A group of shipping firms warned that the International Maritime Organization’s planned net zero rules risk imposing significant costs on the industry. 

The IMO, shipping’s global regulator, has set a goal of net zero for shipping by mid-century and next month aims to formally adopt a plan that would see ships pay levies for at least some of their emissions. The U.S. has criticized the IMO’s Net Zero Framework, labeling it a “global carbon tax.”

The sector faces paying about $20 billion to $30 billion a year by 2030, shippers representing more than 1,200 vessels said in a statement dated September 18. The amount could accumulate to more than $300 billion by 2035 if the global fleet misses targets by as little as 10%, they said.

Despite opposition from the U.S., a majority of countries backed the proposal in an IMO vote earlier this year. The International Chamber of Shipping, which represents more than 80% of the planet’s merchant fleet, also supports the plan.

The IMO’s NZF won’t be effective in helping shipping decarbonize in line with the IMO’s strategy announced in 2023 or ensuring a level playing field, the group of shippers said in the statement. Critical amendments “are needed, including the consideration of realistic trajectories.”

“It is essential that the IMO NZF implements GHG measures that are fit for purpose,” the group of companies, which includes Stolt Tankers, Frontline Plc and Saudi Arabia’s Bahri, the companies told Reuters in a joint statement. It said measures should avoid “excessive financial burdens and inflationary pressure to the end-consumer.”