What is the carbon budget?
The concept “carbon budget” is often used in discussions about the climate crisis to quantify the range of options to emit greenhouse gases which are available to an individual, organization, nation, or humankind in general while limiting global warming to a certain “safe” level. It is useful (if not required) to operationalize such an abstract phenomenon as global warming to easier quantifiable variables such as concentration of greenhouse gases, global surface temperature (in reference to the pre-industrial age), or the quantity of emitted greenhouse gases expressed as equivalent of carbondioxide (ie in tons of CO2e). The amount of carbon dioxide equivalents again can be translated into certain products (e.g. emissions caused by production of a car or destruction of an area of rain forest) or services (e.g. a flight from Stockholm to Bangkok), which again can guide our lifestyle choices.
How is the global carbon budget calculated?
The present and future concentrations of carbon dioxide (as the most important greenhouse gas) develop their harmful effects by the acidification of the oceans and radiative forcing of solar energy (leading to global warming). Since the harmful consequences on the biosphere are primarily mediated by the increase of the global surface temperature, the total carbon emissions budget is “back-calculated” based on the association between the total cumulative amount of emitted greenhouse gases and temperature change (ie global warming) using the Transient Climat Response to Cumulative Emissions (TCRE) equation] [1, p 465, Gleichung 10]
TemperatureChange = Constant * CumulativeEmissions
and certain limits to global warming, which are considered as “safe”, e.g. 2 degrees Celsius.
How is the carbon budget per person and year calculated?
The transformation of the global carbon budget to an individual carbon budget per year is then calculated as follows, for example, for a specific limit of global warming:
CarbonBudgetPerPersonPerYear = GlobalCarbonBudget /(AverageSizeOfHumanPopulation * TimeToNetZeroCarbonEmissions)
For example, the website atmosfair.de  calculates, a carbon budget per person per year of 2.3 tons CO2e based on the WGBU (2009) Special report (cited as in ) based on a total global carbon budget of 750 Gigatons CO2e and a carbon emissions peak around 2015 and net zero emissions around 2050.
CarbonBudgetPerPersonPerYear = 750E9 tons CO2e / (8.2E9 person * 40 years) = 2.286585 tons CO2e/ (person * year)
However, the limitations of the carbon budget concept are:
- Uncertainty: The true total carbon budget corresponding to specific level of global warming and the true “safe” level of global warming are not known. These parameter values have to be estimated and, therefore, are uncertain within specific confidence limits. Therefore, adjusting our lifestyle based on the expected values, may not be sufficient to limit global warming to a safe zone. In other words, the carbon budget estimations to not take into account a margin of error or “buffer” and maybe based on wrong assumptions.
- Violated assumptions: The estimated Carbon Budget Per Person Per Year (as retrieved from atmosfair.de ) is based on outdated assumptions, eg peak carbon emissions in 2015. Therefore, the estimated Carbon Budget Per Person Per Year is likely lower. As explained above, the assumption that global warming of 2 degrees Celsius is still “safe”, and will result in consequences, which are still “controllable” may be wrong.
- No historical responsibility: The Carbon Budget Per Person Per Year assumes all individuals are entitled to and will use the same annual individual carbon budget. However, this assumption does not take into account historical aspects and aspects of justice considering that only a small population of individuals in industrialized countries have emitted the largest fraction of greenhouse gases, and, therefore, have caused global warming. Assuming that all humans have a similar right to use the earth’s resources and humans in industrialized countries have overused the earth’s resources and their offspring are still benefiting from their overuse, they should have a lower carbon budget than in not-industrialized countries [see also 4, not read yet].
- Misinterpretation: The concept of a Carbon Budget may be misunderstood as false signal of safety to continue the current lifestyle.
- Limited scope: The estimated carbon budget per person per year is only applicable until a certain target date (2050 in the calculations above), and the individual annual carbon budget will be net zero after that. Therefore, the only individual annual carbon budget with long-term sustainability will be be zero tons of CO2.
- Climate targets not safe: Considering that the targeted limit of global warming around 2 degrees Celsius, may still leave the earth’s biosphere in the danger zone, potentially triggering positive feedback loops (ie tipping points), one may even postulate that for a certain range in time during which the individual annual carbon budget should be negative, to lead the global biosphere back into the safe zone.
[¹] Schellhuber, HJ (2015). Selbverbrennung (3rd ed.). München: Bertelsmann.
[³] https://www.wbgu.de/en/publications/special-reports (Report not found)
 Moss, J, Kath, R (2018). Historical Emissions and the Carbon Budget. Journal of Applied Philsophy, https://doi.org/10.1111/japp.12307