Conditions and pathways for a climate club to reach an ambitious global treaty

For decades, the world has struggled to agree on how to stop the planet from warming. The prevailing strategy has been a global pact where every nation promises to cut its own emissions, hoping that moral pressure and shared goals will be enough to keep everyone honest. But this approach has a fundamental flaw: it allows countries to benefit from the clean air created by others without paying the cost of changing their own industries. This behavior, known as free-riding, means that even when most nations sign an agreement, the collective effort often falls short of what is needed to prevent dangerous climate change. To solve this, some experts have proposed a different idea: a "climate club." Instead of waiting for everyone to agree at once, a group of ambitious nations would form a tight alliance, agreeing on strict rules and penalties. The logic is that by making it expensive to stay outside the group, the club would eventually grow until every country wanted to join to avoid the penalties.

A team of researchers from Spain and the Netherlands has tested whether this idea could actually work in the real world. They built a computer simulation that acts like a digital laboratory, modeling how 31 different regions of the globe—ranging from single countries like China and the United States to groups of nations like the European Union—would react to the formation of such a club. The simulation does not rely on abstract theories; it uses real economic data, including how much each region produces, how much it trades with others, and how expensive it would be for them to switch to cleaner energy. The researchers ran thousands of scenarios to see which starting conditions would lead to a successful, worldwide agreement and which would cause the club to collapse or remain small.

The study focused on a specific mechanism to make the club work: a border carbon tariff. This is a fee charged on goods coming from countries outside the club if those countries do not have their own strict climate rules. The goal is to ensure that a factory in a club country does not lose business to a factory in a non-club country simply because the latter is allowed to pollute for free. The researchers tested different versions of this system, varying the price of carbon, the strictness of the fees, and how the money collected from those fees was used. They wanted to know if a club started by just a few big players could eventually pull in the rest of the world, or if the economic pain would be too great for outsiders to accept.

The simulations revealed that a global climate club is possible, but only under very specific conditions. The most successful path began with a large, powerful initial group. When the European Union started the club alone, it had a decent chance of growing, but the odds improved dramatically when the EU was joined by either the United States or China. These large coalitions created enough economic pressure to make staying outside the club too costly for other nations. However, the size of the initial group was not the only factor; the rules of the club mattered just as much. The researchers found that the club needed to set a high price on carbon—specifically, a price that reflects the true cost of climate damage, which the study modeled at levels of 200 or 400 dollars per ton of carbon dioxide.

Crucially, the study showed that the way the club handles the money it collects from border fees determines its success. If the club keeps all the money for its own members, it can work, but it is less effective at convincing the world's largest polluters to join. The simulations showed that the club becomes much more attractive if it shares a portion of that revenue with the countries outside the club. This shared money is not a gift; it is specifically earmarked to help those outside nations pay for the expensive transition to clean energy. The researchers found that returning less than half of the collected fees to outsiders created the perfect balance. It lowered the cost of joining for developing nations without draining the benefits for the members. If the club gave away too much money, it lost its own incentive to stay together; if it gave away too little, outsiders could not afford to join.

The study also highlighted the importance of who starts the club. A coalition of the European Union and China proved to be the most effective starting point, bringing the world together faster than a coalition of the EU and the United States. This is because China's trade connections reach deep into Asia and the Global South, regions that the US and EU do not connect with as directly. By including China, the club could exert pressure on a wider variety of economies, making the cost of staying outside the group too high for almost everyone. In contrast, attempts to start a club with just a single small country, such as Switzerland, failed in almost every scenario. A small starter could not generate enough economic pressure or revenue to make the club viable.

The researchers also tested how strict the border fees should be. They found that the fee charged to outsiders should match the carbon price inside the club exactly. This "symmetric" approach ensures that imports and domestic goods compete on a level playing field. A softer approach, where the fee is lower than the internal carbon price, only worked when the initial club was large and the carbon price was low. In most other cases, being too lenient allowed outsiders to avoid the full cost of their pollution, which meant they had no urgent reason to join.

While the simulations are optimistic about the potential for a global club, they come with important caveats. The model assumes that countries outside the club will not fight back with their own trade wars or tariffs. In the real world, a large club imposing fees might trigger retaliation, which could destabilize the system. The researchers acknowledge this risk but note that sharing some of the revenue with outsiders, as their model suggests, could help reduce the anger that leads to such conflicts. Furthermore, the model focuses on the immediate economic costs and benefits of joining, rather than the long-term environmental damage that climate change causes. It assumes that countries make decisions based on their current economic balance sheets, which is how political decisions are often made in the short term.

The study concludes that the path to a global climate agreement is not a matter of waiting for everyone to agree at once. Instead, it suggests a dynamic process where a strong core group, led by major economic powers, sets a high standard and uses trade rules to encourage others to follow. The most promising recipe involves the European Union, China, or the United States forming a tight alliance, setting a high price on carbon, and using the revenue from border fees to help the rest of the world transition to clean energy. If these conditions are met, the simulations suggest that the club could expand to include every nation, creating a stable and effective global system for fighting climate change. Without these specific conditions, however, the club risks remaining small or collapsing under the weight of economic disagreements.