The FOOTPRINTS project is investigating how the Earth system will respond if we meet our emissions targets. The interplay between the Earth's carbon cycle and heat balance will determine how the temperature trajectory will develop. Their state, in turn, depends on how much CO₂ we continue to emit until then and how we achieve our climate targets. Will the climate stabilize even if we use large-scale CO₂ removal measures to achieve our goals? What impact will these measures have on the climate beyond CO₂ removal?

At the end of the project, a comprehensive analysis of the various factors influencing temperature stabilization scenarios should help us understand which of these factors dominate on which time scales. An example of relevant socio-economic factors would be the consumed CO₂ emission budget, i.e., also how much climate change has progressed, the extent of CO₂ removal measures and the mitigation of other climate-relevant impact factors such as greenhouse gases or aerosols. In the Earth system, factors such as the state of the ocean's meridional overturning circulation, natural CO₂ uptake capacities on land (regulated by vegetation) as well as at sea will play an important role. These findings will both inform future research priorities for understanding temperature trends below net-zero and provide initial indicators of what our CO₂ emissions will need to be in order to stabilize global temperature.

Part of the project will be to calculate climate effectiveness FOOTPRINTS of CO₂ removal measures. So far, these measures have been studied mainly in terms of their technological and economic effectiveness. However, if we look at the whole process of producing negative CO₂ emissions, these methods have further climate-relevant effects, e.g., through the emission of greenhouse gases such as methane and nitrous oxide, but also aerosols, or changes in the land surface. The FOOTPRINTS project will initially investigate CO₂ removal measures with high technology readiness, which currently means mainly land-based measures. These include, for example, bioenergy plants combined with CO₂ filtration plants and geological storage of CO₂, accelerated weathering on land, or direct CO₂ removal from the air. Later, we will also investigate marine approaches to CO₂ removal, such as electrochemical weathering or macroalgae rearing combined with biomass sink.

CO₂ removal measures as well as almost all other human activities with an influence on our CO₂ emissions always have further climate-relevant effects. For example, land-based transportation emits not only the greenhouse gas CO₂, but also warmer soot particles (aerosols) and cooling aerosols such as sulfur dioxide. Agriculture, on the other hand, produces large amounts of methane and nitrous oxide, which warm the climate, and ammonia, which has a cooling climate effect. Especially for the short-term climate impacts of our activities, i.e., on a time scale of about ten years, it is important to include the effects of these factors that affect the climate. However, it is also important to understand that CO₂ is by far the longest-lived man-made greenhouse gas and that we must not forget, in the face of all the other factors, that we must get our CO₂ emissions to zero.

As part of the Helmholtz Climate Initiative, we have been addressing precisely this question for the past two years - and our answer is: No. Sonja Simon and her team have used an energy system model for Germany to develop a transformation scenario based on a 1.5ºC CO₂ emissions budget. This scenario takes into account the coal phase-out, energy-efficient building refurbishment and climate-friendly mobility, as well as measures to further reduce CO₂ emissions in industry. Taken all together, the remaining emissions in Germany were around 60 Mt CO₂ per year. These need to be offset in order to get to net zero CO₂. In addition, experts in agricultural soil science, seagrasses and peatlands have calculated the CO₂ removal potential of various methods for Germany. Even in initial calculations, some of which are optimistic, these measures combined could only offset about 25% of these remaining CO₂ emissions. This means that we either need to further reduce our remaining emissions through societal behavioral changes or use technological CO₂ removal measures combined with geological CO₂ storage to achieve CO₂ neutrality.

Interview: Beatrix Boldt