By now, everyone should be aware that it is not the sun, but the Earth and ultimately us humans who are responsible for global warming. For a long time, the Earth's atmosphere was constant in its protection against radiation from space and uncontrolled warming by solar radiation. The absorbing gases in the atmosphere ensured, and still ensure today, that some of the radiation is released back into space and some of it temperates the Earth.

The proportion of carbon dioxide in the atmosphere is important for the functionality of the atmosphere. Until industrialisation, the proportion of carbon dioxide in the atmosphere was constant. The pre-industrial level was around 280 ppm (parts per million) before 1750. By the mid-1950s, the level was already at 315 ppm. The increase was accelerated in particular by the growing use of fossil fuels such as coal, oil and gas, industrial processes and increasing deforestation, with the result that the Mauna Loa measurement series showed a level of over 420 ppm in 2024.

It is therefore clear that the rapidly increasing proportion of carbon dioxide in the atmosphere is responsible for global warming.

In order to restore the functionality of the atmosphere to such an extent that global warming is limited to only 1.5°C by 2050, we must not only reduce emissions to zero, but also actually remove carbon dioxide from the atmosphere. This active removal of greenhouse gases from the atmosphere is referred to as negative emissions.

Globally, we emit approximately 40 gigatonnes of carbon dioxide into the atmosphere every year. 36 Gt are energy-related and 4 Gt are generated by land emissions (biosystems, agricultural use). This is offset by natural CO2 sinks such as moors and forests, oceans and other biosystems. CO2 sinks reduce the amount of carbon dioxide in the atmosphere by approximately 20 Gt per year. A comparison of the figures shows that we emit approximately 20 Gt of additional carbon dioxide into the air each year that is not naturally bound and decomposed.

In order to restore some semblance of balance, we need to reduce emissions to zero and protect and reactivate natural sinks, as well as develop artificial sinks. One method is based on direct air capture (DAC) technology, which filters carbon dioxide directly from the air. A prerequisite for this method is suitable and stable storage sites where the filtered CO2 can be permanently stored (carbon storage). Carbon dioxide is not the most active gas when it comes to forming chemical bonds. Nevertheless, there are possibilities, such as storage in rocks. The various storage capacities in Europe have been investigated in various studies since 1990. Some figures can be found in the EU Geological CO2 storage summary (GEUS and Clean Air Task Force, Karen Lyng Anthonsen and Niels Peter Christensen), published in 2021. Our graphic shows the largest storage capacities in Europe in deep saline aquifers.

There are currently 27 DAC plants worldwide. It is assumed that by 2050, the technology will have advanced to such an extent that approximately 160 large-scale plants with a total filter volume of 75 Mt could be in operation. The drawback of this technology is the high energy requirement for the filtering process. Energy that, self-evidently, must come from renewable sources. Iceland has the first large-scale plant, called Mammouth, from Climeworks, which uses geothermal energy. This means that stable energy sources and suitable storage locations must be available at the sites.

The fact that this technology can only be part of the solution is demonstrated by the limited volume of geologically suitable storage sites, estimated at 14,000 Gt in rock (Fraunhofer Institute). It is currently assumed that by 2050, direct air capture technology and carbon storage will only be able to achieve approximately 8 to 14 percent of the negative emissions required. In addition to storage in natural resources, research is in full swing on CCU (carbon capture usage), which involves storing CO2 in products such as concrete and plastic.

As with the energy mix, the solution therefore lies in technologies for negative emissions from a mix of reforestation, bioenergy use with CO2 capture, accelerated weathering, intelligent soil management, ocean fertilisation, mechanical CO2 filtering and other technologies that have yet to be developed. These technologies are urgently needed, as there is currently a large gap between emissions and negative emissions and climate targets.