In order to be in line with the Paris Agreement, global greenhouse gas emissions must be reduced. For this, different approaches are possible: apart from preventing the formation of greenhouse gases, carbon capture and utilization (CCU) and carbon capture and removal (CCR) are also options. The latter encloses methods that lastingly strip greenhouse gases from the atmosphere, e.g. carbon capture and storage (CCS) or, bioenergy with CCS (BECCS).
The term CCS is used when for example an exhaust gas stream is stripped of its CO2 and the captured CO2 is subsequently stored, e.g. below ground in a depleted natural gas deposit. In the case of BECCS, the natural process of photosynthesis is utilised, in which plants extract CO2 from the atmosphere and store it in their biomass. After harvesting, the biomass can be used for energy, for example in a thermal process. If CO2 is produced during the conversion process, it is captured and stored as part of the BECCS process. Instead of CO2 gas storages a promising alternative option is to store carbon in its pure form as solid carbon. Storing carbon in solid form increases the safety of storage, but capturing solid carbon requires more energy.
However, capturing and safely storing greenhouse gases is currently far more expensive than preventing their formation in the first place. None of the known processes is technologically mature and has been tested on a large scale. It is therefore questionable whether these approaches can contribute to achieving the climate targets of the Paris Agreement.
For the sake of completeness, measures to strengthen natural carbon sinks, such as reforestation or renaturation of peatlands, should not be forgotten. These natural processes can be a supplement to pursued greenhouse-gas-reduction measures but are too slow to be relied on solely.
So, the major lever for greenhouse gas reduction is preventing their formation. Anthropogenic greenhouse gas emissions can be distinguished in terms of the sectors in which they are emitted (see figure). At both global and national levels, the three sectors with the highest greenhouse gas emissions are the energy sector, industry and transportation. The energy sector essentially includes the generation of electricity and heat in power plants. Globally, the building sector accounts for only 6% of total greenhouse gas emissions. In Germany, however, its contribution is significantly higher and amounts to 15%.
Net CO2eq emissions per sector, status 2019 by Fabian Carels, IRENA (2021), BMU (2020) (CC BY-SA)
Options for reducing greenhouse gas emissions mainly come down to replacing fossil energy carriers with renewably generated power. This shift from fossil energy carriers to greenhouse gas neutral renewable energy sources is generally referred to as the Energy Transition. The chart below shows the trend in greenhouse gas emissions in Germany for the period between 1990 and 2019. Since 1990, the emissions were reduced by a total of about 36 %. The energy sector shows the biggest emission reduction, but still has the largest share of total emissions. The savings in the energy sector were mainly achieved by the fact that plants for the generation of electricity from wind and solar energy as well as from biomass were able to gain large shares in the past decades. As a result, CO2-intensive power generation technologies, such as coal-fired power plants, have been partially forced out of the market.
Trend in greenhouse gas emissions in Germany by sector by Fabian Carels, BMU (2020) (CC BY-SA)
The industry sector and the building sector have also been able to reduce their emissions in recent years. In the industry sector in particular, however, the majority of the savings took place before the start of the new millennium. This contrasts with the transportation sector, where emissions have remained almost constant since 1990 despite the development and market introduction of more efficient vehicles and greenhouse gas-neutral fuels of biogenic origin.
