The term geoengineering is actually somewhat similar to the term quantum leap: In the literal sense it refers to something quite different than in general use. Just as a quantum leap is, in fact, only a tiny change in the energy level of an electron and not something earth-shattering, so has geoengineering, in the narrow sense of the word, been part of our everyday life for many millennia. Mining, management of rivers, tunnels, reservoirs or soil-drainage: All these are massive interventions in the subsoil, which, in my opinion, fall under the heading “geoengineering”. Talking today about geoeingineering, people usually mean climate engineering¬ – and so it is not surprising that the topic reliably turns up in the media when major climate conferences are approaching, such as currently in Katowice, Poland, or when it comes to achieving or failing to meet Germany’s self-defined "climate goals".

Roughly speaking, it is possible to distinguish between two classes of climate engineering: One is the active separation of CO2 after burning fossil fuels or after other CO2 emitting processes, and the other is the manipulation of solar radiation influx on the Earth, for example by introducing particles into the atmosphere. For both processes, natural examples exist: Plants extract CO2 from the air and convert it into biomass, be it annually shed leaves or long-lived wood. And volcanoes emit smoke and ash, which enter the stratosphere and lead to a shadowing of the Earth.

The majority of geoscientists and policymakers agree that, in the case of large-scale experiments on the creation of shadowing, knowledge on the possible global and regional consequences is still lacking. Too high is the risk of unintentional side effects of introducing large quantities of soot particles or other substances into the atmosphere. What is often ignored in the debate is: It is not just these unwanted effects that cause concern, but also unresolved and maybe non-resolvable issues of global “governance”. Who should determine how many particles with which calculated effect should be introduced to the atmosphere? Could any state veto it? Who would be liable should something go wrong? Who guarantees the sustainability of the respective measure? How should it be financed - in particular on a permanent basis? The ideas for such radical measures, which repeatedly make headlines, are old and well-known – and they remain fraught with great unexplained risks and a lack of an international legal basis.

The situation is quite different with the sequestration and the storage of CO2. Here, processes that function on an industrial scale already exist, for example in cement production or in the chemical industry. Long-established procedures for the injection of carbon dioxide into the underground are also available – for example, in the production of oil or gas. Scientists at the GFZ have, furthermore, proven that layers of bedrock, suitable for the absorption and storage of CO2 are present in the underground. Our Carbon Capture and Storage (CCS) pilot project in Ketzin has shown that gas can safely be stored in the underground and that it can even be retrieved at a later stage.

So this type of climate engineering is feasible, and, in my opinion, certainly makes sense. CCS can, to a certain extent, contribute to a reduction in CO2, and CCS also plays a role in many scenarios of the Intergovernmental Panel on Climate Change IPCC. However, further action is needed to reduce CO2 levels in the entire atmosphere. In this case, the necessary overall reduction would be much too large for CCS technologies and, here too, potentially harmful side effects must also be taken into consideration. If you think e.g. of negative emissions, in which biomass – green plants on fields or fast-growing trees – are cultivated for conversion to energy with simultaneous CO2 capture, you are neglecting the consequences this would have for soils or ecosystems and food production. Key words are monocultures and the loss of biodiversity such as insect mortality and acreage competition ("food or fuel").

On the other hand, industry has already established processes that sequester CO2. However, there is still no infrastructure for the transportation of the gas to the storage site in the underground, but that, of course, could be solved. In addition, the price plays a role. CCS is still too expensive, but a reduction in the costs for this procedure can be expected, on the one hand, due to up-scaling effects and, on the other hand, resulting from increases in costs for CO2 release.

We are not talking here about permanent storage in the underground. In fact there are interesting research approaches for chemically packing "green hydrogen" rendering it transportable over long distances. One could, for example, generate hydrogen in sunny areas and combine it with CO2 for conversion to an artificial methanol-based fuel. This is of significance for the existing transport infrastructure (ships, trucks, airplanes). Of course, one should work on battery drive systems – but batteries also consume resources and harbor their own risks.

And certainly, the best solution of all is not to give rise to CO2 in the first place – yet at the same time, we still have to take a realistic look at the economy and infrastructure. And needless to say, even though we have demonstrated that CCS is manageable, leakage could possibly occur as a result of improper handling – in this case, however, the effects would then be of local impact only and not, as with other climate engineering ideas, of a global dimension. For these reasons, I speak in favor of the further exploration and the technological implementation of CCS and also of CCU (U stands for utilization). This cannot be done without use of the geological underground. And the expectations on the effects must remain realistic.

Prof. Dr. Dr. h.c. Reinhard Hüttl is Chairman of the GFZ Board and Scientific Executive Director