In our fresh memory, huge hurricanes Helene and Carolina hit the southwest area of the USA, one after another, in 2024, and a tremendously wide area was damaged seriously. In addition, many wildfires broke out in Los Angeles in January 2025. Regarding the weather that induced these wildfires, the World Weather Attribution (WWA) reported that the weather conditions might cause a 35% increase in the probability of wildfire occurrence based on high temperature, dry air, and light rain, compared with that before the Industrial Revolution. In Japan, there were several mountain fires in Iwate, Okayama, and Ehime Prefectures, and their burned areas were 2900, 565, and 442 hectares, respectively, within February 2025. Fortunately, these fires were extinguished by the rain. Contrarily, a tremendous amount of burned wood, as well as a large amount of CO₂ emission during the fire, was produced, although they had grown, followed by capturing a huge amount of CO₂ before the fires.

The Intergovernmental Panel on Climate Change (IPCC) concluded on 9 August 2021, that climate change has been caused by human activities that have produced carbon dioxide (CO₂) since the Industrial Revolution [1]. To reduce atmospheric CO₂ concentrations as a means of mitigating such effects, the so-called Paris Agreement was reached at the United Nations Climate Change Conference (COP20) in 2015. This agreement was based on the requirement to keep the increase in the mean global temperature below 2°C related to the temperature before the Industrial Revolution, preferably less than 1.5°C. China is the world’s largest emitter of CO₂, while the President of the People’s Republic of China, Xi Jinping, has declared that China will be carbon neutral by 2060. The former president of the USA, Joe Biden, rejoined the Paris Agreement on 20 January 2021, whereas the present US president, Donald Trump, quit the Paris Agreement on 21 January 2025. Incidentally, the USA is the world’s second-largest emitter of CO₂. Scientists conclude that this fact will likely lead to dangerous irreversible levels of climate change [2]. People should notice that irreversible climate change cannot be repaired again with money and that we have induced the present climate change since the Industrial Revolution. Even if a carbon-neutral society could be immediately achieved, the accumulated atmospheric CO₂ would not be reduced at present.

CO₂ can be captured from the ambient air or gas via several technologies [3], including absorption [4], adsorption [5–9], and membrane gas separation [5,10]. Absorption with amines is currently the dominant technology. However, amines are organic solvents that chemical reactions must synthesize, and heat treatment is necessary to release CO₂ from the CO₂-amine complexes, resulting in heat decomposition of amines. Eventually, the amine technology is not used worldwide. Contrarily, Membrane and adsorption processes are still in the developmental stages, with the construction of primary pilot plants anticipated in the near future. However, to the best of our knowledge, these methods alone cannot achieve the necessary worldwide reductions in atmospheric CO₂.

On the other hand, we recently developed an innovative method for CO₂ fixation and storage [11]. This method is based on simple chemical reactions involving NaOH and CaCl₂. Using low concentrations of these chemicals prevented the formation of Ca(OH)₂ in the absence of CO₂ but resulted in CaCO₃ formation in the presence of CO₂ bubbling. Additionally, a polyethylene tunnel-based improvement method for CO₂ fixation with NaOH mists was proposed as an “artificial forest” model [12]. In this model, CO₂ penetrates a large polyethylene tunnel and is then converted into CaCO₃, which is stable and harmless. Namely, this CO₂-fixing process is likened to photosynthesis, in which CO₂ is converted to carbohydrates in the plant. The present study has been designed to develop a sustainable energy circulating system based on the integration of an energy circulating unit and CO₂ capturing unit, consisting of low-cost based on simple chemical reactions and simple facilities to spread worldwide.

Our previous studies [11] showed that the NaOH mist can efficiently capture CO₂ in the plastic pet bottle, representing the closed system. In experiments using a chimney model (Fig. 1a), when the chimney contained high CO₂ concentrations, the amounts of NaOH and CaCl₂ in the solution were insufficient to react with all the CO₂ at a gas flow rate of approximately 110 cm³/s (Fig. 1b). Thus, the solution could only capture a relatively small amount of the CO₂ in the chimney model.

To continuously capture a large amount of CO₂ from the ambient air or exhaust gases, a new CO₂ capture model was developed, as shown in Fig. 2. Using a large chamber equipped with many spray nozzles, CO₂ can be efficiently captured by droplets or mists of the NaOH solution. It is possible to expand the chamber in both directions, such as vertical and horizontal directions, increasing the chamber volume. It is easy to increase the number of nozzles that are equipped with pipelines, expanding three dimensions. Thus, it seems possible to use natural caves, mine galleries, used tunnels, and used buildings, which have a large space for the CO₂ reaction with OH^-, instead of new constructions, when the floor, wall, and ceiling are covered with polymer sheets. This unit could also be combined with the NaOH generating unit, which produces electricity.

This system is applicable to thermal power plants, chemical plants, large ships, combustion operations, incinerators, and automobiles. For example, the incinerator could produce electricity, integrating with the generator, as shown in Fig. 3. The thermal power plant is already established, so it is easy to integrate the thermal power unit with the present CO₂ capturing unit. In addition, as the electrolysis unit can produce HCl, the addition of HCl can convert Na₂CO₃, which is formed from CO₂ and NaOH to pure concentrated CO₂ and NaCl. Similarly, CaCO₃ can be converted into CO₂ and CaCl₂ by the addition of HCl. The focused CO₂ could be used for further chemical reaction materials. The synthesis of methanol from CO₂ is practically important because methanol is a primary raw material for the production of numerous other chemicals [13]. Methane is produced from CO₂ [14], and hydrocarbons are made from the ambient air by using a photo catalyzer in the presence of H₂O [15–17]. These facts indicate that the presently proposed CO₂-capturing technology is applicable to produce energy from CO₂.

Electrolysis of NaCl solution produces H₂ and Cl₂ from the anode and cathode, respectively, as follows:

2Na⁺ + 2Cl^- + 2H₂O → H₂↑+ Cl₂↑ + 2OH^- + 2Na⁺

CO₂ easily reacts with OH^- to produce CO₃^2-.

CO₂ + 2OH^- → CO₃^2- + H₂O

CO₃^2- reacts with Ca²⁺ to produce CaCO₃.

CO₃^2- + Ca²⁺ → CaCO₃↓

The total reaction is:

H₂O + CO₂ + Ca²⁺ 2Cl^- → H₂↑ + Cl₂↑ + CaCO₃↓

Using the electrolysis of NaCl solution in a CO₂ fixing unit, CO₂ is converted to CaCO₃ along with H₂ and Cl₂ productions followed by CaCl₂ addition. Namely, the electrolysis of 2 mol NaCl produces 1 mol H₂, 1 mol Cl₂, and 1 mol CaCO₃. When low-cost electricity could be provided for the electrolysis, the cost reduction of CO₂ fixation could be obtained along with H₂ production which has high potential as fuels and chemical materials.

The H₂ produced from the electrolysis based on the thermal power plant could be supplied not only to the hydrogen generator, which produces electricity but also to the chemical industry as fuel or starting materials of methane and ammonia [18,19]. Neglecting the energy loss due to the electrolysis and hydrogen generator, the integrated system is a complete energy circulation system. At the same time, the total energy might exceed the energy loss by the thermal power (Fig. 4A). Solar power panels are also capable of CO₂ capturing unit, as shown in Fig. 4A. In this case, the integrated system consists of a CO₂ capturing unit and solar power panel. The system is very simple without a thermal power unit (Fig. 4A). Similarly, not only renewable energy such as hydro power, wind power, geothermal power, and biomass plants but also even nuclear plants, could be applicable. The CO₂ capturing from a thermal power unit is not necessary. In addition, by incorporating the storage battery into the integrated system, the system would be independent not only of weather conditions but also of time (Fig. 4B). This system must be very beneficial for the mass reduction of CO₂ from the atmosphere. In addition, the system could be applied to the brown H₂ production industry from brown coal and H₂O, exhausting a large amount of CO₂ which could be captured, and to the methane fermentation which produces simultaneously methane and CO₂ to remove CO₂.

In our proposed CO₂ fixation flow sheets (Figs. 3 and 4), the relationships between the energy-generating unit and the CO₂ fixation unit are shown. Namely, only human beings found that burning fossil fuels, coal, oil, and natural gas, produces energy as well as wood, resulting in the accumulation of CO₂ in the atmosphere. On the other hand, the accumulated CO₂ could be removed by our technology without environmental concerns, as shown in Fig. 5.

Plants capture large quantities of CO₂ based on photosynthesis and are distributed on the large parts of the earth, not only on the land surface but also in the shallow sea. However, the planet’s large forest, the Amazon, which greatly contributes to the removal of atmospheric CO₂, is continuously shrinking because of commercial development and serious fires. The plant can spontaneously capture CO₂ from the extremely low concentrations of CO₂ in the ambient air and produce wood, leaves, fruits, and crops under sunlight. Eventually, as these natural products consist of organic compounds such as cellulose, lignin, and carbohydrates, they are basically flammable materials that potentially have high energy which was obtained from the concentrated thin sunlight energy.

Recently, we discovered that the high penetrability of CO₂ in the gas phase caused “Pseudo-osmosis” against polymer elasticity not only cellulose membranes but also plastic membranes based on the CO₂ concentration gradient [20]. In this phenomenon, CO₂ can penetrate through the polymer membranes, such as latex and cellulose, while O₂ and N₂ whose penetrability is extremely lower compared with that of CO₂, can’t penetrate the membrane. Therefore, when a latex glove partially filled with air is left in a glass bottle filled with high-concentration CO₂ gas, the latex glove expands spontaneously. The CO₂, air, and polymer membranes resemble water, impermeable compounds, and semipermeable membranes, respectively, in the osmosis. We named this discovery “pseudo-osmosis”. This characteristic of CO₂ contributes to CO₂ absorption from the air or water by plants as well as their pores.

The dead plants were converted to coals by geothermal heat and pressure deep underground in the Carboniferous period, while the remains of plants and animals were converted to oil in liquid condition and natural gas in the gas condition, respectively. Namely, these fossil fuels are high-energy materials, although they produce a large amount of CO₂ at the time of combustion. Consequently, we have used valuable fossil fuels since the Industrial Revolution because of their easy handiness, whether people living on the earth are wealthy or not. We make a certain amount of waste based on papers and foods in our daily lives, while our integrated CO₂ capturing system is very beneficial for us to dispose of waste without environmental concerns.

Cyanobacteria have been thought that large quantities of CO₂ were produced about 2.7 billion years ago by the photosynthesis of chlorophylls with CO₂ and sunlight in the presence of H₂O, in the shallow ocean, resulting in stromatolite, CaCO₃, formation. Even at present, this organism still lives in Shark Bay, Australia. In the food chain, a large amount of phytoplankton breeds under sunlight, and CO₂ exists in the ocean followed by the zooplankton prosperity which leads to animal foods. These processes contribute obviously to CO₂ fixation in the ocean, although the organisms produce CO₂ via their respiration [20]. Of course, organisms including humans release CO₂ via the metabolism of carbohydrates, the TCA cycle, which are produced by plants. Interestingly, we found that not only sodium carbonates, NaHCO₃, and Na₂CO₃ [21], but also amines [22] accelerated glucose consumption in cultured cells. These results indicate that the CO₂ balance is reserved in nature but also organisms if it were not for CO₂ production based on using the fossil fuel.

Recently, plastic waste has been shown to be a significant environmental pollutant, and microplastics have been found to affect marine organisms [23]. A small portion of the plastics used daily in human activities is recycled, while the remainder is simply treated as waste. Many of these materials could be incinerated but are typically sent to landfills. However, if the present CO₂ fixing system becomes available, this waste could be readily disposed of by burning without environmental concerns and with the potential to generate energy.

CO₂ also dissolves in the oceans to form H₂CO₃, HCO^3- and CO₃^2-, there is approximately 50 times as much carbon dissolved in the oceans in the atmosphere [24]. However, even though a certain amount of Ca²⁺ (0.4%) is dissolved in the ocean, CaCO₃ formation does not take place in nature. This fact indicates that this reaction does not take place under natural inorganic conditions without organisms. Indeed, limestone, CaCO₃, consists of the fossils of Fuslinids, belonging to protozoa in the Paleozoic and Mesozoic eras, several hundred million years ago, and at present, coral reefs and shells are formed by coral and shellfishes, respectively. In our experiments, the bubbling of the atmosphere into the seawater did not produce obvious CaCO₃, whereas white precipitates of CaCO₃ were formed by the addition of a small amount of NaOH (unpublished data). This experimental fact indicates that limestone and coral reef formations were carried out by living organisms based on biological reactions, not chemical reactions. These living organisms are indeed capturing spontaneously CO₂ from the atmosphere or oceans on large parts of the earth. Therefore, environmental conditions where coal and plants can live continuously must be reserved to prevent climate change.

In our previous study [11], we found that CO₂ is converted into CaCO₃ as a result of NaOH and CaCl₂. CaCO₃ is a main component in limestone or coral and is almost insoluble in water and harmless to organisms. Contrarily, for CO₂ storage, geo-sequestration by injecting CO₂ into underground geological formations, such as oil fields, gas fields, and saline formations, has been suggested [25,26], although these systems are still projects for the future. Considering CO₂ condensation, transportation, and storage technologies, the total CO₂ storage cost might be extremely high, compared with our integrated CO₂ technology.

It is impossible not to use fossil fuels, such as coal, oil, and natural gas, in our daily lives because the present acquired civilization is based on the consumption of fossil fuels since the Industrial Revolution. However, it seems possible to preserve the present daily lives using continuously fossil fuels if our proposed sustainable circulating energy system, which does not emit CO₂ into the atmosphere, could be incorporated into our society immediately. Adopting the present system could improve climate change without social confusion.