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.
Based on the 18 global climate models with different horizontal resolutions from the High Resolution Model Intercomparison Project (HighResMIP) of CMIP6, precipitation over China in the period of 1961-2014 was compared against the observation of CN05.1. Results show that all of the models can reproduce the spatial pattern of mean annual precipitation and the seasonal variation of dry winter and wet summer over China. Compared with the model with low-resolution, the high-resolution model can significantly improve the spatial distribution of precipitation, and the bias in the Tibetan Plateau, North China and South China is significantly reduced. The annual cycle of precipitation is also better simulated by the high-resolution compared to the low-resolution models, with lower bias in January, from September to December, and the annual precipitation. In addition, for the first and second leading modes of interannual and interdecadal, most of the low- and high-resolution models fail to simulate the first mode of interdecadal scale. But for the second mode of interdecadal, the first and second mode of interannual, the performance of about half models are improved with the increase of horizontal resolution.
South-South Cooperation to address climate change is a solemn promise made by Chinese Leader at the Paris Climate Conference. It is also a concrete practice that embodies the spirit of the Nineteen Major Conferences, “to be a participant, contributor and leader of global ecological civilization”. As an important foreign aid program of China, there is a lack of relevant research. In practice, the choice of foreign aid countries is usually based on the leaders’ visiting places and some important international conferences, which is lack of systematism and continuity, being also in line with the characteristics of the primary stage of foreign aid. In order to promote the sustainable development of South-South Cooperation to address climate change more efficiently and systematically, and produce better environmental and social benefits, a methodological discussion was carried out on the selection of priority aid countries for South-South Cooperation based on quantitative assessment in this paper. The evaluation index and weight were determined by expert collective scoring and analytic hierarchy process. The stage threshold method was used to convert the performance of different countries in the evaluation index into quantitative score, and the comprehensive score and ranking of countries were calculated. The results show that South Africa, Pakistan, Bangladesh, Cambodia and Indonesia are among the top five priority aid countries, and 20 priority aid countries are listed. In general, the top 20 priority aid countries belong to the 77+ China Group under the United Nations mechanism, 90% of them are participating countries in the “Belt and Road” Initiative, and 70% are members of the Asian Infrastructure Investment Bank. The assessment conclusions are consistent with the major cooperation initiatives initiated by China. The high degree of relevance can not only support the development of South-South Cooperation to address climate change, but also echo and promote the development of other international agendas led by China. This paper attempts to introduce quantitative evaluation research methods in the traditional qualitative description and evaluation fields, and expand the application of research methodology. The conclusions of the paper can provide reference for relevant departments to choose priority aid countries for South-South Cooperation to address climate change.