Energy is a resource in nature that supports human survival and social progress. Ever since humans utilized fire for the first time, energy, water, and food have become the three elements that humans cannot live without. Two driving forces, technological progress and social civilization promote energy development. The global energy structure has gone through two transformations: the first transformation realizes the energy revolution from wood to coal and the second transformation completes the shift from coal to petroleum and natural gas. Nowadays, mankind is experiencing the third significant transformation that converts from conventional fossil fuels to new energy. Based on the energy development law, the energy resource, in terms of its form, has shifted from the solid phase (wood + coal) and liquid phase (petroleum) to the gas phase (natural gas). In terms of the carbon number, energy resource has shifted from high-carbon (wood + coal) and medium-low carbon (petroleum + natural gas) to carbon-free (new energy). The future development will go along with the three major trends - resource type carbon reduction, production technology intensification, and utilization method diversification. At present, the global energy is undergoing a low-carbon revolution in fossil energy, a large-scale revolution in new energy, and an intelligent revolution in energy management. The three energy revolutions are proceeding simultaneously to accelerate the formation of "new coal", "new oil and gas", and "new power grid." Human beings live on the same earth and under the same sky but breathe “air” with different carbon dioxide contents. As mankind stepped into industrialization, carbon dioxide emissions have continued to increase, resulting in environmental hazards including global warming, glaciers melting, and sea level rising. The environment on which mankind depends for survival is facing unprecedented threats and challenges. According to statistics, since the year of 1850, the concentration of carbon dioxide in the atmosphere has risen from 280× 10^-6 to 450×10^-6, the global temperature has increased about 0.9 °C to 1.2 °C and the sea level has risen by 20 cm^[1,2,3]. Especially in the past 30 years, the global warming and sea level rising have accelerated, the rate of temperature rise has increased 0.2 °C every 10 years and that of sea level rise has reached 0.32 cm/a^[4,5,6]. By the end of this century, if the global temperature rise reaches 2 °C, the sea level would rise 36 to 87 cm. By that moment, 99% of coral reefs will vanish, about 13% of the ecological system on land will be destroyed, and many plants and animals will be on the edge of extinction^[7]. Cutting down the emissions of greenhouse gases such as carbon dioxide and mitigating global warming have become the goal shared by all mankind. In October 2018, the Intergovernmental Panel on Climate Change of the United Nations (IPCC) proposed the goal of “carbon-neutral” that will limit the global temperature rise to 1.5 °C by the end of the century.

The carbon dioxide emissions caused by human activities mainly come from fossil fuel consumption. Developing new energy and realizing energy transformation by reducing fossil fuel consumption and constructing a green, low-carbon energy system is one of the important measures to lower carbon dioxide emissions and implement global carbon neutral. This article summarizes the main experience and approaches of major countries towards carbon neutral, analyzes the global distribution of carbon dioxide emissions, and proposes the challenges and countermeasures facing global carbon neutrality. New energy has become the dominant role in the third energy transformation and will take the lead in the implementation of carbon neutral. In response to the challenges and opportunities facing China's carbon neutral, this paper proposes a roadmap and implementation path for China to achieve carbon neutral, and provides a reference for the smooth realization of the carbon neutral goal by 2060.

Carbon is one of the major elements in living matter and a significant component of organic matter. It exists in the atmosphere, terrestrial ecosystem, marine ecosystem, and lithosphere on the earth in the forms of carbon dioxide, organic and inorganic matter. Carbon circulates within the atmosphere, terrestrial ecosystem, marine ecosystem, and lithosphere on the earth through carbon fixation and carbon release^[8]. Carbon fixation refers to the carbon dioxide absorption via photosynthesis by plants, the carbon dioxide dissolution into the ocean from the atmosphere, the carbon dioxide absorption by saline soil in arid regions, the formation of carbonic rock, and the artificial technology that converts carbon dioxide to chemicals or fuels, etc. Carbon release mainly comes from the respiratory actions of plants and animals, consumption of fossil fuels, and the decomposition of carbonic rock in the lithosphere. This article defines the carbon dioxide that can be fixed and available as the “gray carbon”; carbon dioxide that cannot be fixed, used, and remains in the atmosphere as “black carbon”. Since humans stepped into industrialization, the consumption of fossil fuels has drastically increased, releasing carbon within fossil fuels in the lithosphere into the atmosphere. As a result, the carbon dioxide concentration in the atmosphere continuously increases and the carbon circulation cycle balance is destroyed, causing “black carbon” content in the atmosphere to constantly increase. The purpose of carbon neutral is to reduce the concentration of “black carbon” in the atmosphere, gradually restore carbon cycle balance on the earth, protect the ecosystem that humans depend on, and build a habitable earth.

The Special Report on Global Warming of 1.5°C published by IPCC pointed out that carbon-neutral means that carbon dioxide emissions of an organization within one year are balanced through carbon dioxide elimination technology, which is also known as net zero CO₂ emissions^[7]. The carbon neutral target is to reduce the global carbon dioxide emissions by about 45% in 2030, compared to 2010, and to reach net zero CO₂ emissions and realize net zero carbon dioxide emissions in 2050.

The primary task of carbon neutral is to control global warming within 1.5 °C by the end of this century. Carbon neutral can not only mitigate climate changes but also is a radical measure for humans to protect the ecological environment. It also helps to protect the biological diversity and ecosystem, and prevent more species from extinction. Carbon neutral has accelerated the low-carbon and green transformation in the energy resource system and provided an emerging economic growth point for the whole world. According to the Energy Transformation 2050 published by International Renewable Energy Agency, carbon neutral can bring an extra 2.4% GDP growth for global economics and additional 7 million employment positions in the energy industry, etc.^[9].

As of January 2021, according to the statistics of the United Kingdom’s Energy & Climate Intelligence Unit^[10], there have been 28 countries in the world realizing or making a commitment to carbon neutral targets (Table 1). Among them, Suriname and Bhutan have achieved carbon neutral; six countries including Sweden, the United Kingdom, and France have committed to being carbon neutral in law; six countries and regions including the European Union, Canada, and South Korea are formulating relevant laws; fourteen countries including China, Australia, Japan, and Germany commit to being carbon neutral. The year of 2050 is the major time node for the world to achieve carbon neutral. Except the 2 countries that have already been carbon neutral, Finland has committed to being carbon neutral at the earliest time in the year of 2035. Other 99 countries are discussing carbon neutral targets, among which Uruguay plans to set the target in 2030 while other countries plan to set it in 2050.

The two countries that have achieved carbon neutrality have the characteristics of small land area and extremely high forest coverage. Among them, the forest coverage rate of Suriname is 90% and that of Bhutan is 72%. In the carbon neutral process, the European Union has been the most active participant who is willing to build the first carbon neutral continent. The European Green Deal published by the European Commission in December 2019 mentions that the greenhouse gas emissions will be reduced by 50% to 55% in 2030 compared to 1990 and achieve the goal of carbon neutral by 2050. The Green Growth Strategies proposed by the Japanese government in December 2020 is regarded as the progress chart for Japan to be carbon neutral in 2050. Regarding the countries that have already committed to being carbon neutral, except the European Union and Japan that have already made the specific route map to be carbon neutral, the carbon neutral paths of other countries are still in the making. This article analyzes and concludes the approaches done by major countries and region as follows.

(1) Gradually withdraw from coal power generation plans. Except Germany, the European Union countries that have pledged to be carbon neutral have a relatively small amount of coal resources and small territorial area. These countries have all terminated the coal power generation. Germany announces that it will withdraw from coal power generation by 2040. The countries that have relatively abundant coal resources and have a relatively high ratio of electricity consumption taken up by coal (such as Australia) have not yet confirmed the plan to quit coal power generation.

(2) Accelerate the application and generalization of the new energy industry such as solar, wind, hydrogen energy. Photovoltaic power generation will become the main electric power source of the European Union and Japan and offshore wind power will usher in explosive growth. It is estimated that by 2050, the offshore wind power in the European Union and Japan will have more than a 25-fold increase. As for hydrogen energy, the European Union focuses on the production and preparation of green hydrogen; Japan fully develops the industry chain for hydrogen energy; South Korea has legislated on hydrogen energy, extending its application to transportation, metallurgy, power generation, and other fields.

(3) Develop carbon sequestration and carbon conversion technologies. Germany will reinitiate the carbon dioxide capture and sequestration project and meanwhile utilize its ample natural gas pipeline network facilities to rapidly develop electricity-to-gas technology, and convert carbon dioxide into methane for pipeline network transportation. Japan is developing carbon dioxide capture and resource recovery technologies. By 2030, the price of carbon dioxide recovery fuel will be equivalent to that of conventional jet fuels, and by 2050, the price of carbon-dioxide-made plastic products will be equivalent to that of existing plastic products.

(4) Introduce a pricing mechanism for carbon and increase the cost of carbon emissions. The European Union started to implement Emissions Trading Scheme (EU ETS), which is the first carbon emissions trading system participated by several countries in the world. This trading system uses the Cap-and-Trade Rules, trading carbon emissions via administrative license transactions, based on limiting total greenhouse gas emissions. By prescribing limits and setting up trading plans, this trading system also sets limits to each member country to distribute the emissions reduction targets to enterprises, and define the emissions reduction upper limits to enforce the implementation.

According to the statistics by the International Energy Agency, the global energy-related carbon dioxide emissions in 2019 are the same as in 2018 (330×10⁸ t). The top 5 carbon emitter countries are China, the United States, India, Russia, and Japan, whose carbon emissions are 98×10⁸, 48×10⁸, 23×10⁸, 15×10⁸, and 11×10⁸ t respectively^[11,12]. Carbon emissions in Asia mainly come from China, India, and Japan. Carbon emissions in America come from the United States, Canada, and Brazil; those in Europe come from Russia, Germany, and the United Kingdom. Carbon emissions in Africa come from South Africa, Egypt, and Algeria. Carbon emissions in Oceania come from Australia.

Fossil fuel consumption is the major factor responsible for increased carbon dioxide emissions. Coal consumption has always been the largest source of carbon dioxide emissions since 2003 (Table 2). In 2019, the consumption of coal, petroleum, natural gas, and other consumption took up 44%, 34%, 21%, and 1% of the total count of carbon dioxide emissions, respectively. The electric power industry is the largest carbon emission industry that accounts for 38% of the total emissions, followed by transportation, industry, construction, and other fields, accounting for 24%, 23%, and 9% of the total carbon emissions^[12].

It has been the global consensus that carbon neutral can be a solution for global climate changes, but its imple-mentation still faces challenges from all aspects including politics, energy resources, technology, market, and energy structure.

2.2.1. Politics

Achieving carbon neutral is a global target that needs the cooperation of all countries in the world. The permanent member of the UN Security Council should take the lead on the carbon neutral target and however, the United States and Russia have not yet pledged to be carbon neutral. Among the top 5 carbon emission countries in the world, India has not yet committed the time to be carbon neutral. Angola, Iran, Iraq, South Sudan, Turkey, Yemen, and other countries initially signed the Paris Climate Agreement but have not yet assigned formal legislative approval. Another 99 countries discussing carbon neutral targets, which have yet to be adopted^[9].

2.2.2. Energy resources

Developing new energy to replace fossil fuels is a primary measure to achieve carbon neutral. The global distribution of new energy including solar and wind energy varies greatly with respect to time and space, which brings challenges to the large-scale development of new energy. The global solar energy resources are mainly concentrated near the equator and between the Tropic of Cancer and Capricorn, where the Saharan region of northern Africa is the most abundant. The east and south sides of the Africa continent, the northwest region in China, and Australia are also the regions with rich solar energy. Wind energy resources are mainly distributed in East Asia, Southeast Asia, Central Asia, and the regions from 30°S to 30°N in America, together with the north and the east region in China, Mongolia, northeast region in Australia, south of the Sahara Desert in Africa, and other regions. The global terrestrial solar energy and wind energy vary significantly across different regions and seasons^[13].

2.2.3. Technology

The maturity of new energy technologies decides the progress of the carbon neutral process. The overall price of power generated by solar, wind, and other new energy is still higher than that of power generated by coal. Also, the stability of peak-valley is far from satisfactory and the peak regulation technology still needs further innovation. Fields such as heavy industry and long-distance transportation can hardly achieve electrification. Even though hydrogen fuel cell is the optimum choice, some key technologies are still in the demonstration and prototype stage and have not yet reached generalization and large-scale industrial application. Compared to hydrogen production from traditional fossil energy (also known as “gray hydrogen”), Hydrogen production from renewable energy sources (also known as “green hydrogen”) has a higher cost and corresponding carbon dioxide capture and sequestration technology are still in the demonstration stage. Although low-carbon technology transfer can significantly help reduce carbon emission and control the warming effect, the commitment, made by developed countries, about funding and low-carbon technology assistance to developing countries has not yet been fulfilled.

2.2.4. Market

In the carbon neutral process, the generalization and application of new energy depend on cost advantages and application convenience. At present, the cost of new energy has dropped by each year but is still less competitive with fossil energy. Particularly, in 2020 when the global crude oil price collapsed, the cost advantages of fossil fuels caused an adverse effect on new energy transformation^[14]. The incomplete corollary equipment for new energy also poses an inconvenience. The problems such as the charging piles not being popularized and the insufficient amount of hydrogen refueling stations have pushed up the cost of using new energy vehicles.

2.2.5. Energy structure

Fossil fuels still account for a major part of global energy consumption with new energy having a relatively small proportion. The global energy consumption in 2019 is 144×10⁸ t of oil equivalent, where coal accounts for 27%, petroleum 33%, natural gas 24%, and new energy 16%^[15]. In the carbon neutral process, the consumption proportion of coal, petroleum, and other high carbon content fossil fuels needs to be cut down significantly while that of new energy needs to be increased. So far, the consumption proportion of fossil fuels remains high, bringing challenges for energy transformation.

The strategies to reduce carbon emissions and achieve carbon neutral can be distinguished into four major approaches, carbon replacement, carbon reduction, carbon sequestration, and carbon cycle.

Carbon replacement refers to replacement by electricity, heat, and hydrogen. Replacement by electricity is the utilization of hydroelectricity, photoelectricity, wind electricity, and other “green electricity” to replace thermal power. Replacement by heat is the utilization of photo- thermal resources, geothermal resources to replace fossil fuel heating. Replacement by hydrogen is using “green hydrogen” to replace “gray hydrogen”.

Carbon reduction mainly includes energy saving and efficiency improvement. In the construction industry, the approaches mainly include improvement of electrical appliance and facility efficiency, installation of solar photovoltaics outside buildings, reduction of the embodied carbon emissions of cement and steel, and so on. In the transportation industry, the approaches mainly involve the utilization of more efficient power systems and lighter materials. The emissions of the “black carbon” can thus be reduced fundamentally.

Carbon sequestration refers to the collection of carbon dioxide generated by large-scale thermal power generation, steelmaking plants, chemical plants, etc., and then transport to a suitable place, using technical means to isolate and seal the carbon dioxide from the atmosphere for a long time. Geological sequestration is the main form of carbon sequestration, the sequestration places are usually oil and gas reservoirs, deep underground saltwater layers, and abandoned coal mines. In the future, after oilfield and gas fields being depleted, the existing surface and the underground facility can be applied to sequestrate carbon dioxide, which may become one of the main approaches. The “black carbon” content in the atmosphere can be reduced by technical measures.

The carbon cycle includes artificial carbon conversion and forest carbon sink. Artificial carbon conversion means using chemical or biological measures to convert carbon dioxide into useful chemical products or fuels including methanol synthesis from carbon dioxide, the electrocatalytic reduction from carbon dioxide to prepare CO or light hydrocarbon products (C₁—C₃), etc. Forest carbon sink refers to the absorption of atmospheric carbon dioxide by plants through photosynthesis and the fixation of carbon dioxide in vegetation and soil to reduce the concentration of atmospheric carbon dioxide. The recycling function of “gray carbon” comes into play.

Aiming at the four main carbon neutral strategies of carbon substitution, carbon emission reduction, carbon sequestration, and carbon cycle, based on the maturity of technology or the price competitiveness of conventional fossil energy, the trend of carbon dioxide emission reduction under the global carbon neutral target from 2020 to 2050 is predicted (Fig. 3). From 2020 to 2030, the carbon dioxide emission reduction rate is relatively low. The main reasons for this are: the price advantage of new energy remains unapparent, the new energy has not yet gone into large-scale application, and the carbon sequestration technology has not yet been refined. From 2030 to 2050, as the related technologies develop, the cost of new energy becomes competitive with that of fossil fuels, together with landings and promotions of new energy projects. Thus, carbon dioxide emissions can be largely reduced. Once the carbon sequestration technology reaches the requirements of popularization and application, it will largely contribute to carbon neutral. Overall, carbon replacement will become the backbone force during the carbon neutral process. It is predicted that by 2050, its contribution will take up 47% of the global carbon neutral efforts, while carbon reduction, carbon sequestration, and carbon cycle take up 21%, 15%, and 17%, respectively.

New energy refers to the non-fossil carbon-free renewable clean energy that is further developed and utilized based on new technologies, replacing conventional energy. The main types include solar energy, wind energy, biomass energy, hydrogen energy, thermal energy, ocean energy, nuclear energy, new material stored energy, and so on^[16]. Compared with conventional carbon-containing fossil energy, new energy differs from theoretical technology, utilization cost, environmental influence, management method, and other aspects. As the new energy technology rapidly develops, together with the improvement of the internet plus, artificial intelligence, new materials, and other technology, the new energy industry is in its rising phase and will gradually step into its golden period of development. Developing new energy and driving the energy structure transformation is the key to achieve carbon neutral. The acceleration of new energy development and utilization has become the driving force of global energy growth. New energy will gradually replace fossil fuels and play a key role in the carbon neutral process.

Throughout the world's energy development process, the history of human utilizing energy has undergone two transformations from wood to coal and from coal to oil and gas, respectively. Now human beings are experiencing the third transformation that swifts from fossil fuels to new energy. The clean and low-carbon features of new energy meet the needs of carbon-neutral development, turning new energy into the leading role in the third energy transformation.

Since 1925, global energy has become cleaner. Except for biomass energy, the development of new energy has been accelerating. From 1925 to 2019, global energy needs increased from 14×10⁸ t of oil equivalent to 144×10⁸ t equivalent, which is a ten-fold increase. The new energy proportion in global energy resources has grown from 0.6% to 15.1%, having a 24-fold increase^[17] (Fig. 4).

In the recent decade, the global energy technology revolution significantly accelerates. The cost of photovoltaic power generation and wind power generation largely reduces, promoting the green transformation of the energy system. According to the IRENA report, since 2019 the levelized cost of electricity from photovoltaic power (PV), concentrating solar power (CSP), and onshore and offshore wind power have dropped about 82%, 47%, 39%, and 29%, respectively^[18]. In 2019, 56% of the large-scale installed capacity of the new energy power generation that is newly put into operation and connected to the grid, can generate power at a price lower than conventional fossil fuels. From 2010 to 2019, the energy from photovoltaic power increased from 32 TW•h to 699 TW•h with an annual increase of 240%; the energy from wind power increased from 342 TW•h to 1404 TW•h with an annual increase of 45% (Fig. 5).

In views of energy production and consumption structure, world energy structure has been divided into four parts by coal, oil, gas, and new energy. According to the research prediction, the year of 2030 will be the turning point for new energy as the cost of many types of new energy will drop to a competitive level with fossil fuel and the energy de-carbonization trend will go stronger. As predicted in 2030, global primary energy resource will reach a peak value of 156×10⁸ t oil equivalent at an annual average increase rate of 1.2% with 19% of coal, 28% of petroleum, 26% of natural gas, and 27% of new energy (Fig. 6)^[15]. In 2025, it is predicted that the increase in the need for petroleum will slow down while in 2030 the need for petroleum will enter its platform period. Since natural gas is characterized by its low carbon content, it is expected to be the only fossil fuel that will grow in long term.

As predicted, after 2030 the cost of new energy will be lower than that of fossil fuels. From 2030 to 2050, the global total primary energy consumption will be maintained at a relatively stable level. In 2050, the global total primary energy consumption will be basically the same as in 2030, with coal accounting for 4%, petroleum 14%, natural gas 22%, and new energy 60%. The world energy consumption structure will embrace fundamental changes, forming a "one big and three small" structure with new energy as the mainstay, and new energy will exceed coal, petroleum, and natural gas and become the main energy.

Solar energy, wind energy, hydro-energy, nuclear energy, and hydrogen energy are the main forces of new energy, to achieve low-carbon emissions. Since 2019, the average cost of new energy power generation has been lower than that of gas power generation but overall, it is still 16% higher than that of coal power generation^[19]. As predicted by 2030, building a photovoltaic or wind power project can have an average level of investment lower than building a coal power generation plant in most cases. Almost all Asia-pacific markets can have costs of photovoltaic and wind power generation lower than that of coal power generation^[19]. In 2050, new energy power generation can satisfy 80% of the global electricity demand, in which half of the total power output can be cumulatively taken up by photovoltaic and wind power generation^[9].

“Green hydrogen” is the reserve force of new energy, helping industry, transportation, and other fields to further reduce carbon emissions. The electricity price takes up 60% to 70% of the cost of hydrogen production by water electrolysis. However, as the electricity price drops dramatically, the cost of “green hydrogen” will drop quickly. In about 2030, “green hydrogen” is expected to have a greater cost advantage than fossil fuels^[20]. In 2050, the proportion of global hydrogen energy in terminal energy consumption is expected to reach 18%. By then, “green hydrogen” technology is fully developed and can be applied to the fields that can hardly achieve zero carbon emission via electrification^[21,22,23], which mainly includes steel, oil refining, synthesis ammonia and hydrogen for industrial use, and long-distance transportation fields including heavy trucks and ships.

Artificial carbon conversion technology is the bridge connecting new energy and fossil energy and can effectively reduce carbon emissions from fossil fuels. It converts the excess power into chemical products and fuels for storage, thus cutting the peak and filling the valley of the new energy power grid.

Power to gas is the main form of artificial carbon conversion. It can rearrange carbon dioxide to produce methane and is viewed as the key to Europe to achieve energy transformation. According to the prediction, in 2050, 10% to 65% of the energy consumption in the European industry comes from the power to gas technology while the proportion in the heating industry and transportation industry is 30% to 65%^[23,24].

Chinese government commits to achieve carbon neutral and will make policies to promote the carbon neutral process. In November 2020, Xi Jinping delivered a speech in the United Nations General Assembly: “China will scale up its nationally determined contributions by adopting more vigorous policies and measures, and strive to reach the peak of carbon dioxide emissions by 2030, and strive to achieve carbon neutrality by 2060”^[25]. Then, in December, the white paper China's Energy Development in the New Era was released, comprehensively expounding the main policies and major measures of China’s energy security development strategy in the new stage and new era.

The report Research on China's Long-term Low Carbon Development Strategy and Transformation Path pointed out that in around 2025, China’s carbon dioxide emissions will enter peak platform period and China will strive to stably reach the peak value by 2030, limiting the peak value of carbon dioxide consumed by fossil fuels within 110×10⁸ t and setting the carbon dioxide emissions in 2035 to be significantly lower than peak year^[26]. Based on the reduced levels of peak carbon dioxide emissions, this research predicts China's carbon emissions in 2060 under three scenarios: low, medium, and high (Fig. 7). Under the low-level scenario, carbon dioxide emissions will be reduced to 40% of the peak value with emissions of 44×10⁸ t; under the medium-level scenario, carbon dioxide emissions will be reduced to 30% of the peak value with emissions of 33×10⁸ t; under the high-level scenario, carbon dioxide emissions will be reduced to 20% of the peak value with emissions of 22×10⁸ t. The remaining emissions will be consumed via approaches that mainly include carbon dioxide sequestration and utilization, artificial carbon conversion, and forest carbon sink. The medium-level and high-level scenarios have greater demands of carbon neutral technologies including carbon dioxide sequestration, utilization, artificial carbon conversion and forest carbon sink, and therefore inputs of these fields should be increased.

Compared with other countries, China will face many challenges on the road to carbon neutrality, such as large carbon emissions, fossil energy-based energy consumption, and short buffer time from carbon peak to carbon neutrality. China is the largest carbon dioxide emitter in the world. In 2019, China’s carbon dioxide emissions account for 29.8% of the global total emissions, even higher than the sum of the US (14.4%), India (7.0%), and Russia (4.7%). At present, fossil fuels such as coal, petroleum, and natural gas are still dominant in the energy consumption in China, especially for coal taking up more than half of the total amount. China’s total energy consumption in 2019 was 34×10⁸ t of oil equivalent, where coal accounts for 58% and petroleum accounts for 19%^[18]. The period for China from peak carbon emissions to carbon neutral is only 30 years, meaning carbon emissions need to drastically drop to go carbon neutral once peak emissions are reached. The committed period made by European Union from peak carbon emissions to carbon neutral is 60 to 70 years, where buffering time is twice as long as China’s. Based on China’s national conditions, carbon neutral models of other countries cannot be directly duplicated, and it is necessary to develop a carbon neutral implementation path in line with China's resource endowment and national conditions. On the way to becoming a carbon neutral country, China needs to work together in power, industry, construction, agriculture, and other fields to reduce "black carbon" emissions and make "gray carbon" available (Table 2).

4.2.1. Promoting efficient and clean use of coal

The abundant coal resources in China are the main energy type and crucial industry raw materials. Vigorous promotion in efficient and clean use of coal can not only effectively control carbon dioxide emissions, but also give play to the main role of coal in ensuring national energy security. The efficient and clean use of coal includes the safe, efficient, and green mining operation, pollution control and purification of coal combustion, New clean coal combustion, advanced coal-fired power generation, and clean and efficient coal conversion, etc. Underground coal gasification is an important approach for clean utilization and can radically change the utilization modes of mining and use of medium and deep coal, reducing negative environmental influence caused by coal development and utilization. We should strive to realize the gasification and utilization of China's onshore coal resources with a buried depth of 1000 to 3000 m, and the coal resources gasification mining productions of gas such as methane and hydrogen are estimated to be 272 to 332×10¹² m^3[27]. 50% of the total coal consumption in China is used for power generation, and therefore a clean and efficient coal power generation is the top priority of efficient and clean utilization of coal. The modern coal chemical industry is mainly based on clean energy and fine chemicals, including coal gas, coal oil, coal chemicals, and other products.

4.2.2. Speeding up clean energy replacement

Speeding up implementation of clean energy replacement, optimizing energy structure, and building a clean, low-carbon, safe and efficient energy system is the crucial measure for China to achieve carbon neutral. Based on technology innovation, the cost of solar energy, and wind energy can be further reduced. The wind power-photoelectric-stored energy coupling mode can replace thermal power to take stored energy’s advantages in quick response, round-way regulation, and energy buffering, improving new energy system adjusting ability and stability. Using photothermal-geothermal coupling mode to replace coal-fired heating energy can bring the respective advantages of solar photothermal energy and geothermal energy, forming complementary heating energy.

4.2.3. Enhancing the function of natural gas as a partner and a bridge in the low-carbon transformation

Natural gas is a low-carbon clean energy, a partner and bridge for the transition from high-carbon to zero-carbon energy, and is the mainstay of fossil energy and the cornerstone of energy security in achieving carbon neutrality. In a carbon neutral context, China’s demand for natural gas robustly increases. It is estimated that by 2035, the demanded quantity can rapidly rise to (6500-7000)×10⁸ m³. Numerous natural gas production bases with a 10 billion cubic meter production level will be built, in which the Sichuan, Ordos and Tarim basins are the key areas, boosting conventional gas production. Emphasis will be put into making breakthroughs in unconventional natural gas exploration and development and improving the industrial layout and policy systems of gas storage and import channels to maintain the safe use of natural gas.

4.2.4. Vigorously developing the “green hydrogen” industry and its industrial chain

China needs to accelerate the construction of the hydrogen energy industry and promote the implementation of the “Hydrogen China” strategy, just like coal, oil and gas, and other industries. China has a strong demand for hydrogen energy, but most hydrogen products are still made from fossil fuels (“gray hydrogen”). Replacing “gray hydrogen” with “green hydrogen” can effectively reduce carbon dioxide emissions. As predicted by China Hydrogen Energy Alliance, in 2030 China will be in the mid- stage of the development of hydrogen energy market with an average annual demand of 3500×10⁴ t, accounting for 5% of terminal energy consumption; in 2050, China’s average annual demand for hydrogen hits 6000×10⁴ t, 70% of the hydrogen sources is “green hydrogen”, accounting for at least 10% of terminal energy consumption^[28], and can reduce carbon dioxide emissions about 7×10⁸ t. Besides, the overall development of hydrogen storage, transportation, hydrogen fuel cells, hydrogen fueling stations, and other industrial chains will be accelerated to deeply integrate with the oil and gas industry. By utilizing existing infrastructure including natural gas pipeline network and oil and gas fueling stations, the natural advantages of oil and gas companies are given full play in hydrogen production, hydrogen processing, and other industrial chain nodes to realize the joint construction of “oil, gas, hydrogen, and electricity”, and promote the high-quality development of the hydrogen industrial system.

4.2.5. Increasing the application and promotion of carbon dioxide burial and sequestration

Carbon dioxide burial and sequestration can largely reduce carbon dioxide emissions and is the matching technology of clean utilization of fossil fuels. China’s coal-dominant resources endowment decides the necessity to accelerate the application and promotion of carbon dioxide burial and sequestration, maximizing its functions in the carbon neutral process and promoting the efficient and clean use of coal. In the future, the depleted oilfield, gas field, and the underground “paddy field” after development can be utilized to form an “artificial carbon dioxide field” that buries ad sequestrates carbon dioxide (Figs. 8 and 9). So far, PetroChina has carried out technological breakthroughs including carbon dioxide flooding in the Jilin, Xinjiang, and Daqing oilfields, with an annual oil displacement production of almost one million tons, making a new breakthrough in carbon dioxide oil flooding technology.

China offshore has great potential for submarine carbon dioxide geological sequestration with a total sequestration capacity of 2.5×10¹² t^[29]. According to preliminary predictions, the effective carbon dioxide sequestration capacities of the saltwater layer and reservoirs in the deep Ordos Basin are 133×10⁸ t and 19.1×10⁸ t, respectively. The effective carbon dioxide sequestration capacity of the gas oil reservoir, deep saltwater layer, and coal layer in the Tuha Basin is 44×10⁸ t. The carbon dioxide absorption and sequestration capacity of the coal layer in the Qinshui Basin is estimated to be 1280×10⁸ t with an adsorption capacity over 96%^[30,31,32]. In addition, carbon dioxide oil flooding and gas flooding can not only achieve carbon dioxide burial and sequestration but also enhance oil and gas recovery^[33].

In the future, the completed oil and gas fields in Songliao, Bohai Bay, Ordos, Daqing, and other large oil and gas areas can be built into demonstration bases for burial and sequestration of "artificial carbon dioxide gas fields".

4.2.6. Developing carbon conversion and forest carbon sinks

Developing carbon conversion that converts carbon dioxide into chemical products and fuels can turn waste into “wealth”. The “liquid sunshine” technology proposed by Dalian Institute of Chemistry and Physics, Chinese Academy of Sciences lets “green hydrogen” react with carbon dioxide and obtains methanol. Producing 1 ton of methanol can fix 1.375 tons of carbon dioxide. China's methanol production capacity is about 8000×10⁴ t, which is mainly produced from natural gas and coal. If all methanol is produced using the "liquid sunshine" technology, hundreds of millions of tons of carbon dioxide can be fixed^[34].

Forest carbon sinks should be vigorously developed. China's major forest zones, such as the southwest and northeast regions, have a large carbon sink capacity. From 2010 to 2016, China’s terrestrial vegetation has an annual average carbon fixation capacity of 11×10⁸ t, which is about 45% of China's annual emissions over the period^[35]. Afforestation can play a beneficial role in the process of carbon neutral.

4.2.7. Establishing market mechanisms to control carbon emissions

A national carbon emissions trading market will be established and improved to control carbon emissions through market mechanisms. The establishment of a carbon market and an increase in the cost of fossil carbon utilization will help reduce the consumption of fossil energy fundamentally and reduce carbon dioxide and air pollutants emissions. At present, China's carbon emission trading market is still in the early stage of construction. It is necessary to further improve and complete the relative rules of the carbon emission trading market, implement the construction of relevant infrastructure, clarify the standards and specifications of relevant parties, and improve the national carbon emission trading market system.

The world today is undergoing profound changes unseen in a century. The ecological environment is related to the survival and sustainable development of mankind and requires all countries to work together to meet challenges. Carbon neutral is not only a consensus reached by mankind in response to global climate change but also a shared purpose committed by countries all around the world. Carbon replacement, carbon reduction, carbon sequestration, and carbon cycle are the four major approaches of achieving carbon neutral, in which carbon replacement is the backbone force to realize carbon neutral and is expected to contribute 47% of the carbon dioxide emissions reductions in 2050.

The process of carbon neutral accelerates the transformation of global energy from fossil energy to new energy. New energy has become the main part in the third energy transformation and will play a leading role in carbon neutral in the future. The year of 2030 is predicted to be the turning point of new energy development, in which the cost of new energy will drop to be able to compete with fossil energy; new energy will be promoted and applied on a large scale from 2030 to 2050, and the downward trend of carbon emissions will accelerate. In 2050, most countries and regions in the world will achieve carbon neutral, and by then new energy will become the dominant energy resource. It is estimated that before 2100, the energy consumption structure will transform from the current “four evenly distributed” structure to the new “one big and three small” structure (The “one big” part is the new energy while the other “three small” parts are coal, petroleum, and natural gas). In the future, China will gradually develop towards the new trend of the world energy consumption structure and realize the leap from the "one big and three small" (The “one big” part is the coal while the other “three small” parts are petroleum, natural gas, and new energy) to the new "three small and one big" structure (the “three small” parts are coal, petroleum, and natural gas while the “one big” part is the new energy). The development and industrialization of hydrogen energy, energy storage of new materials, controllable nuclear fusion and other disruptive technologies will be accelerated to realize China's strategy of "energy independence" with new energy as the focus and make contributions to a livable and green planet. This paper only represents the current understanding of the stage. With the change of science and technology and the world pattern in different periods in the future, the understanding of carbon neutral will continue to innovate and develop.