Study: Can Artificial Photosynthesis Limit Global Warming?

CO₂ emissions have to be reduced as soon as possible. If not, CO₂ will have to be removed from the atmosphere in the future to lower global warming. Planting new forests and biomass, as well as new technologies for artificial photosynthesis, could be contributing factors to this. An HZB physicist and a researcher at the University of Heidelberg have projected how much surface area such solutions would need. Although artificial photosynthesis is capable of binding CO₂ more efficiently than the natural model, there are no large modules available that are stable over the long period. The researchers published their calculations in “Earth System Dynamics”.

After a number of years during which global emissions at least stagnated, they increased again slightly in 2017 and 2018. Germany has also noticeably missed its climate targets. So as to keep global warming below 2 °C, only around 1100 gigatons of CO₂ may be discharged into the atmosphere by 2050. In order to restrict global warming to 1.5°, only just under 400 gigatons of CO₂ may be discharged globally. By 2050, emissions will have to drop to zero even. At present, however, 42 gigatons of CO₂ are added yearly.

Almost all the various scenarios require “negative emissions”

The Intergovernmental Panel on Climate Change (IPCC) has numerically simulated different scenarios. Only in very optimistic scenarios can the climate target still be realized by means of direct and radical measures in all sectors (agriculture, transport, energy, construction, etc.). In the less optimistic situations, the international community will have to take extra measures starting in 2030 or by 2050 at the latest: “negative emissions” must be implemented by removing large amounts of CO₂ from the atmosphere and storing them permanently so as to balance the carbon budget. One instance of negative emissions is extensive forestation—forests bind CO₂ in wood provided it is not later used as fuel. But CO₂ could also be eliminated from the atmosphere and bound using artificial photosynthesis.

Recently, physicists have calculated how this might work. Dr. Matthias May of the HZB Institute for Solar Fuels is an expert in artificial photosynthesis. Dr. Kira Rehfeld is an environmental physicist at the University of Heidelberg researching climate and environmental variability.

Natural photosynthesis: a surface area the size of Europe would have to be forested

In a median scenario, no less than 10 gigatons of CO₂ per year would have to be eliminated from the atmosphere beginning around 2050 to balance the climate carbon budget. Forestation and cultivation of biomass for lowering CO₂ vie for the same areas as are needed for agriculture, however. With just more biomass alone, it is as a result hard to reach this scale, for natural photosynthesis is not a predominantly efficient process. Leaves are able to utilize a maximum of 2% of the light for changing CO₂ and water into new chemical compounds. The two physicists contend that in order to bind 10 gigatons of CO₂ per year in the forest, around 10 million km² of Earth’s fertile areas would have to be planted with new forest. This corresponds to the area of continental Europe (stretching to the Urals).

With artificial photosynthesis, an area the size of the State of Brandenburg could suffice

Materials systems presently being studied for artificial photosynthesis might bind CO₂ with significantly higher efficiency. Already at present, on a lab scale, photo-electrochemical systems composed of semiconductor materials and oxides can utilize about 19% of the light to split water, for example, and thus realize part of the photosynthesis process. However, the material system visualized by May and Rehfeld is not about creating hydrogen with sunlight, but instead about binding CO₂ molecules and changing them into stable chemical compounds. “However, this is a relatively similar problem from the point of view of physical chemistry”, says May.

The prerequisite, however, is that it will be possible to create by 2050 large-scale, durable modules that use solar energy to change atmospheric CO₂ into other compounds. The necessary area for this solution can be calculated. Assuming efficiency of 19% and 50% system losses, about 30,000 km² of modules could be adequate to extract 10 gigatons of CO₂ from the atmosphere yearly. This matches the approximate area of the German Federal State of Brandenburg.

These kinds of modules could be placed in non-agricultural regions—in deserts, for example. In contrast to plants, they require hardly any water to operate, and their efficiency does not suffer when exposed to intense solar radiation.

Dr. Matthias May, Expert in Artificial Photosynthesis, HZB Institute for Solar Fuels

The extracted CO₂ could be changed to formic acid, oxalate, or alcohol and integrated with other compounds (such as calcium chloride) to form solid minerals that can be stored or even used in the form of plastic as a building material.

Focus on development, not on miracles

Even if May and Rehfeld are swayed that such solutions should be deliberated more closely, they caution against depending on technical miracles. This is because these systems still only work at the smallest scale, are costly, and not stable in the long haul. Altering this requires large investments in research and development.

It might be possible to develop such modules, but even if we could then build them, we estimate that the conversion will cost at least 65 euros per ton of CO₂. The extraction of 10 gigatonnes of CO₂ thus results in costs of 650 billion euros each year. Moreover, negative emissions can only be the last resort to slow dramatic climate developments. The best thing now would be to drastically reduce emissions immediately—that would be safer and much cheaper.

Dr. Matthias May, Expert in Artificial Photosynthesis, HZB Institute for Solar Fuels