More than 20 countries backed a charge on the shipping industry’s greenhouse gas emissions at a global finance summit held in Paris last week.

France, Spain, Norway and Portugal are among 22 nations that support “the principle of a levy on the GHG emissions” of the international shipping sector, according to a summary of discussions and information provided by a French official. It’s also backed by the European Commission.

Charging shippers for their emissions isn’t a new idea — it’s previously been pitched by countries to the International Maritime Organization, shipping’s global regulator, where it remains under discussion. Commodity trading titan Trafigura Group is also supportive and the International Chamber of Shipping, which represents more than 80% of the merchant fleet, has previously called for a levy.

Although substantial, the list of supporters currently doesn’t include the US or China, the world’s two largest economies. Treasury Secretary Janet Yellen said her country would look at a proposal for a maritime tax.

The shipping industry transports more than 80% of world trade and is responsible for about 3% of man-made CO2 emissions. Making it pay for its pollution would help bridge the price gap between the relatively cheap fossil fuel the sector relies on today and those propellants it could use in the future, such as clean versions of methanol and ammonia.

But getting an agreement at the IMO, which has more than 170 member states with various competing interests is easier said than done. The organization’s Marine Environment Protection Committee is meeting in early July, where it’s expected to revise its strategy for cutting greenhouse gas emissions, but not to make any final decision on a global carbon levy.