At present, there are about 20 definitions of energy, but in essence, they are all elaborated from different angles around "energy" and "resource" ^[1⇓⇓-4]. By the form of energy, there are light energy, thermal energy, mechanical energy, electrical energy, etc. By the carrier of energy, there are coal, oil, natural gas, water, sunlight, wind, etc. The reason why energy is recognized and utilized by human beings is inseparable from the progress of human society. With the development of social revolution, technological revolution and industrial revolution, the available high- quality energy forms and advanced energy technologies are evolving. Since the discovery and use of fire by ancient humans in the Paleolithic Age, the dominated energy resources in human consumption have transformed from firewood to coal and then to oil and gas. The transformation from low energy density, low-end technology mining, low-efficiency utilization and high carbon emissions to high energy density, high-end technology mining, high-efficiency utilization and low-carbon emissions promotes the gradual transformation of human civilization from primitive civilization, agricultural civilization and industrial civilization to green ecological civilization, and makes human activities gradually develop from understanding and transforming nature to protecting nature.

Fossil energy, as the main energy source since the industrial revolution, has helped mankind to complete industrialization and modernization. However, the traditional way of energy use and the rapid development of human society have gradually changed the original natural environment of the Earth, resulting in a global ecological crisis represented by climate change, which has become the primary issue of common concern to the whole world. Under the background of the global pursuit for carbon neutrality in response to climate change, the traditional high-carbon energy system dominated by fossil energy is transforming into a new green and low- carbon energy system dominated by renewable energy ^[5]. An era of "big energy" featuring multi-energy complementarity, multi-energy integration and multi-energy transformation is dawning. In the future, energy technology innovation and energy transformation will profoundly change the lifestyles and ideas of human beings. The historical research on the Earth system, human development and energy evolution will help us learn from the past and build a better future.

There are mainly three "sources" of "energy" on the Earth, i.e., the energy of celestial body outside the Earth, the Earth energy and the energy of interaction between the Earth and other celestial bodies. The energy of celestial body outside the Earth is mainly solar energy. Apart from direct radiation, solar energy also provides a productive basis for wind energy, water energy, biomass energy and mineral energy. Plants store solar energy as chemical energy through photosynthesis. Coal, oil and gas are solar energy fixed by ancient plants and animals. The Earth energy includes atomic energy stored in the crust, geothermal energy, etc. Atomic energy includes nuclear fission energy and nuclear fusion energy, while geothermal energy mainly stays in the form of underground hot water, underground steam, dry hot rock mass, and so on. The energy of interaction between the Earth and other celestial bodies is mainly tidal energy, generated by the periodic rise and fall of sea level caused by the moon's gravitational changes (Fig. 1).

The energy can be classified by the above sources, but also the properties of the carriers in which energy is stored. According to the law of conservation of energy, energy can be changed from one form to another or more forms, or from one carrier or system to another, but it cannot be created or destroyed. Strictly speaking, the "energy saving" usually mentioned should refer to the saving of the energy carrier resources, rather than the energy itself. Many oil companies that "yield" oil are actually the "transporter" of energy. According to the types of these carrier resources, energy is usually divided into fossil energy and non-fossil energy, and is further divided into renewable energy and non-renewable energy according to whether it can be reused in the short term (Table 1).

Fossil energy consists of coal, oil and natural gas, in which the chemical energy stored came from the ancient solar energy captured via photosynthesis by ancient plants on the Earth hundreds of millions of years ago. Ancient plants convert CO₂ and H₂O into higher-energy (reduced) carbon, which provides energy for living things and is chemically recombined into the structural materials of living things. Subsequently, after a long process of biochemical and geological evolution, these biological raw materials are transformed into fossil energy, and the energy in them mainly exists in the form of chemical bonds between C and O atoms and between C and H atoms ^[6]. Taking natural gas as an example, it is mainly composed of CH₄, in which the combination between C atoms and H atoms is relatively "loose". If it is mixed with oxygen or mixed with air containing 20% oxygen, the H atoms will be separated and combine with the O atoms to form a stable H₂O molecule^[5], and C atoms will combine with O atoms to form stable CO₂ molecules. Compared with the hydrocarbon molecules before combining, H₂O and CO₂ have lower potential energy, and more energy will be released in the form of heat. During the combustion of natural gas, C in a reduced state with high energy becomes C in an oxidized state with low energy, and at the same time, C that was buried in sediments hundreds of millions of years ago returns to the atmosphere ^[7]. In consideration of the formation of fossil energy compared to human history, fossil energy cannot be restored and regenerated in a short period of time, so it belongs to non-renewable energy.

Non-fossil energy mainly includes atomic energy, geothermal energy, wind energy, hydroenergy, solar energy, biomass energy, tidal energy, etc. Their energy supply mechanisms are different due to their different energy sources.

Atomic energy, or nuclear energy, is the energy released by changes in atomic structure. The process of fission of heavy nuclei is called nuclear fission, and the process of combination of light nuclei is called nuclear fusion. A chemical reaction in the usual sense only involves the change of the bonding relationship between atoms and the atomic nucleus does not change. However, in a nuclear reaction, the relationship between neutrons and protons in the nucleus has changed. Under the action of the nuclear force, protons and neutrons gather together and have potential energy. When the mass of the newly formed nucleus is less than the sum of the masses of all the nuclei that make up it, the lost mass becomes heat according to Einstein's mass-energy equation. Since the square of the speed of light is a huge number, the energy change caused by a small change in mass can release energy millions of times greater than a chemical reaction. The nuclear ore as the carrier of atomic energy is a non-renewable mineral resource, so atomic energy is a non-renewable energy. At present, human mainly use uranium atoms for power generation based on nuclear fission. The energy released by the fission of a single U235 atom is 1.2×10⁸ times higher than the energy released by the combustion of a single CH₄ molecule. Controlled nuclear fusion technology is still in the preliminary laboratory stage ^[6].

Geothermal energy mainly comes from the decay of radioactive elements inside the Earth and the heat convection and heat conduction of the mantle and core. At present, it is mainly used to generate hot steam to drive turbines for power generation. Thermal energy resources are limited to areas of tectonic activity and represent only a small fraction of the total potential available geothermal energy. In the future, effort will be put in the exploration of enhanced geothermal systems (EGS), which injects a thermal medium into the ground to generate electricity by harvesting energy from contact with hot rocks. Traditional oil and gas sector has the natural advantages of integration with EGS due to its hot water resources generated during the oil and gas formation and its well facilities. In addition, the use of supercritical CO₂ to replace water as a heat transfer medium to realize CO₂ storage and geothermal power generation will also be an important research direction ^[10-11].

Tidal energy is the energy generated by the gravitational forces between the Earth, the Sun and the Moon. The gravitational field of the Moon causes sea levels to rise and fall in the areas closest to and farthest from the moon, triggering tidal motion. The kinetic energy from rising and falling tides drives the blades of underwater turbines to produce electricity. Tidal energy is a renewable resource, but it is limited to areas with abundant tidal resources.

Solar energy is used by directly converting the electromagnetic radiation energy of the sun into electric energy, thermal energy or chemical energy, etc. The dominated ways of solar energy utilization are photovoltaic power generation and solar thermal power generation. The former uses the photoelectric effect of the semiconductor interface to directly convert light energy into electricity, while the latter uses a large mirror array to focus sunlight to obtain heat energy, which is then used to produce steam that drives turbines to generate electricity. The photovoltaic power generation technology is mature, but the battery production process has high energy consumption and pollution, and is greatly affected by the intensity of sunlight, making it difficult to connect to the grid of traditional batteries. The solar thermal power generation technology features a clean process, and it can store solar energy in the form of thermal energy to delay the power generation time and achieve continuous power supply. It can also be operated in conjunction with conventional thermal power generation by adding heat storage units or supplementary combustion, but the technology is still in the stage of innovation and improvement.

Hydroenergy is an indirect energy generated by solar radiation. About 50% of the solar energy intercepted by the Earth is absorbed by the surface, in which about half of the energy is used for the waterevaporation process ^[6], mainly seawater evaporation. However, the ability of the atmosphere to retain water vapor is limited. When the water vapor falls to the surface above sea level in the form of rain, the water is endowed with potential energy, which is converted into kinetic energy as it flows down to the ocean. Humans guide kinetic energy on the path of water flow to do useful work. Currently, hydroenergy is mainly used to generate electricity and it is a kind of renewable clean energy. Around 200 BC, human beings began to build water mills to utilize hydroenergy ^[12]. In 1878, France built the first hydropower station of the world. Subsequently, hydropower developed rapidly in the 20th century and the world hydropower generation accounted for about 16% of the world's electricity generation in 2020.

Wind energy is another indirect energy generated by solar energy. The solar radiation heat causes the uneven thermal expansion and flow of air on the surface to generate kinetic energy in two ways. In one case, when air sinks from a high altitude to a low altitude, gravity pushes the air to do work, and in the other case, when air moves from an area of high pressure to an area of low pressure, the pressure gradient pushes the air to do work. The kinetic energy can be used to drive the rotation of wind turbine blades to generate electricity, but the conversion efficiency has an absolute upper limit of 59.3%, also known as the Betz limit. The efficiency of modern wind turbines has reached 80% of Betz extreme, but only 48% kinetic energy is used. In 2000 BC, human beings had begun to build sailboats and wind mills. In 1887, Charles F. Brush built the first wind turbine of the world to generate electricity ^[5]. Wind power generation capacity accounted for about 6% of global power generation capacity in 2020.

Biomass energy refers to the solar energy contained in biomass. Most biomass energy comes from organic matter produced by green plants through photosynthesis. In the broad sense, biomass includes all living organisms that can grow (plants, animals and microorganisms). In the narrow sense, biomass includes all kinds of agricultural and pastoral products planted and bred by human beings, as well as feces and organic waste produced by human and livestock. The use of biomass energy is essentially to release the energy stored in biomass. The ways of biomass energy utilization mainly include direct combustion of firewood (heat energy), biomass liquefaction (thermochemical conversion), biogas fermentation (biochemical conversion), etc.

The evolution of the universe and the Earth has provided many material prerequisites including energy and global environmental conditions for the emergence of human beings ^[13]. The development of human beings has transformed the evolution of the Earth. In this process, the development and utilization of energy drives the stepwise upward evolution of human civilization, but the progress of civilization in turn accelerates the evolution of energy utilization types and the development of utilization technologies and brings about the global fossil energy crisis and climate change, thus affecting the upgrade of human civilization (Fig. 2). The relationship between the Earth, human beings and energy needs to be analyzed from three scales. First, it is necessary to analyze the Sun-Earth-Moon system on an ultra-long time scale of cosmic evolution to reveal the evolutionary history of the Earth. Second, it is necessary to analyze the relationship between various spheres of the Earth and the relationship between mankind and the Earth on a long time scale of the Earth system to reveal the origin of life and the history of human development. Third, it is necessary to analyze the development of human society and civilization on a short time scale of human social development to reveal the evolution history of energy.

According to the Big Bang theory, the universe came into being via expansion of a singularity with extremely high density and temperature about 138×10⁸ years ago ^[14], during which hydrogen and helium were directly produced. About 50×10⁸ years ago, the gas-dust vortex dominated by hydrogen molecules contracted to form a "solar nebula". Affected by the flattening of the centrifugal force of rotation, the primordial sun was formed in the central area of rapid contraction and high density, while the rest gradually expanded around the sun along the equator to form a "nebula". The sun continued to heat up due to the conversion of the kinetic energy of matter contraction into heat energy, and hydrogen was transformed into helium through nuclear fusion. Then the sun gradually entered the stage of "main sequence star" (prime). The dust particles in the nebula collided and bonded with each other to form larger material clumps. Under the action of gravity, centrifugal force and frictional force, these material clumps collided again to form eight planets including the Earth ^[15-16]. The sun attracted the planets around it since its mass accounted for more than 99% of the mass of the solar system. Less than 1×10⁸ years after the formation of the solar system, the embryo of the Moon collided with the Earth to generate huge energy, making the Earth in a molten state. The silicates from the Earth surface and the material from the embryo of the Moon together formed the materials that surrounded the Earth. Among these materials, the heavy material cooled down and returned to the Earth, some disappeared into space, and the rest formed the moon ^[17-18]. The appearance of the moon altered the Earth's gravitational field, caused the formation of tides on the Earth's surface, changed their distance and rotational speed, affected the orbital period of the Earth, and also affected the Earth's climate by balancing the Earth's inclination. About (46-45) ×10⁸ years ago, the Sun-Earth-Moon system was formed, and the planets of the solar system were running in their respective orbits, providing a safe cosmic environment for the evolution of the Earth. The sun itself gradually stabilized and heated up, providing light and heat to the Earth and maintaining the temperature of the Earth's surface. The moderate mass of the Earth itself (0.000 3% of the mass of the solar system) keeps it at a moderate distance from the sun^[16-17] and makes the surface temperature of the Earth moderate, which provides conditions for the existence of liquid water and atmosphere, and thus provides the material and energy prerequisite for the existence of life.

Regarding the onset of the long time scale (i.e., the astronomical age of the Earth), since there may have been multiple "magma oceans" on the Earth due to the early planetesimal collision events, which completely obliterated the geological records of the original continents, the rocks and minerals that made up the Earth's original crust were not preserved intact. A tiny zircon crystal found in western Australia is the oldest known piece of our planet, dating to about 44×10⁸ years ago ^[19]. The lead-lead dating method was used to calculate the ratio of lead isotopes in the most primitive chondrites on the South Pole of the Earth, and it was indirectly inferred that the Earth was formed 45.5×10⁸ years ago. The earliest geologic records of life on the Earth come from the Isua volcanic and sedimentary rocks in western Greenland ^[15]. Accordingly, the evolution of the Earth system is described in three parts, i.e., "astronomical stage", "biosphere evolution stage" and "anthroposphere evolution stage".

Due to the lack of direct evidence and geologic records, the evolutionary characteristics of this stage are described based on the current human cognition and the general consensus of the scientific community ^{[14⇓⇓⇓⇓⇓-20]}. During the growth of the primitive Earth, the collision of planetesimals or the decay of long-lived radioactive substances inside and the release of gravitational potential energy caused a warming process, which made the upper part of the Earth in a molten state (magma ocean). In this context, siderophile elements such as iron, nickel, cobalt and manganese underwent gravitational differentiation with lithophile elements such as silicon, aluminum, magnesium and calcium to form the core and mantle. The surface layer of the mantle gradually differentiated into a thin crust. Thus, the layered structure of the Earth was gradually formed. In the "Late Big Bang" stage (44-38) × 10⁸ years ago, the collision of planetesimals triggered a collisional outgassing process on the Earth surface, releasing a large number of water molecules and other volatile compounds, which formed the original atmosphere dominated by gaseous silicates. As the Earth surface cooled, a large amount of silicates solidified rapidly, leaving behind a thick atmosphere of CO₂ and H₂O. Then the temperature of the atmosphere continued to drop sharply, and the water vapor condensed to produce precipitation, forming the hydrosphere on the Earth.

About 38×10⁸ years ago, there was no oxygen in the gradually cooling primeval atmosphere of the Earth, so there was no ozone layer to absorb ultraviolet rays in the sunlight, making life difficult to survive. However, the formation of hydrosphere provided conditions for the generation of life, and the original ecosystem began to form gradually ^[14-15]. About 27×10⁸ years ago, cyanobacteria performed aerobic photosynthesis in the form of microbial mats, which started the process of oxygen accumulation in the Earth atmosphere. The early oxygen was basically consumed by biological respiration, weathering of reducing substances, and the interaction of H₂ and H₂S with O₂ in volcanic gases. Until the "Great Oxygenation Event" 25 × 10⁸ years ago, Fe²⁺ was oxidized to Fe³⁺ and precipitated widely on the seabed, which led to the development of banded iron-bearing formations worldwide, forming 70% of the iron ore of the world. At the same time, it caused the early ocean to contain a high proportion of sulfuretted hydrogen, which delayed the great development of life on the Earth. As Proterozoic eukaryotic organisms gradually appeared, the oxygen and ozone concentrations of the Earth gradually increased and an effective ultraviolet barrier was formed. In addition, the development of seabed banded iron-bearing formations ended and the plate activity completely freed the seabed water layer from the trouble of sulfide ^[14]. About (7.5-6.0) × 10⁸ years ago, after another oxygenation event on the Earth, a large amount of organic carbon was buried, which made the inorganic carbon ¹³C heavier. The carbon released from the mantle to the surface was reduced to organic carbon, releasing free oxygen and forming an oxidized atmosphere, which provided conditions for the Cambrian life explosion about 5.4×10⁸ years ago. With the thickening of the continental crust, the continuous increase in oxygen content and the emergence of several great ice ages, the Earth's biosphere began to evolve from microorganisms, plants, animals to humans. Meanwhile, the Earth system also began to evolve from the primitive lithosphere to a multi-sphere coordinated development system of lithosphere, atmosphere, hydrosphere and biosphere^[21-22]. In addition, the five major biological events of mass extinction at the end of the Ordovician, the Late Devonian, the end of the Permian and the end of the Cretaceous ^[23-24] produced a large amount of organic matter and provided suitable conditions for its preservation, providing a material basis for the formation of fossil energy. The formation of coal was mainly concentrated in three coal-forming periods ^[25], i.e., the period from late Carboniferous to Permian when spore plants turned into coal, mainly bituminous coal and anthracite; the period of Jurassic and Early Cretaceous when gymnosperms turned into coal, mainly lignite and bituminous coal; and the period from late Cretaceous to Neogene when higher plants such as angiosperms turned into coal, mainly lignite. The formation of oil and gas was mainly concentrated in the Cambrian, Ordovician, Silurian, Devonian-Early Carboniferous, Late Carboniferous-Middle Permian, Jurassic, Cretaceous, and Paleogene-Neogene. The early source rocks have a high degree of evolution, mainly generating natural gas, while the middle and late source rocks mainly generate oil ^[26] (Fig. 3).

The anthroposphere, also known as the noosphere, is used to mark the intervention of human beings in the modern Earth system. The lithosphere, atmosphere, hydrosphere and biosphere discussed above are the environment of the anthroposphere and have a global scale, so they are called global environment. The modern Earth system is a system composed of the anthroposphere and the global environment. There are different views on the difference between the concept of the anthroposphere and the biosphere, the time of anthroposphere formation (the time of the great geographical discovery) and the concept of Anthropocene geology ^[27⇓-29]. However, after a long period of evolution, human beings have significantly transformed nature by subjectively and actively carrying out social material production and consumption on the basis of understanding nature, and have become one of the driving forces for changes in the Earth system. The range of human activities from 12.5 km below the surface of the lithosphere ^[30] to the solar system space has gone beyond the traditional concept of the biosphere. Therefore, the concept of the anthroposphere is used to illustrate the impact of the development of human civilization on other spheres in the Earth system. At the same time, with reference to the hypothesis of Kardashev Scale of the cosmic civilization proposed by the Russian astrophysicist Nikolai Kardashev in 1964 based on energy utilization rate ^[31], the evolution of the anthroposphere is illustrated from the perspective of human energy utilization methods and efficiency (Fig. 4). In the nearly 45×10⁸ years before the emergence of human beings, the Earth system was in a primitive natural evolution stage, forming lithosphere, atmosphere, hydrosphere and biosphere successively. After the emergence of human beings around 250×10⁴ years ago, the formation of primal character marked the beginning of primitive civilization. According to the conceptual framework of three types of civilizations proposed by Nikolai Kardashev, human civilization has experienced the development of primitive civilization, agricultural civilization and industrial civilization, and is sitting at 0.73 on the Kardashev Scale ^[32]. The anthroposphere has evolved from one of the species in the biosphere in the early stage to the current global sphere ^[33], and has significantly influenced the natural evolution of other spheres in the Earth system. It is developing from a low-end form of relying on, adapting to, understanding and transforming nature to a high-end ecological civilization stage of sharing and caring for nature. The prominent sign could be the mastery of controlled nuclear fusion to build an "artificial sun". Humans will become the masters of the Earth in Type I civilization, and species will not go extinct because the Earth enters the next ice age or changes in the Earth's environment such as rising temperatures. Type II civilization and Type III civilization are still only scientific hypothesis. Although the influence of the anthroposphere has broken through the Earth system, and the space station and spaceflight technology have been developed, the potential of human beings to utilize chemical energy has reached the limit. The exploration of deep underground space and deep outerspace is still in its infancy. Sustainable development is limited by material technology, battery technology, gene technology, controlled nuclear fusion technology, etc.^[34-35]. The key scientific issues that the anthroposphere needs to solve to achieve the leap into Type I civilization, such as unlimited energy, sustainable life, high-performance materials and advanced artificial intelligence, have not been solved yet. Controlled by the stepwise upward evolution of modern Earth system, the industrial civilization has developed to a bifurcation point and entered the abrupt change period. Transforming human needs and survival patterns and promoting the transformation and development of energy utilization will be the main way for the development of human society from industrial civilization to ecological civilization.

As the foundation and driving force for the progress of human civilization, energy has a bearing on the survival and development of human beings ^[36]. The research on the development history of human society and the evolution history of energy can provide reference for mankind to cope with the challenges of global environmental changes. The development of human society can be divided into survival stage (the emergence of human beings - before industrial civilization), development stage (industrial revolution - early 21st century), and sustainable development stage (mid-21st century - future). Corresponding to these three stages, the evolution of energy utilization has experienced the firewood era, coal era, and oil and gas era, and is transforming to the new energy era.

From 170×10⁴ years ago when Homo erectus yuanmouensis started to use fire to 1.8×10⁴ years ago when the Upper Cave Man mastered the fire-making technology, the earliest and longest "technological revolution" was initiated, which promoted human society to develop from the Paleolithic age to the age of agricultural civilization and go through three agricultural technological revolutions^[37]. During this period, firewood became the main energy source for the development of human society, which promoted the evolution of human beings and the development of social technology, and made the social production power develop from human power to animal power, wind power and hydraulic power (Fig. 5).

After the development of the Dark Ages in Europe, with the invention of the steam engine, steam power began to replace human and animal power on a large scale. For the first time, human society truly had a powerful internal driving force beyond the biosphere. Two technological revolutions and one industrial revolution occurred successively, which led to the rapid development of metallurgy, manufacturing, mining, transportation and other industries and accelerated the process of world industrialization.

In 1859, Americans first drilled the first commercial oil well in Pennsylvania, opening the modern oil industry. With the invention of high-power generator and internal combustion engine, oil gradually replaced coal as the third-generation main energy source because of its higher calorific value and easier transportation. Human society entered a stage of rapid industrial and technological development and ushered in four technological revolutions, three industrial revolutions, one agricultural revolution and four management revolutions ^[38], which promoted the world to enter the electrical age and the information age successively and develop towards the intelligent age. The energy supply system for social production also evolved from steam power to diversified power sources composed of gas power, electric power, wind power, hydraulic power, nuclear power, etc. (Fig. 6).

New energy generally refers to renewable energy that is developed and utilized with new technologies other than traditional energy. The utilization of new energy by human beings runs through the survival and development stages of human social development (Fig. 7). Before the 17th century, it was mainly characterized by the direct use of new energy to generate mechanical force. From the 18th century to the early and mid-20th century, human beings began to explore new energy supply mechanism and utilization technology. In the early 21st century, stimulated by the fossil energy crisis, new energy utilization entered a stage of large-scale development. In the future, as human society enters the era of wisdom and the atomic era, human beings will gradually make breakthrough in the efficient, low-cost, large-scale utilization technology and energy storage technology of new energy and establish a global smart energy network. It is expected that in the 23rd century or before, human beings are likely to finally master controlled nuclear fusion technology, completely get rid of the energy utilization mode dominated by chemical energy utilization, and open the advanced energy utilization mode that directly utilizes nuclear energy and biomass energy. At that time, traditional fossil fuels will also return to their material properties^[35,39].

From the ecological perspective of human social development, the biological attributes of human beings determine that relevant ecological laws will act on human beings. In this context, the rapid development of human society will bring about global environmental changes. As proposed by Pierre François Verhulst, under the assumption that the environmental resources on the Earth are limited and the initial population size is less than the maximum environmental capacity, the population size follows a three-stage growth pattern of the Logistic curve ^[33,40]. In the first stage, sufficient environmental capacity drives the rapid development of the system. In the second stage, as the scale of the human system grows, the limitation of environmental conditions will obviously hinder the increase of the system growth rate. In the third stage, when the system scale develops to an extent close to the environmental capacity, the system growth rate approaches zero. The growth mode of the Logistic curve indicates that the environment has an immediate restriction on the population size, but due to the subjective initiative of human beings, the impact of changes in environmental resources on the development of human society is relatively lagging behind. From the perspective of the Earth system, the so-called global change means that human activities have changed the natural operation of the Earth's surface system. Human beings with intelligence have become an unprecedented force in the Earth system and are changing the trajectory of the natural evolution of the Earth system. In particular, through the extensive use of fossil energy, humans have changed the natural carbon cycle path of the Earth system, which has seriously affected the sustainable development of human society.

Carbon, which did not exist in the primitive universe, was created by the fusion reaction of helium in a star that fused three helium nuclei together to form one carbon atom. When a star exploded into a nebula at the end of its evolution, various elements inside it dispersed into space and were captured by the Earth in the form of dust, particles and celestial bodies, thus starting the carbon cycle on the Earth. Carbon atoms participate in chemical reactions through various valences such as +4, +2, 0 and -4^[14-15] and have a variety of bonding ways, in which the C-C bond is extremely strong. Carbon can form inorganic compounds, and also organic compounds through covalent bonds of shared electrons between atoms. It is an element with the most complex existence form on the Earth. It is precisely because of it extremely high bonding ability and unique properties that carbon becomes the main chemical energy carrier of solar energy and the skeleton element of all kinds of life on the Earth, as well as the main component of greenhouse gases. The carbon cycle can be subdivided into inorganic carbon cycle, short-term organic carbon cycle and long-term organic carbon cycle, which intersect and occur simultaneously^[41] (Fig. 8).

The inorganic carbon cycle is mainly a closed-loop cycle of inorganic carbon between the atmospheric carbon pool, oceanic carbon pool and lithospheric carbon pool. A cycle spans from millions to tens of millions of years, keeping the Earth in a relatively stable temperature range. The CO₂ in the atmosphere dissolves in rainwater, and then erodes terrestrial rocks through chemical weathering or is directly absorbed by the ocean to form carbonate deposits on the seafloor, which undergo sedimentary diagenesis to form carbonate rocks. The subduction collision between oceanic crust and continental crust makes carbonate rock melt into magma and degassing occurs. Then it is converted into CO₂ again and returned to the atmosphere (Fig. 8a).

The short-term organic carbon cycle is a closed-loop cycle between the atmospheric carbon pool and the biosphere carbon pool. A cycle spans from decades to hundreds of years. Plants, as primary producers, use chlorophyll to absorb solar energy through photosynthesis and convert low-energy CO₂ and water into sugars with high chemical energy. Then inorganic carbon enters the biosphere for the first time in the form of organic carbon. As consumers, organisms obtain energy to sustain life through the food chain. The remains and excretions of animals and plants are decomposed by microorganisms and release CO₂ (Fig. 8b).

The long-term organic carbon cycle is a closed-loop cycle between atmospheric carbon pool, biosphere carbon pool and lithosphere carbon pool. A cycle spans tens of millions of years. Its initial process is similar to that of the short-term organic carbon cycle, but the remains of plants and animals are buried deep in the ground before being broken down by microbes, where they undergo a long physical and chemical process to turn into fossil fuels such as coal, oil and gas, which are then burned during plate movement, releasing CO₂ (Fig. 8c).

Carbon is one of the earliest elements used by human beings, and the use of carbon brought human beings into the age of civilization. However, human civilization in turn becomes a new element of the Earth carbon cycle, changing the closed-loop path of the Earth carbon cycle (Fig. 9). In the era of primitive civilization, humans were still part of the biosphere. Limited by the number of humans and the scale of fire utilization, the extra CO₂ emitted could be quickly regulated by the Earth carbon cycle. However, in the era of agricultural civilization, with the development of farming technology and the increase of population, the use of firewood caused the carbon that should have remained in the biosphere for hundreds of years to be discharged into the atmosphere in advance. Moreover, the reduction of forest area caused by deforestation weakened the carbon storage capacity of forest carbon pools. As a result, CO₂ concentration in the atmosphere began to rise 7000 years ago, but the rise was very slow, so it did not completely change the Earth's carbon cycle. Since the 18th century, human beings have entered the era of industrial civilization. The extensive use of fossil energy accelerates the release of organic carbon in deep formations, and a large amount of CO₂ released in advance enters the Earth system. However, forests with declining carbon storage efficiency can only absorb about 30% of the CO₂ emitted by humans. In addition, the ocean is acidified due to the absorption of excess CO₂, which limits the carbon storage efficiency of the ocean carbon pool, so it can only absorb about 30% of the CO₂ emitted by humans. In this case, the remaining 40% or so of the CO₂ enters the atmosphere and does not participate in the carbon cycle in the short term. As a result, the CO₂ volume fraction in the atmosphere increased from 280×10^-6 before the Industrial Revolution (1760) to 400×10^-6 in 2015, exceeded 410×10^-6 in 2019, and reached 415×10^-6 in 2021. In the past 70 years, the growth rate of CO₂ concentration in the atmosphere was about 100 times that at the end of the last glacial period^[42-43], which has become a direct factor affecting the chain reactions such as global temperature and climate change.

Global climate change has become a serious threat to mankind in the 21st century. With the continuous growth of population size and the rapid development of society, the scale of the anthroposphere is approaching the environmental capacity of the Earth. As of January 2022, the total population of 230 countries in the world was 75.97× 10^{8 [44]}, of which the developing countries accounted for about 80%. In the coming decades, with continuous population growth and more people in developing countries shifting to a modern lifestyle, the world greenhouse gas emissions will continue to increase at a rate of 510×10⁸ t/a. As a result, the energy budget of the Earth system will be unbalanced, extreme weather events and epidemic diseases will become increasingly frequent, and human beings will face unprecedented challenges in the sustainable development ^[45].

The continuous increase in the concentration of greenhouse gases such as CO₂ increases the net energy absorption of the Earth, which leads to the continuous warming of the geosphere. In 2012, Hansen and Sato performed a simulation based on climate data over the past 80 × 10⁴ years, which revealed that when the CO₂ concentration equivalent in the atmosphere doubled, the equilibrium change in the annual mean of global average surface temperature would be (0.75±0.15)°C. Thus, it is estimated that the global average surface temperature will rise by 2.5 to 3.5 °C ^[5-6]. The observation data of global average temperature since 1880 revealed that from 1880 to 1930, global average temperature was stable, with a value change of around 0 °C; from 1930 to 1980, global average temperature rose slowly, with a value change of 0.1-0.5 °C; and from 1980 to 2020, the global average temperature rose at a higher rate, with a value change of 0.5-1.2 °C. It indicates that 70% of the temperature increase occurred in the past 40 years, the Earth surface temperature is higher than at any time in the past 140 years, and the risk of irreversible climate change is increasing ^[46] (Fig. 10). In 1977, American economist William Nordhouse first proposed a global temperature rise control target of 2 °C. In 2015, it was formally proposed to limit the global average temperature rise to the pre-industrial level (2 °C) by the end of this century in the Paris Agreement. However, according to the Special Report on Global Warming of 1.5°C (SR15) completed by the Intergovernmental Panel on Climate Change (IPCC) after a two-year effort in 2018, limiting the global temperature rise to 1.5 °C is more effective in reducing the global climate risk than limiting it to 2 °C ^[47]. In 2021, a resolution to "limit the global temperature rise to 1.5 °C" as one of the goals to ensure the sustainable survival of human beings on the Earth was reached in the United Nations Climate Change Conference ^[46-47].

According to IPCC assessment ^[47], the past 40 years saw a trend of increasing and intensifying extreme events such as extreme low temperature, extreme high temperature, extreme drought, extreme precipitation, and volcanic eruptions ^[48] (Fig. 11). About 190 extreme weather events occurred globally in 2021. The "lungs of the Earth", the Amazon rainforest, is approaching the tipping point of ecological collapse and is transforming from a carbon sink to a carbon source. With continuous decline of ice extent, the Arctic Sea will face the phenomenon of no ice in summer. The thermal expansion of sea water and the melting of land ice have caused the global sea level to rise rapidly. Compared with the 2.1 mm in the period from 1993 to 2002, the annual global average sea level rise increased by more than 100% to 4.4 mm in the period from 2013 to 2021. The pH value of the global open ocean surface has dropped to its lowest level in 26 000 years, and the ability of ocean carbon pools to absorb CO₂ has decreased accordingly. Up to now, 9 of the 15 "climate tipping points" in the world have been activated. Extreme weather may accelerate and trigger the "domino" effect of the Earth system, thereby directly or indirectly affecting the survival and sustainable development of human beings in the form of natural disasters and diseases. So far, 16 major epidemics have broken on human beings, leading to more than 10 × 10⁸ deaths. The outbreak of SARS epidemic in 2002-2003 caused 919 deaths in a short period of time, with a case fatality rate of 11%. The COVID-19 epidemic has caused more than 600×10⁴ deaths worldwide since its outbreak in 2019 (Table 2). Humanity is facing the most severe test since the World War II. As the environmental crisis brought about by global climate change continues to develop, extreme weather with greater intensity and wider impact in the future will bring more uncertain risk factors to all spheres of the Earth system. All countries in the world have generally recognized that in order to avoid the threat of global extreme weather, human beings should stop the behavior of releasing 510×10⁸ t CO₂ equivalent of greenhouse gases to the global atmosphere each year and strive to achieve "net zero" emissions by the end of the century, but also should eliminate the CO₂ that has been emitted in human history to achieve “net negative emissions” ^[49].

Under the goal of carbon neutrality, the transition from carbon-based energy to non-carbon-based energy is underway. In this context, the transition to a low-carbon and zero-carbon energy system will accelerate ^[50-51]. Fossil energy will gradually transform from main energy to guaranteed energy, and new energy will gradually become the main clean energy ^[52-53]. Transformative breakthroughs will be made in energy technology. Disruptive technologies such as CCUS (carbon capture, utilization and storage)/CCS (carbon capture and storage), hydrogen energy and fuel cells, biophotovoltaic power generation, solar power generation, optical storage smart microgrid, super energy storage, controlled nuclear fusion, and future internet for smart energy will gradually be conquered. The energy concept will change, from simply meeting the energy needs of human beings to green energy use that protects and shares the Earth. Building a green energy community will become an important part of building a community with a shared future for mankind. Pursuing green innovation, contributing the green energy and constructing the green homeland will become three goals of the new energy technology innovation system.

The renewal and transformation of energy concept requires re-understanding and re-development of energy science theory. Under the goal of carbon neutrality, the establishment of a green energy system is critical. The proposal of energy science ^[24] broke the traditional single type of energy research paradigm, and human for the first time explored the scientific issue of the interaction and co-evolution of the Earth, energy and human beings from a systematic perspective. The study on the formation and distribution of various energy sources, evaluation and target area selection, development and utilization, orderly replacement, and development prospects, etc. on the temporal and spatial scales can reveal the symbiotic distribution relationship and development law of various energy carriers in the Earth system, which is of great significance to improving the discipline system of energy research. Carbon neutral science expands the research scope of energy science. It aims at the dynamic balance between carbon emissions caused by human activities and the Earth carbon cycle system by using carbon-free new energy to replace fossil energy in an orderly manner based on economic and industrial policies and energy technology. As an interdisciplinary subject between energy science and social science, it explores the scientific laws of minimizing the impact of human activity footprints on the natural environment ^[54-55]. Besides talent cultivation and technological and management innovation, it is necessary to carry out innovative development of energy science and carbon neutrality, which will provide theoretical guidance for sustainable development and utilization of Earth energy, clean and green development, and building a livable Earth.

As scientific innovation is an endless frontier and technological development is an infinite power, scientific and technological innovation is the main driving force to achieve carbon neutrality. Low-carbon and ecological development have become prominent features of the new round of scientific and technological revolution. Green, intelligent and sustainable development has become major themes. The development of core technologies for carbon emission reduction, zero carbon emission and negative carbon emission has become an important direction and key goal of scientific and technological innovation in the energy transition. The development of energy system towards carbon neutrality will be promoted through the energy substitution, cross-border integration and energy transition, and the "new energy" + "smart energy" system with clean, carbon-free, intelligent and efficient core will be constructed through the development of carbon industrial system and hydrogen industrial system.

Green development embodies the value pursuit of a community with a shared future for mankind. It has become the common concern and expectation of the world and implies the integration and convergence of global collaboration and cooperation. Building a green energy community requires human beings to establish a new model of energy technology innovation and energy cooperation and development based on win-win, mutual trust, synergy, participation and sharing within the framework of the "global village" concept, to jointly address challenges such as climate change. Human beings need to respect nature, conform to nature and care for nature, actively promote the realization of global sustainable development goals, help promote the construction of a community with a shared future for mankind, and jointly build an ecologically livable green Earth.

China has grown from a “small energy country” with a total energy output of 0.2×10⁸ t of standard coal in 1949 to a “big energy country” with a total energy output of 41.7×10⁸ t of standard coal in 2021. In the future, to leapfrog to "energy power" and gradually realize "energy independence", China must guarantee the national strength in energy. To speed up the energy revolution and build a powerful energy nation, China should adhere to the concept of "green innovation, green energy and green homeland", open mind to slip the leash of traditional concepts, break through the technical bottlenecks that restrict industrial upgrading to develop the advanced technology, and rely on high-level self-reliance of science and technology to achieve high-quality self-reliance in energy. China should secure the cornerstone of the supply of fossil energy and forge a strong growing pole for green and sustainable development of new energy. Energy development has three major trends, i.e., low-carbon fossil energy, large-scale new energy and intelligent energy system. A high-quality green energy system of cleaning, low-carbon, safety, efficiency and independence should be built in "three steps", in order to realize the strategic transformation of China energy production and consumption structure from the type of fossil energy domination to the type of new energy domination.

A new green energy system will be built in three steps. (1) From 2021 to 2035, efforts are taken to clean fossil energy and accelerate the development of new energy. The clean coal utilization and shale oil/gas exploitation are industrialized and the renewable and nuclear energy resources are developed in a large scale to increase the proportion of new energy and ensure the supply of oil and gas. (2) From 2036 to 2050, equal efforts will be made on fossil energy and new energy. The scale of new energy will be expanded, with the coal consumption reduced. In-situ recovery of shale oil and oil-rich coal will be industrialized to release a large amount of potential resources. Revolutionary breakthroughs will be made in key technologies such as energy storage and controlled nuclear fusion. The "Hydrogen China" society will be basically shaped. (3) After 2051, new energy dominates the market and will become cheaper and extensive. A smart energy system will be established. The energy mix will transform revolutionarily from the type of fossil energy domination to the type of new energy domination. The status of fossil energy and new energy will reverse historically.

To secure the supply of fossil energy, efforts should be made as follows. First, coal is utilized efficiently in a cleaner manner and with ultra-low carbon emissions, so as to play a reserve and fundamental role in China's long-term strategic energy security. Second, oil is stabilized and unconventional oil serves as a strategic replacement, in order to satisfy the "urgent need" in national energy security and secure raw materials for the people livelihood. Third, gas, both conventional and unconventional, is expanded, in order to act as the best "partner" of new energy in ensuring national energy security. Fourth, new energy is actively developed, and multiple energy resources are coordinated in both centralized and distributed patterns, so as to play the role of “replacement” and “main force” in ensuring national energy strategy.

Based on the new generation of digital and intelligent information technologies, a flexible, stable and smart energy system is established. The system is characterized by the intelligent coordination of power supply, power grid, load, and energy storage, the complementation of multiple energy sources including coal, oil, gas, electricity and hydrogen, the integration of power grid, heat grid and fuel grid, and flexible responses to both production and consumption. It can help realize the connection and integration of energy supply side, distribution side, and demand side. Moreover, the smart "big energy" management is adopted to improve the energy efficiency, and various energy entities are encouraged to independently access to the energy system and participate in energy transactions, so as to promote the energy efficiency and service levels.

A gas reserve system combining strategic and commercial reserves and comprising underground and above- ground storages is established to keep gas as "stabilizer" in the large-scale development and utilization of renewable energy. An oil reserve system featuring "basic guarantee and peak shaving is constructed according to the strategy of "exploring oil for reserve", with an expected capacity of 10×10⁸ t proved reserved. Moreover, a multi-energy reserve system is pursued, which incorporates mechanical energy, electromagnetic energy, electrochemical energy and other energy sources in addition to conventional oil and gas, and it can be centralized or distributed, depending on the energy characteristics and the energy demand under normal and abnormal economic and social conditions.

An industrial system of "green electricity, green hydrogen, green heating and low-carbon" is built along with the new energy development and energy reserve system construction. The electricity sector transforms from the "grey power" dominated by thermal power to the "green power" dominated by new energy power. The "grey hydrogen" sector dominated by fossil energy will shift to a "green hydrogen" sector dominated by new energy. Particularly, in northwest China, the wind, solar and thermal energy and novel energy storage technologies are actively promoted to secure the "West-to-East Hydrogen Transmission" project; in southeast China, ocean energy and hydrogen production from seawater are expanded to meet the hydrogen demand in this region. The geothermal energy in oil and gas fields is fully utilized to build a geothermal sector that can provide both heating and cooling for the residents. The "grey carbon" utilization is improved, and a full-chain carbon sector with CCUS/CCS as the core and covering carbon capture, carbon transportation, carbon flooding, carbon storage, carbon products, and carbon finance is created.

Ten questions are raised on the future of energy from the perspective of universe evolution, Earth system evolution, and human society development and energy evolution. (1) How will humans manage to maintain the Sun-Earth-Moon system through thousands of generations before the sun becomes a red giant after 56×10⁸ years? (2) How can humans avoid the disintegration of civilization caused by the sixth mass extinction that may occur periodically of the Earth? (3) How does human being use the current environment and remaining resources of the Earth to support the development of human beings from industrial civilization stage to ecological civilization stage? (4) What kind of green "geoscience" and "geoengineering" is needed as global climate change increasingly threatens mankind? (5) The process of human development is to break the limits of time and space. Which technological bottlenecks will need to be overcome first for human beings to go to space? (6) With the progress of human society and energy transition, besides carbon neutrality, is there any need for other elements to be "neutralized" or "compensated"? (7) For the ultimate energy in the future, in addition to mastering controlled nuclear fusion technology, are there other green energy options with higher energy density? (8) Will artificial photosynthesis technology, which uses "artificial leaves" to produce "super fuels" and fix CO₂, be widely used in the future? (9) Humans can gradually replace the energy properties of fossil fuels, but what can be used to replace their material properties to sustain human development? (10) It has taken thousands of years for new energy to be used on a large scale, but will it take a long time for it to be used in a smart and efficient way?

The origin of life and the formation of energy are closely related to the Earth system. There are three scales of co-evolution. On an ultra-long time scale, the evolution of the Sun-Earth-Moon system has provided energy sources and extraterrestrial environmental conditions for the formation of the Earth system. On a long time scale starting from the astronomical age of the Earth, the evolution of the Earth system has gone through the astronomical stage, biosphere evolution stage and anthroposphere evolution stage, providing the material preconditions such as energy resources and suitable sphere environment for life birth and human development. On a short time scale starting from the birth of human beings, human has established an energy era represented by coal, oil, gas and new energy, and used energy to promote social progress, so that the anthroposphere has broken through the Earth system and expanded to extraterrestrial civilization, which pushes the Earth civilization to leap from planetary civilization to stellar civilization.

There are three processes in the carbon cycle, i.e., inorganic carbon cycle, short-term organic carbon cycle and long-term organic carbon cycle, which record the co- evolution process of the Earth, energy evolution and human development. The biological attributes of human beings determine that the consequences of human efforts to transform the Earth will affect human beings. Due to immoderate utilization of fossil energy and global sphere reforming activities, human has broken the natural balance and closed-loop path of the Earth carbon cycle, resulting in the increase of greenhouse gases and global climate change, which seriously affects the sustainable development of mankind. As the Earth surface temperature has risen to its highest level in 140 years, "limiting the temperature rise to 1.5 °C" has become a global goal to ensure the sustainable development of mankind. In this context, energy transition is inevitable and the vision of carbon neutrality will surely be realized.

Building a global green energy community is a fundamental measure to create the new energy system under the goal of carbon neutrality. China is speeding up its energy revolution and developing a powerful energy nation. It is necessary to secure the cornerstone of the supply of fossil energy and forge a strong growing pole for green and sustainable development of new energy. China energy production and consumption structure will achieve a revolutionary transformation from the type of fossil energy domination to the type of new energy domination, depending on a high-level self-reliance of science and technology and a high-quality green energy system of cleaning, low-carbon, safety, efficiency and independence.

The strategic goal of carbon neutrality accelerates the arrival of the new energy era. The end of Stone Age was not attributed to the shortage of stones. Similarly, the shortening of the fossil energy era is not due to the shortage of resources. The transition from traditional fossil energy to non-fossil new energy is inevitable. Energy development has three major trends, i.e., low-carbon fossil energy, large-scale new energy, and intelligent energy system. We should open mind to tap into the boundless power of science and technology and strive to achieve energy independence, relying on the green innovation, contributing the green energy and constructing the green homeland.