That has now changed. Last Friday, an I.M.O. committee meeting in London agreed to a blueprint that sets the industry on a course to reduce carbon emissions by container ships, tankers and other vessels by at least 50 percent by the middle of the century compared with 2008 levels, with the eventual goal of phasing out fossil-fuel use.

Billed as the maritime organization’s “climate strategy,” the accord was adopted by more than 100 member nations, over objections from the United States. Although the European Union and some countries had sought even greater reductions, the plan as adopted was hailed by many environmental groups as ambitious.

“I think it’s a really important development for this sector,” said Daniel Rutherford, who directs the aviation and marine programs at the International Council on Clean Transportation, a watchdog group. “Until this past week, it didn’t have a climate target.”

But he and others noted that the next few years would be critical. The I.M.O. gave itself until 2023 to develop and implement specific measures to reach the target. These will probably include stricter design and operation standards for ships to increase efficiency, as well as programs to introduce low-carbon fuels or carbon-free propulsion.

“If you don’t make early investments and policies, then it becomes very challenging to meet the I.M.O. goals,” Dr. Rutherford said.

The I.M.O. may also take steps to introduce so-called market-based measures that would set a price on carbon. This would create economic incentives for shippers to adopt new fuels and technologies, which are likely to be costly.

An analysis by researchers at the University College London Energy Institute described the agreement as a signal to the industry and investors “that a clear switch away from fossil fuels is now on the cards.”