The water cycle in the Earth’s climate system is constantly evolving and renewing. In recent years, under the dual influence of global warming and human activities, the hydrological cycle has undergone significant changes, which has aroused widespread concern in society and academia. The hydrological cycle is the core of hydrometeorology and climate system, which links between the climate subsystems. In recent years, scholars at home and abroad have made great achievements in understanding the characteristics of global and regional hydrological cycle changes by using various observation methods and sea-land-air coupling models. The deep understanding of the physical mechanism and driving factors of water cycle change improve the ability to predict its possible change in the future. In this paper, the changes in the hydrological cycle related to hydrometeorology in the past 20 years, causes of the changes in the global, regional and watershed hydrological cycle fluxes, and the prediction of future changes are comprehensively reviewed. Finally, some problems that should be paid attention to in future hydrological cycle change research are discussed. In particular, three questions worth further attention are emphasized. These questions are the changes of hydrological cycle elements with global warming at different spatio-temporal scales, the detection and attribution of the causes of hydrological cycle changes at different spatio-temporal scales, and the interaction and coupling model between terrestrial hydrological cycle and atmospheric water cycle.

For 754 homogenized daily maximum temperature series across rapid warming China during past 40 years, dynamic return periods and return levels were investigated based on various nonstationary Generalized Extreme Value (GEV) models, and new method of evaluation for high temperature extremes was proposed. By this method, the extremality of extreme climate events can be better explained. This method also enhances the comparability between climate extremes, thus more historical information can be well preserved. Transformed applications of dynamic return periods between stationary and nonstationary models can provide more accurate and reliable assessment about current state of climate extremes in climate change studies. Finally, some long-existing disagreements and mistakes in understanding “once-in-N-year” extreme events (i.e., as a value or a probability) between academic circles and public opinions are clarified effectively via the new method.

In view of the impact of tropical cyclones (TCs) on coastal areas of China, it is of great significance for disaster prevention and reduction to study the changes of TC activities landing in China in the future under global warming. Based on outputs from the global climate model HadGEM2-ES in CMIP5, this study carried out dynamic downscaling simulation of the regional climate over East Asia in historical period and in the future under three representative concentration pathways (RCPs) with low, intermediate and very high greenhouse gas emissions (RCP2.6, RCP4.5 and RCP8.5), and tested the model’s ability of simulating the activities of TCs landing in China and TC-related large-scale environmental fields by using the regional climate model RegCM4. The changes of track, intensity and frequency of TCs landing in China during 2030-2039, 2050-2059 and 2089-2098 under three RCPs were also projected. The results are as follows. The model can reasonably reproduce the spatial structure of large-scale atmospheric circulation field over East Asia and the characteristics of TCs landing in China in the historical period (1986-2005). The average intensity and number of TCs landing in China under three RCPs will increase in varying degrees in the future, especially the number of typhoons and above will increase significantly. The most prominent changes appear in the RCP8.5 scenario, by the end of the 21st century (2089-2098), the average intensity, annual mean number of typhoons and above will increase by 7.56% and 1.05, respectively. The tracks of TCs landing in China under three RCPs in the future are prone to move northward in varying degrees. Moreover, the greater the global warming, the more obvious the northward tendency, which may be related to the significant warming offshore China and the weakening of vertical wind shear in the future. The prediction results of this study show that coastal areas of China, especially in the middle and high latitudes, are likely to face increasingly severe TC disasters in the future, so it is necessary to find the countermeasures of disaster prevention and reduction as soon as possible.

Based on the observation data of meteorological stations in the Qinling-Huaihe region from 1970 to 2019, with daily average temperature stability ≥10℃continuous days as the main indicator, and January average temperature as the auxiliary indicator, with the help of Thin Plate Spline (TPS) and the ordinary Kriging interpolation method based on Digital Elevation Model (DEM), to explore the response and changes of the subtropical northern boundary in Qinling-Huaihe region to global warming and warming hiatus. From 1970 to 2019, the daily average temperature stability≥10℃continuous days in Qinling-Huaihe region and the January average temperature showed an upward trend of varying degrees, but the distribution years of the high and low values were different. In the past 50 years, the subtropical northern boundary has shown an obvious trend of moving to high-altitude areas in the Qinling section. Among them, the southern slope of the Qinling Mountains has been uplifted by 153.3 m in total, and the northern slope has been uplifted by 148.8 m. During the period of warming hiatus, the elevation changes of the subtropical northern boundary of the southern and northern slopes of the Qinling Mountains were highly synchronized, but they were quite different from 1980s to 1990s. From 1970 to 2019, the subtropical northern boundary moved northward by more than 1.3 latitudes in the Huaihe River section, and even reached 3 latitudes near 115°E, which is similar to the northernmost position that the subtropical northern boundary of China has reached in the past 2000 years. From the perspective of different ages, the northward movement of the subtropical northern boundary was most obvious during the 1990s, parts of the lower Yellow River basin have been gradually transformed from warm temperate to subtropical.

Based on the simulations of 9 CMIP6 models and observation data, the performance of the CMIP6 models in simulating the terrestrial leaf area index (LAI), gross primary productivity (GPP) and net primary productivity (NPP) over Asia is evaluated in this paper. The evaluation results indicate that the CMIP6 models can reasonably reproduce the temporal and spatial characteristics of LAI, GPP and NPP over Asia. Moreover, the multi-model ensemble mean (MME) simulation outperforms the individual models. The spatial correlation coefficient between the MME simulated and observed LAI, GPP and NPP are 0.90, 0.81 and 0.89, respectively, and the root-mean-square error of the MME simulation with respect to the observation is around 0.5. On this basis, the changes of LAI, GPP and NPP over Asia under the SSP1-2.6, SSP2-4.5 and SSP5-8.5 scenarios are further projected. In general, the MME projections show an increasing trend by the end of the 21st century for the LAI, GPP and NPP over Asia, with larger increases under high emission scenario than under low emission scenario and larger increases over the mid-high latitudes than over the low latitudes of Asia. Regionally averaged, the largest increases in LAI, GPP and NPP by the end of the 21st century are expected in North Asia, where the LAI, GPP and NPP are projected to increase by 68%, 106% and 90% under SSP5-8.5 and by 23%, 29% and 26% under SSP1-2.6, respectively. The smallest changes are anticipated in Southeast Asia, where the projected increases in LAI, GPP and NPP are 15%, 34% and 39% under SSP5-8.5 and 3%, 10% and 11% under SSP1-2.6, respectively. These results signify a greening of Asian ecosystem and a strengthening of carbon sequestration in the context of future global warming.

China is exploring the total volume control system of carbon emissions to speed up its progress of carbon peaking and carbon neutrality. Allocation of provincial carbon emission allowances (CEA) is the key to China’s implementation of the total volume goal of carbon emissions. This paper reviews the research on allocating China’s provincial CEA from three aspects: allocation criteria, methods, and results. The results show that taking both principles of fairness and efficiency into account is the consensus of many types of research. However, there are still disputes on the interpretation and measurement of the principle of fairness. The index and optimization methods are most widely used in allocating China’s provincial CEA. The former can take into account the interests of multiple parties, while the latter can improve allocation efficiency. The hybrid method has great potential for development due to its multi-method advantages. The game method is rare due to its’ lack of transparency. There is still a gap between the emission reduction responsibilities of each province in previous allocation results and the actual situation of each province. In addition, previous allocation results only focus on a specific year’s CEA allocation or a specific period’s cumulative CEA allocation. They lack attention to the year-by-year CEA allocation. It is suggested that follow-up research should pay more attention to the construction process of intertemporal dynamic allocation methodology, which takes into account both fairness and efficiency, considers regional heterogeneity and consumer emission responsibility, adopts a multi-method combination, and is compatible with the particularity of individual cases.

Carbon quota allocation is the key to carbon emissions control. It is of great significance to explore provincial-level allocation schemes that are more conducive to high-quality development under the national “dual carbon” goal. Based on egalitarianism, historical responsibility, capacity to pay, emission efficiency and feasible allocation criteria, 10 allocation schemes were designed using the comprehensive indicator method; Taking high-quality development as standard, the Malmquist index model was used to evaluate these schemes. The results show that various provinces in China have diversified carbon quotas under different allocation schemes, and most regions obtain the maximum quota according to scheme F of emission efficiency criteria, capacity to pay criteria, historical responsibility criteria and feasibility criteria. From the national perspective, scheme F has the best economic and high-quality development performance, which is the relatively optimal choice. From the regional perspective, each province has different high-quality economic development performance under different allocation schemes. Most regions achieve the optimal development according to scheme G of comprehensive egalitarianism principle, emission efficiency principle and feasibility principle. The selection of emission efficiency indicators is robust to the results of allocation scheme.

The transport sector is one of the major sources of greenhouse gas emissions. The proposal of “dual-carbon” targets puts forward higher requirements for carbon emission reduction in the transport sector. The statistical data base for energy consumption statistics and greenhouse gas emission measurement in China’s transport sector is relatively weak. There is no precise data on greenhouse gas emission of transport sector. The boundary, scope, and methodology of greenhouse gas emission accounting need to be further clarified. At present, the accounting method for greenhouse gases has not been unified in the transport sector at the national level. Drawing on the experience and practice of domestic and overseas greenhouse gas emissions accounting boundaries and calculating methods for the transport sector, this paper puts forward the greenhouse gas emissions accounting methods applicable to different modes of transportation in China. Aiming at the existing problems in the transport greenhouse gas accounting, corresponding policy recommendations in establishing energy consumption and emissions accounting method system, setting up the transport energy consumption and carbon emissions data sharing mechanism, strengthening transportation energy consumption emissions accounting methods training, improving data quality management are proposed in this study, to provide a reference for the continuous development of greenhouse gas emissions accounting in China’s transport sector.

Climate change and biodiversity loss are both important and intractable environmental problems, the synergistic promotion of which has become a global consensus. For this purpose, major countries have carried out a lot of practical explorations in strategic planning, ecological space, green finance, technological innovation and so on. China has also begun to pay attention to the synergy while vigorously pursuing the carbon peaking and carbon neutrality goals. In order to become an important participant, contributor and leader in global ecological governance in the future, China should incorporate biodiversity conservation into its climate policy system, focus on incorporating NbS into national development strategies such as Urbanization and Rural Revitalization. And green finance and digital technology should be promoted. Besides, China should increase public awareness and social participation in the synergy.

As climate change intensifies and the global climate governance process accelerates, carbon peaking has become the core of global climate action, with an increasing number of countries setting carbon neutrality targets and taking actions. China announced the “dual carbon” goals at the 75th General Debate of the United Nations General Assembly. Several developed economies that have already peaked also proposed carbon neutral commitments. Adopting both holistic and focal perspectives which highlight phases and equity respectively, this paper analyses the course and characteristics of carbon peaking, as well as carbon neutrality targets and plans of major developed economies including the EU and the US. It is found that developed economies usually experienced a relatively long climbing period (58-136 years) and plateau phase (4-20 years) in the process of carbon peaking. For these economies, at the time of carbon peaking, the energy mix was dominated by oil and natural gas, which accounted for 57%-77% of primary energy consumption. In addition, per capita emissions, historical cumulative emissions and per capita GDP were all at a high level. The decoupling of economy growth and carbon emissions were achieved around the carbon peak. Carbon neutrality pathways of developed countries generally focus on energy transition, apply diverse policy instruments, and emphasize low and negative carbon technology innovation. Based on the policy outlook of developed economies, it is difficult to reduce absolute emissions to zero when achieving carbon neutrality, and offset by carbon removal methods is required. Through a comparative analysis, this paper finds that China’s carbon peaking and carbon neutral targets are ambitious climate commitments that require greater efforts than those of developed economies. To support the implementation of China’s carbon neutrality goal, it is recommended to use a comprehensive integrated policy framework, accelerate climate legislation, promote energy transition in a just transition approach, and strengthen the development and application of renewable energy and energy efficiency technologies.

Germany is a pioneer in the global energy transition. Both China and Germany have the characteristics of “rich coal, poor oil and little gas” resource endowment, high dependence of oil and natural gas on foreign countries, and uneven energy demand and distribution. Summarizing the experience and lessons from the energy transition process of Germany is of great significance for China to promote energy transition and achieve the carbon peaking and carbon neutrality goals.

This paper introduces the evolution of electricity structure in China and Germany, elaborates the mechanism of energiewende in China and Germany, and analyzes the experience of Germany power system flexibility regulation taking March 19, 2022 as an example. Research shows that the sound legal system provides an important guarantee for Germany energy transformation, the German coal commission and relevant financial support have effectively solved relative problems in the process of coal to renewable, the liberalized power market is a prerequisite for developing a new power system with renewable energy as the main body, accurate prediction and management of wind and photovoltaic power generation are the key to renewable energy consumption, coal power and the interconnected European grid are important guarantees to ensure the stability of power supply in Germany, the overly aggressive strategy of removing coal and denuclearization are important reasons for the current German energy crisis.

In view of this, based on the experience of Germany’s energy transition and the lessons of the 2022 Germany energy crisis, this paper draws the following inspirations: adhere to the primary goal of energy security, vigorously develop new energy. Accelerate the construction of “double carbon” legal system to provide legal guarantee for promoting energy transformation. Continuously promote the reform of the power market and provide strong support for the construction of a new power system. Strengthen the accurate prediction and management of wind power and photovoltaic power generation, and improve the consumption level of wind power and photovoltaic power. Strengthen international cooperation and promote energy transformation in a coordinated manner.