CO₂ Budget

Since both data and models are always associated with uncertainties, the results of model calculations are always given with probabilities for their occurrence.
- A 67% probability of occurrence means that we achieve the targeted value in 4 out of 6 cases and miss it in 2 cases.
- A 50% probability of occurrence means that we only achieve the targeted value in 3 out of 6 cases and miss it in 3 cases.
A CO₂ budget based on a 50% probability is slightly larger than a 67% one, as more uncertainties are accepted and the chance of exceeding the +1.5°C limit, or the specified temperature limit, increases.
However, to mitigate the effects of global warming, it is important to avoid any additional tenth of a degree. For some island states, an additional tenth of a degree on top of the +1.5°C means doom, and possible tipping elements are more likely to reach their point of no return. In Germany, for example, heatwaves would become more frequent and long-lasting. 

• The CO₂ budget is a measurable quantity, through which we can trace whether the global community, but also Germany, has sufficiently reduced the CO₂ emissions. It provides countries and sectors (such as transport, energy, industry etc.) with planning security for their climate protection measures.

• The budget also plays a role in court, as the Federal Constitutional Court referred to the calculated German CO₂ budget in its 2021 judgment on the constitutional complaint against the German Climate Protection Act:
  "A substantial consumption of the CO₂ budget already by 2030, however, increases the risk of serious losses of freedom, because it shortens the time span for technical and social developments, with the help of which the transition from the lifestyle still extensively associated with CO₂ emissions to climate-neutral behaviors could be carried out in a freedom-preserving manner." - Federal Constitutional Court (2021)

The German Bundestag approved the Paris Agreement in September 2016. The Paris Agreement came into force in November 2016, and thus Germany is legally obliged to comply with the agreement. Therefore, the SRU proposed 2016 as the starting point for the calculation and derivation of the national CO2 budget from the global CO2 budget. Other fairness criteria are of course also justifiable. 1990, for example, saw the publication of the 1st IPCC assessment report, or in 1992, the UN conference took place in Rio de Janeiro, by which time it was internationally recognized that CO2 has harmful effects on the planet and humanity.

The global CO₂ budget can be distributed among states according to various principles; ethical, political, and climate scientific considerations play a major role. The Paris Agreement emphasizes shared but differentiated responsibilities. Without international agreement, each country risks choosing the most favorable key for itself, thereby increasing the likelihood that the global budget will be exceeded as a whole. The SRU recommends a distribution based on population share (as of 2016) as a fair and practical basis. A per capita distribution is ethically well-founded and politically viable. The SRU writes on this: "With the principle of distribution according to population share, a maximum CO₂ budget for Germany results, the exceeding of which, in the opinion of the SRU, cannot be ethically justified without corresponding agreements with other states. But the SRU also considers criteria such as the inclusion of historically cumulative emissions or the development rights of economically poorer countries to be well-founded." Methods that assign a CO2 budget based on their current emission share, such as "Grandfathering", would favor economically-technologically advanced countries and disadvantage other countries, as it would result in a larger CO2 budget for Germany than the per capita approach, since economically-technologically poorer countries are less responsible for climate change. The SRU summarizes its analysis: "Therefore, the choice of a distribution principle according to population share already represents a compromise between different views and is generous in favor of industrialized countries. Given the challenges of the upcoming decarbonization, the remaining German CO₂ budget now appears small. However, this does not ethically justify choosing an alternative principle that favors Germany."

Source for the quotes