A Green--Golden Rule for Climate Policy
This paper proposes a "green–golden rule" for climate policy that ties admissible public deficits to the measurable reduction in greenhouse-gas emissions valued at the social cost of carbon, thereby overcoming the operational flaws of traditional investment-based fiscal rules while aligning current borrowing with intergenerational climate benefits.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Climate policy faces a stubborn paradox: the costs of action are paid today, while the benefits of a stable climate arrive mostly for future generations. This timing mismatch creates a political and economic dilemma. Governments, naturally focused on the immediate needs of current voters, often hesitate to spend heavily on measures that will pay off decades later. Public debt offers a potential solution to this problem. By borrowing money today to fund climate action, a government can shift the financial burden forward, aligning the payment with the generation that eventually enjoys the cleaner air and stable temperatures. In theory, this makes climate policy fairer across time.
However, most countries have strict rules to prevent governments from borrowing too much. These fiscal rules are designed to stop leaders from over-spending for short-term popularity at the expense of the future. Unfortunately, these same rules often block the very debt needed to fund climate action. The standard fix for this problem is the "golden rule" of public finance, which allows governments to borrow freely for long-term investments like building roads or schools, while still limiting borrowing for everyday spending. But this traditional rule has a fatal flaw in practice: it is incredibly difficult to define exactly what counts as a long-term investment. Because the definition is fuzzy, governments can easily relabel ordinary spending as investment to bypass borrowing limits, a practice known as "creative accounting." This ambiguity has made the golden rule unreliable.
A team of researchers from the Potsdam Institute for Climate Impact Research and the University of Potsdam has proposed a new approach called the "green–golden rule." Instead of trying to define investment by the type of project or the money spent, their rule ties borrowing power directly to the actual result achieved: the reduction of greenhouse gas emissions. The researchers argue that unlike a building or a bridge, which can be hard to categorize, the amount of carbon removed from the atmosphere is a precise, measurable quantity. One ton of carbon dioxide is identical to any other ton, making it a perfectly uniform asset.
The core of their proposal is simple yet radical. Under this new rule, a government would be allowed to borrow a specific amount of money for every ton of carbon emissions it successfully reduces. The value of that borrowing power would be set by the "social cost of carbon," a figure that estimates the economic damage caused by each ton of emissions. If a government reduces emissions, it earns the right to borrow money to pay for that reduction. Crucially, once the emissions are reduced and the borrowing limit is unlocked, the government can spend that borrowed money on anything it chooses—whether that is more climate projects, education, or healthcare. The rule rewards the outcome, not the specific method used to get there.
The researchers tested this idea using a theoretical model of how governments make decisions. They found that when a government is allowed to borrow based on the actual reduction of emissions, it naturally chooses the most efficient path. Because the borrowing limit is tied to the benefit achieved rather than the cost of the project, governments are incentivized to find the cheapest ways to cut emissions. If a project is too expensive to be worth the benefit, it simply won't generate enough borrowing power to be worthwhile. This creates a self-correcting system where fiscal discipline and climate efficiency work together rather than fighting each other. The model shows that this rule can guide a government, even one that is biased toward the present, to make the right choices for the future without needing complex oversight or case-by-case approvals.
To see how this would work in the real world, the team applied their rule to the European Union and Germany. They assumed a value for the social cost of carbon of 200 euros per ton. Under this assumption, if the EU were to raise the price of carbon in the transport and building sectors to 200 euros per ton, the green–golden rule would justify an additional annual deficit of about 72 billion euros for the entire European Union. For Germany alone, the same price would unlock about 24 billion euros in additional borrowing space each year. These figures represent roughly 0.4 percent of the EU's total economic output and 0.56 percent of Germany's. The researchers emphasize that this money would come with no strings attached regarding how it is spent, provided the emissions reductions were actually achieved.
The study also explored two ways to implement this rule. One version, the quantity rule, would calculate borrowing limits based on the exact number of tons of emissions reduced compared to a baseline scenario. This offers flexibility in choosing which policies to use but requires a difficult calculation of what emissions would have been without any action. The second version, the price rule, ties borrowing limits directly to the price of carbon. This is easier to administer because it relies on a clear price signal rather than complex estimates of future emissions, though it limits the government's ability to use non-price tools like regulations. The researchers found that both versions could work, but the price rule offers a more straightforward path for implementation.
The authors are careful to note that their findings rely on specific assumptions, particularly regarding the value placed on carbon and the responsiveness of the economy to price changes. They also point out that the size of the borrowing room depends heavily on international cooperation. If a country acts alone while others do not, the benefits of its actions might be diluted by "carbon leakage," where emissions simply move to other countries, reducing the effective value of the reductions. In such cases, the borrowing power granted by the rule would be smaller. Nevertheless, the study suggests that for regions like the EU, where international mechanisms like carbon border taxes are being developed, the green–golden rule could provide a powerful tool to overcome political resistance to high carbon prices.
By shifting the focus from what is spent to what is achieved, this proposal attempts to solve the operational problem that has plagued previous fiscal rules. It removes the ambiguity that allows for creative accounting by using a quantity that is impossible to fake: the actual amount of carbon removed from the atmosphere. The researchers conclude that this approach offers a way to reconcile the need for fiscal responsibility with the urgent need for climate action, turning the debt that usually restricts policy into a resource that enables it. The result is a system where the government is rewarded for doing the right thing, and the cost of that right thing is shared fairly with the generations who will inherit the benefits.
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