Energy storage is needed to compensate for the intermittent nature of solar and wind power. Alternatively, overcapacity can be built up in the renewable energy system. Overcapacity can effectively reduce the “winter gap” in regions with strong seasonal fluctuations and help to balance the local energy supply. Once the energy transition is complete, the surplus can be used during peak production periods to reverse anthropogenic carbon flows.
Since 1988, atmospheric CO₂ concentrations have been above 350 ppm, which is considered the long-term safe climate threshold. CO₂ concentrations above 350 ppm for more than a century can trigger tipping cascades in the dynamic Earth system, leading to a series of selfreinforcing feedbacks with catastrophic and irreversible consequences for human society and the integrity of the biosphere.
Having limited peak warming by achieving net-zero emissions, it is imperative to begin removing excess CO₂ above 350ppm from the atmosphere and store it permanently and safely. This means that all CO₂ emissions accumulated since 1988 must be removed from the atmosphere. As the risk of exceeding the 1.5 °C limit increases, the scale, and speed of removal will continue to increase. Even ‚difficult to avoid‘ emissions must be absorbed or offset by additional negative emissions to achieve net-zero emissions, on top of what is needed to return to a safe climate state. It is therefore crucial to minimise remaining emissions and prioritise societal sources for a complete phase-out of fossil fuel emissions.
The emissions required for the transition and any delay in climate action will increase the amount of CO₂ that needs to be removed. It is estimated that, in a best-case scenario, about 1500 Gt CO₂ or 400 Gt carbon (GtC) have to be removed from the atmosphere.
As the risk of triggering tipping cascades above 350 ppm increases with time, it is necessary to reverse the anthropogenic carbon flux and sequester atmospheric CO₂ at a rate equivalent to the rate of emissions over the last century. Biomass growth through photosynthesis naturally sequesters atmospheric CO₂ at between 2.5 and 4.3 GtC/yr. While biomass carbon sequestration can be enhanced by engineered systems, biogenically mediated sequestration is ultimately limited by land, nutrients, water and the inherent growth dynamics of biological systems. The scale
and rate of biogenic carbon sequestration alone will not be sufficient to prevent climate change. Despite this limitation, pyrolysis of waste biomass to biochar can help remove excess carbon as a first step.
Technological processes have already been developed for direct air capture and for removing CO₂ from seawater. Unlike biogenic capture, technical CO₂ capture could be scaled up in the long term, but requires large amounts of energy. As long as fossil fuels are burned, technical capture will be partially offset by emissions from energy production, and it is far more efficient to use society‘s resources to replace carbon-emitting sources. The strategy for a rapid return to 350 ppm is to first switch to 100% renewable energy, stabilise or reduce society‘s final energy demand, and match energy demand to renewable supply to minimise energy storage. At this stage, the atmosphere will begin to be cleaned up through technological pathways powered by zero emission energy. Bio-based carbon sequestration can be enhanced during the energy transition, helping to minimise overshoot and offset remaining process-related emissions.
Most importantly, solutions need to be developed for the safe and permanent storage of large quantities of captured CO₂. Carbon capture and storage (CCS) typically involves capturing CO₂ from industrial sources with high concentrations of CO₂ and transporting it to a long-term storage site, usually underground. Except the special case of BECCS (Bioenergy with Carbon Capture and Storage), CCS aims at reducing industrial emissions rather than achieving negative emissions. The world‘s potential underground storage capacity is estimated at 3,000GtC, although the actual usable capacity is orders of magnitude smaller. Injection of CO₂ into depleted oil and gas fields has been used primarily
to enhance oil and gas recovery, resulting in emissions in excess of sequestration. Other solutions include injection into deep saline aquifers and sediments, or mineral carbonation of basalt formations by CO₂ injection. Storage of gaseous/supercritical CO₂ requires large amounts of energy and equipment, e.g. to compress the CO₂, transport it by ship or pipeline to sites with suitable geological formations, and inject it at depths of more than 800 metres. Once stored deep underground, leakage remains a concern. In addition to the technical challenges, all these activities impose an additional financial burden, which is not alleviated by the economically viable use of the captured CO₂, which is treated as a waste rather than a potential resource.
To overcome the challenges of CO₂ storage, both in terms of volume and cost, the captured CO₂ should be converted directly into solid and stable carbon-rich materials with added value. The majority of these are construction materials: concrete, aggregates, bricks, and asphalt account for about 95% of the total mass. In fact, the mass of buildings and infrastructure on Earth is estimated at 1,100 Gt. Building materials also dominate material flows (40 Gt/year). Polymers, on the other hand, make up only a small proportion of anthropogenic materials (about 8 Gt or 0.7% of the mass), but are still important because of their shorter residence time.
The authors argue that excess carbon from the atmosphere should be converted into bulk materials to replace fossil carbon. Carbon could be added in large quantities to building materials to replace mineral aggregates and fillers, which make up the largest volume fraction. Carbonaceous fillers do not have a significant effect on the performance of concrete when used in quantities below certain limits.
In a first step, CO₂ is separated from air and/or water using renewable energy. The CO₂ is then combined with H₂ to form methane (CH4) or methanol (CH3OH).CH3OH can be used to make many carbon-based materials. These conversion steps can be carried out through a global network of activities using local and seasonal renewable energy sources. The methane is then pyrolysed on site to produce H₂ and partially reduced carbon, which can be converted into materials.
Alternative pathways can convert CO₂ locally via photosynthesis followed by pyrolysis to biochar, or technically via CH4 or CH3OH in biomass if there is a local surplus of renewable energy. The biomass pathway may initially dominate, but will eventually decline due to natural limits to biomass production. Solid carbon from various sources is converted into additives and fillers for construction materials. Some carbon is combined with silicon to form SiC, while waste SiC becomes an aggregate and filler for construction materials. Ultimately, carbon-rich building materials end up in surface landfills or shallow underground reservoirs, which become the ultimate sink for CO₂ removed from the atmosphere. Landfilling carbon-rich building materials, rather than storing carbon dioxide directly, solves the critical problems of flammability and medium-term return to the atmosphere.
The transition from a carbon-emitting to a carbon-sequestering society requires not only significant technological advances, but also new business models, regulations, governance, and management systems.
Compared to other solutions for mitigating global warming (particularly underground CO₂ storage), the production of high value-added carbon-based materials offers several advantages, including long-term stability, high solid carbon storage density, decentralised implementation and substitution of current CO₂-emitting materials.
An advantage of the ‚mining the atmosphere‘ approach is the prospect of developing a strong economy based on capturing carbon dioxide from the atmosphere and converting it into useful carbon-rich materials. As the high demand for renewable energy remains a disadvantage and may become a bottleneck, materials science and engineering will need to develop processes that can effectively and efficiently use decentralised renewable energy surpluses, which will mostly be available as highly variable peak energy. The systemic integration of ‚mining the atmosphere‘ pathways into the future renewable energy system, into existing metabolic processes and into global and regional economic systems
must be thoroughly investigated with regard to their ecological sustainability. Other issues to be addressed include the design of appropriate business models, economic incentives and regulatory frameworks for a global carbon-binding society.
