Maintaining bitcoin and other cryptocurrencies causes about 0.3 percent of global CO₂ emissions. That may not sound like a lot, but it’s more than the emissions of Switzerland, Croatia, and Norway combined. As many cryptocurrencies crash and the FTX bankruptcy moves into the litigation stage, regulators are likely to scrutinize the crypto world more than ever before. This presents a perfect opportunity to curb their environmental damage.

The good news is that cryptocurrencies don’t have to be carbon intensive. In fact, some have near-zero emissions. To encourage polluting currencies to reduce their carbon footprint, we need to force buyers to pay for their environmental harms through taxes.

The difference in emissions among cryptocurrencies comes down to how they create new coins. Bitcoin and other high emitters use a system called “proof of work”: To generate coins, participants, or “miners,” have to solve math problems that demand extraordinary computing power. This allows currencies to maintain their decentralized ledger—the blockchain—but requires enormous amounts of energy.

Greener alternatives exist. Most notably, the “proof of stake” system enables participants to maintain their blockchain by depositing cryptocurrency holdings in a pool. When the second-largest cryptocurrency, Ethereum, switched from proof of work to proof of stake earlier this year, its energy consumption dropped by more than 99.9 percent overnight.

Bitcoin and other cryptocurrencies probably won’t follow suit unless forced to, because proof of work offers massive profits to miners—and they’re the ones with power in the system. Multiple legislative levers could be used to entice them to change.

The most blunt solution is to ban cryptocurrency mining altogether. China did this in 2018, but it only made the problem worse; mining moved to other countries with even less efficient energy generation, and emissions went up. The only way for a mining ban to meaningfully reduce carbon emissions is to enact it across most of the globe. Achieving that level of international consensus is, to say the least, unlikely.

A second solution is to prohibit the buying and selling of proof-of-work currencies. The European Parliament’s Committee on Economic and Monetary Affairs considered making such a proposal but voted against it in March. This is understandable; as with a mining ban, it would be both viewed as paternalistic and difficult to implement politically.

Employing a tax instead of an outright ban would largely skirt these issues. As with taxes on gasoline, tobacco, plastics, and alcohol, a cryptocurrency tax could reduce real-world harm by making consumers pay for it.

Most ways of taxing cryptocurrencies would be inefficient, because they’re easy to circumvent and hard to enforce. To avoid these pitfalls, the tax should be levied as a fixed percentage of each proof-of-work-cryptocurrency purchase. Cryptocurrency exchanges should collect the tax, just as merchants collect sales taxes from customers before passing the sum on to governments. To make it harder to evade, the tax should apply regardless of how the proof-of-work currency is being exchanged—whether for a fiat currency or another cryptocurrency. Most important, any state that implements the tax should target all purchases by citizens in its jurisdiction, even if they buy through exchanges with no legal presence in the country.

This sort of tax would be transparent and easy to enforce. Because most people buy cryptocurrencies from one of only a few large exchanges—such as Binance, Coinbase, and Kraken—auditing them should be cheap enough that it pays for itself. If an exchange fails to comply, it should be banned.

Even a small tax on proof-of-work currencies would reduce their damage to the planet. Imagine that you’re new to cryptocurrency and want to become a first-time investor. You’re presented with a range of currencies to choose from: bitcoin, ether, litecoin, monero, and others. You notice that all of them except ether add an environmental tax to your purchase price. Which one do you buy?

Countries don’t need to coordinate across borders for a proof-of-work tax on their own citizens to be effective. But early adopters should still consider ways to encourage others to come on board. This has precedent. The European Union is trying to influence global policy with its carbon border adjustments, which are designed to discourage people from buying carbon-intensive products abroad in order to skirt taxes. Similar rules for a proof-of-work tax could persuade other countries to adopt one.

Of course, some people will try to evade the tax, just as people evade every other tax. For example, people might buy tax-free coins on centralized exchanges and then swap them for polluting coins on decentralized exchanges. To some extent, this is inevitable; no tax is perfect. But the effort and technical know-how needed to evade a proof-of-work tax will be a major deterrent.

Even if only a few countries implement this tax—and even if some people evade it—the desirability of bitcoin will fall globally, and the environmental benefit will be significant. A high enough tax could also cause a self-reinforcing cycle that will drive down these cryptocurrencies’ prices. Because the value of many cryptocurrencies rely largely on speculation, they are dependent on future buyers. When speculators are deterred by the tax, the lack of demand will cause the price of bitcoin to fall, which could prompt more current holders to sell—further lowering prices and accelerating the effect. Declining prices will pressure the bitcoin community to abandon proof of work altogether.

Taxing proof-of-work exchanges might hurt them in the short run, but it would not hinder blockchain innovation. Instead, it would redirect innovation toward greener cryptocurrencies. This is no different than how government incentives for electric vehicles encourage carmakers to improve green alternatives to the internal combustion engine. These incentives don’t restrict innovation in automobiles—they promote it.

Taxing environmentally harmful cryptocurrencies can gain support across the political spectrum, from people with varied interests. It would benefit blockchain innovators and cryptocurrency researchers by shifting focus from environmental harm to beneficial uses of the technology. It has the potential to make our planet significantly greener. It would increase government revenues.

Even bitcoin maximalists have reason to embrace the proposal: It would offer the bitcoin community a chance to prove it can survive and grow sustainably.