Each country should be held responsible for how much it is to blame when the world overshoots the 1.5掳C warming limit 鈥� by assigning a 鈥渘et-zero carbon debt鈥� if it has pumped out more than its fair share of emissions, a team of climate scientists is proposing.
The aim is to encourage those with greater responsibility for the overshoot to make amends by, say, removing carbon dioxide from the atmosphere or helping other countries reach net-zero faster.
鈥淭here is a need for collective action,鈥� says team member at the International Institute for Applied Systems Analysis in Austria. 鈥淲ho should bear the effort?鈥�
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The idea of net-zero carbon debts is an extension of the idea of a global carbon budget for limiting warming to a specific target, and that the global budget should be divided fairly among countries based on their populations.
The concept of carbon budgets came to the fore in 2009, when climate scientists showed that the relationship between CO2 emissions from human activity and the resulting warming is roughly linear. This means it is possible to calculate how much more CO2 can be emitted before a given level of warming is likely to be passed.
The simplicity of this led to the carbon budget idea being widely adopted by climate scientists and communicators. For instance, it has been highlighted in reports by the Intergovernmental Panel on Climate Change (IPCC).
In 2020, the IPCC estimated that the remaining budget for a 50-50 chance of staying below 1.5掳C was about 500 gigatonnes of CO2, and by 2023 it was down to 250 Gt of CO2, so the budget is going to run out very soon.
鈥淚f we know that鈥檚 approaching, the work we are doing must be resilient to that,鈥� says Pelz. 鈥淚f it is not resilient to that, then everything we鈥檙e doing now is redundant.鈥�
His answer is that once a country has emitted its fair share of the global carbon budget 鈥� almost all rich countries already have 鈥� all future emissions should be counted as its carbon debt. This debt will continue to accrue until a country achieves net-zero emissions, with a country鈥檚 total carbon overshoot up to this point being its net-zero carbon debt.
Pelz and his colleagues have estimated the likely net-zero carbon debts for broad regions based on current policies and pledges. North America is likely to end up with the biggest debt, with sub-Saharan Africa having almost none.
The researchers say they are just putting out the idea for discussion, and whether it becomes part of climate agreements and how exactly net-zero carbon debts are defined is up to others.
鈥淭his is an attempt to kickstart a conversation around who is going to end up paying for this overshoot,鈥� says team member at Imperial College London. He聽thinks every country has a moral duty to take action in proportion to its climate debt. There is no legal duty, though, he says.
In theory, that could change. Countries could be obliged to compensate for their carbon debt as part of future climate agreements. But the idea of making carbon budgets part of international agreements has from the biggest emitters, and net-zero carbon debts aren鈥檛 likely to go down any better.
鈥淚t鈥檚 a neat idea, but for me it butts up against the realpolitik of where the world is,鈥� says at the University of Leeds in the UK.
That said, estimating net-zero carbon debts is at least informative 鈥� and the world is going to be a very different place by 2050. A coalition of willing countries could have the means to persuade the unwilling to pay their debts, such as carbon border tariffs like those being introduced by the European Union. Net-zero carbon debt is an idea whose time may yet come.
PNAS
