessment of SLCFs. This work summarizes the major conclusions on SLCFs, especially those since AR5, including the definition of SLCFs, changes in emissions and abundances of SLCFs, the effective radiative forcings of SLCFs and climate responses, projected future changes in climate and air quality under Shared Socioeconomic Pathways (SSPs), and the impact of COVID-19 lockdown on climate. We also discuss the uncertainties associated with the AR6 conclusions as well as the implications for climate and air quality in China.

This work extracts the chapter seven of the IPCC AR6 working group I on the Earth’s energy budget, climate feedback, and climate sensitivity, and gives a concise summary on the new findings and conclusions on the topic. AR6 suggests that the effective radiative forcing (ERF) from anthropogenic activity over the industrial era is 2.72 [1.96-3.48] W/m². Changes in well-mixed greenhouse gases and aerosols contribute to the total anthropogenic ERF with 3.32 [3.03-3.61] W/m² and -1.1 [-1.7--0.4] W/m², respectively. The net climate feedback parameter is assessed to be -1.16 [-1.81--0.51] W/(m²∙℃), and clouds remain the largest contribution to overall uncertainty in climate feedbacks. Equilibrium climate sensitivity (ECS) and transient climate response (TCR) are effective measures that can be used to assess the response of global mean surface air temperature to forcing factors. The best estimates of ECS and TCR are 3.0 [2.0-5.0]℃ and 1.8 [1.2-2.4]℃, respectively.

The water cycle plays an important role in global and regional climate changes, and problems such as shortage of freshwater resources, expansion of subtropical drylands, and frequent occurrence of extreme droughts and floods, which are closely related to water cycle changes in the context of global warming, are becoming increasingly prominent and seriously constrain the sustainable development of ecosystems and human society. In the IPCC Sixth Assessment Report (AR6), Working Group I established a separate chapter, namely Chapter 8, for an integrated assessment of global water cycle changes. AR6 suggested that human activity has significantly altered the global water cycle since the mid-20th century, including an overall increase in atmospheric moisture and precipitation intensity, changes in the global drought patterns, and the poleward shift of storm tracks in the Southern Hemisphere. Changes in the water cycle that have occurred were influenced by a variety of anthropogenic forcing including greenhouse gases, aerosol and land use change, while future water cycle changes will gradually be dominated by greenhouse gases.

Although climate change is a global phenomenon, its manifestations and consequences are different in different regions, and therefore climate information on spatial scales ranging from sub-continental to local is important for the impact and risk assessments of climate change. To respond to this, the WGI report of IPCC AR6 Chapter 10 assess how to link the global to regional climate change. Regional climate change is the result of the interplay between regional responses to both natural forcings and human influence, responses to large-scale climate phenomena characterizing internal variability, and processes and feedbacks of a regional nature. This chapter emphasized how to distill regional climate information from multiple observational datasets, ensembles of different model types, process understanding, expert judgement and indigenous knowledge. The distillation attribute multi-decadal regional trends to the interplay between external forcing and internal variability. Human influence has been a major driver of regional mean temperature change since 1950 in many sub-continental regions of the world. The choice of the reference period and signal-to-noise threshold is important to robustly assess the future emergence of anthropogenic signals, as well as past emergence results. Human influence has contributed to multi-decadal mean precipitation changes in several regions, internal variability can delay emergence of the anthropogenic signal in long-term precipitation changes in many land regions. Distilling regional climate information from multiple lines of evidence will increase the fitness usefulness and relevance for decision-making.

Compared to the IPCC AR5, in the AR6, evidence of observed changes in weather and climate extremes and their attribution to human influence have been strengthened. Human-induced climate change has been affecting many weather and climate extremes worldwide. With further global warming, the frequency and intensity of hot extremes, heavy precipitation, agricultural and ecological droughts in some regions, and the proportion of intense typhoon (hurricane) are projected to increase. Projected percentage changes are larger for the frequency of rarer events. These findings again raise the necessity and urgency to address climate change as well as weather and climate extremes.

The Climatic Impact-Driver (CID) framework is developed in the IPCC Sixth Assessment Report (AR6). CIDs are physical climate system conditions (e.g., means, events, extremes) that affect an element of society or ecosystems. The CID framework includes seven categories, thirty-three climate factors, and each factor can be assessed using different evaluation indices for diffeent affected sectors. The major features of CIDs include their time scale variety and irreversibility, mutation and tipping points, the time of emergence, compoundness, and their dependence on affected system elements. The CID framework is helpful for making more objective, neutral and comprehensive assessments on the impacts and risks of climate change.

Policy makers and the public are increasingly concerned about the impacts of climate change, and this requires richer, fine-scale information on current and future climate conditions at regional scales. The Atlas chapter coordinates with other chapters of Working Group I (WGI) report in the IPCC Sixth Assessment Report (AR6) to assess basic information on observations, attributions, and projection of regional climate change, and establishes an online interactive atlas system. The Atlas consists of two parts. The Atlas chapter assesses climate change in each region based on new reference regions, and focuses on observed trends and attributions of surface temperature and precipitation, and projected future changes. The Interactive Atlas, a new component of the AR6 WGI report, provides comprehensive information on observed and predicted climate change and climate change attribution over time in the form of interactive maps based on observed, global (CMIP5 and CMIP6), and regional (CORDEX) model projections.

The IPCC AR6 Working Group I gives new understandings of the predictability, irreversibility and deep uncertainty of various elements of the climate system. This paper summarizes the above three aspects of cryospheric changes in global and regional scales. In general, on the hemispheric and global scales, regardless of the emission scenario, all cryospheric elements could have certain predictability in the 21st century, i.e., they all change in the direction of melting or degradation, mostly irreversible. However, on the regional scale, short time scales and long-term commitment, different cryospheric elements may exhibit large internal variability, poor predictability and even deep uncertainty.

In the IPCC Sixth Assessment Report, the latest monitoring and simulation results indicate that the current rate of sea level rise is accelerating (3.7 mm/a) and will continue to rise in the future, showing an irreversible trend. Under low emission scenarios (SSP1-1.9) and high emission scenarios (SSP5-8.5), global mean sea level (GMSL) is projected to rise by 0.15-0.23 m and 0.20-0.30 m by 2050, respectively. By 2100, GMSL is projected to rise 0.28-0.55 m and 0.63-1.02 m, respectively. Antarctica ice sheet instabilities are significant sources of uncertainty affecting future sea level rise projections. Regional relative sea level rise is an important driving factor affecting extreme still water levels.

Marine Ice Sheet Instability (MISI) and Marine Ice Cliff Instability (MICI) and the acceleration of ice streams will increase the estimated GMSL in the future. After 2100, with the continuous heat uptake of deep ocean and the continuous loss of ice sheet mass, sea level rise will last for thousands of years (high reliability).

A major deficiency of present-day sea-level studies is the prediction of high sea-level scenarios at the end of the 21st century. Under the impacts of climate warming and polar amplification effect, the collapse of Antarctic ice shelf may accelerate. The intensification of hydrofracturing process and ocean stratification can increase the melting on and beneath the ice shelf, respectively. However, these physical processes have not been well implemented in the models.

By the end of the 21st century, the tidal amplitude in most coastal regions of the world will change significantly. Human factors such as land reclamation and different land management policies in coastal areas will affect the impact of global sea level rise on tidal amplitude. Therefore, the impact of ice sheets on global and regional sea level changes has important practical and long-term significance for China’s future coastal infrastructure and ecological environment protection.

IPCC AR6 was formally approved at the 14th joint session of Working Group I of the IPCC and accepted by the 54st session of the IPCC on August 2021. Assessment content related to the ocean circulation of chapter 9 “Ocean, cryosphere and sea level change” is synthesized in this paper. Compared to the former IPCC reports, AR6 further confirmed the influence of human activities on ocean circulation, and provided the projected results based on the newest numerical simulations. AR6 pointed out that, surface salinity contrasts are increasing (virtually certain), and fresh ocean regions will continue to get fresher and salty ocean regions will continue to get saltier in the 21st century (medium confidence). The upper ocean has stably stratified since at least 1970 over the vast majority of the globe (virtually certain), and the upper ocean stratification is projected to increase (virtually certain) while the mixed layer depth is projected to mostly shoal under high emissions scenarios (low confidence). The frequency of marine heatwaves has doubled since the 1980s (high confidence) and the duration has become longer (medium confidence), furthermore the projection shows such trend will continue. Of the four eastern boundary upwelling systems (EBUS), only the California current system has experienced upwelling-favorable wind intensification since the 1980s (medium confidence), and the EBUS will change with a dipole spatial pattern of reduction at low latitude and enhancement at high latitude (high confidence). Under all SSP scenarios, the Atlantic Meridional Overturning Circulation (AMOC) will decline over the 21st century (very likely). The decline will not involve an abrupt collapse before 2100 (medium confidence). AR6 has added the high resolution numerical simulation experiments which could resolve mesoscale eddy, such experiments could effectively improve the simulation of Sea Surface Temperature (SST), air-sea flux and dynamic sea-level change.

Assessments of IPCC AR6 and SROCC show that the global warming rate in High Mountain (HM) regions has increased recently. Since the 1980s, the warming rate in the High Mountain Asia (HMA) has been significantly higher than the average of the global mountains and that in other high mountains. The warming is generally altitudinal dependent, but the mechanism keeps complex and there are large regional differences. Except for the Rocky Mountains, the warming magnitude in other high mountain areas will increase with altitude to varying degrees. Although the global annual precipitation in mountain areas in the past few decades has shown no obvious trend, it is projected that the annual precipitation in many Northern Hemisphere mountain regions will increase by 5%-20% by the end of the 21st century, with great spatial and temporal discrepancies of extreme rainfall. The frequency and intensity of extreme rainfall will increase over the Qinghai-Tibet Plateau and the Himalayas. The decrease of annual maximum snow water equivalent in mountains is stronger in the altitude zone that solid precipitation is transforming towards liquid precipitation, and the change of mountain snow in the future is not only related to the emission scenario, but also closely related to the altitude. Global mountain glacier mass loss from 2010 to 2019 was greater than that in any other decade since observational records began. Although the rate of mass loss in HMA is smaller, the total ice volume loss is second only to that in the Southern Andes among global main mountain regions. It is projected that mountain glacier retreat will continue for decades or hundreds of years in the future, and the corresponding contribution to the sea level rise will be the largest in HMA among the four HM regions. The permafrost temperature and the thickness of active layer in mountains have been found increased and decreased, respectively. In the future, mountain permafrost will become more unstable with continued and accelerated degradation. Even under the low greenhouse gases emission scenario, the permafrost area on the Qinghai-Tibetan Plateau is expected to decrease by 13.4% to 27.7% by the end of the 21st century. However, from the perspective of the completeness and confidence level of the above assessment, there are still huge gaps in observations and researches in the mountains.

The observing networks in mountains do not always follow standard observation procedures and are often not dense enough to capture fine-scale changes and potentially large-scale patterns. The discrepancies between satellite retrieval data and ground-based observations exist widely, especially for snow cover, which is still a recognized challenge. Therefore, immediate actions should be taken to strengthen the density of mountain observations, especially in three-dimensional space, in accordance with WMO standards, so as to improve the capacity of climate monitoring and services in mountain areas. In terms of information extraction from observations, refined, three-dimensional and accurate climate monitoring and assessment information is urgently needed to provide highly applicable scientific support for disaster management and climate change response.

Projections of future climate change are made through global climate models, regional climate models, or their simplified versions, which are dynamically consistent in the way of representing physical processes and hence link changes in mountains with large-scale atmospheric forcing. However, existing models that specialized for mountain studies usually cover only a single mountain range, and there have been no initiatives such as model inter-comparison programs or coordinated downscaling trials addressing problems existing in mountains climate change.

As a result, the direction of the future researches for mountain climate change lies, on the one hand, in the perspective of natural science, that is to continue to extract reliable information from observation data, support to improve the model resolution and simulation capability for complex underlying surfaces and quantify the contribution of mountain climate change in energy, water and carbon cycle and feedbacks from both macro and micro levels. On the other hand, from the perspective of supporting social sustainable development, to identify the climate change indicators that affect the stability of mountain society and ecosystem, supporting adaptation and mitigation of mountain climate change.