Delegates of the International Maritime Organization (IMO), the U.N. agency responsible for shipping safety and pollution, announced this month that 170 of its members had agreed to reduce carbon emissions from shipping to no more than half of 2008 levels by 2050.

Shipping in Changing Climates, a research consortium, called the deal “major progress” toward bringing shipping in line with the goals of the 2015 Paris climate agreement. But why has it taken two years after Paris to reach such a deal?

Shipping and airlines were the only greenhouse-gas-emitting industries not mentioned in the Paris agreement. This was, in part, because assigning emissions is hard. To whom should you designate emissions for shipping Chinese goods, made with South Korean components, across the Pacific, to American consumers?

But similar problems did not stop airlines agreeing on an industrywide limit within a year.

Diplomats argue the slow progress is because any caps would affect exporters, too. If regulators move too aggressively, they may reduce the competitiveness of seaborne trade.

For instance, Brazil, a big exporter of iron ore to China, fears overzealous caps will drive shipping costs higher, helping its competitor, Australia, whose ores travel a quarter as far as Brazil’s.

The idea of slowing vessels down draws complaints from countries that export perishable goods, like cherries and grapes, as Chile does.

Others fear that powerful lobbyists have hijacked the process. Transparency International, an NGO, has raised “serious concerns” about the IMO being unusually influenced, for a U.N. body, by corporate interests.

A report by research firm InfluenceMap found that at a recent meeting of the agency, 31 percent of nations were represented, in part, by direct business interests.

The IMO announcement also faced opposition from countries such as Saudi Arabia and the United States, who consistently oppose efforts to combat climate change.

Even including the agreed cuts, shipping, which currently accounts for almost 3 percent of total global carbon emissions, will probably see its share of emissions rise in coming years, although new technology and reduction in speeds could change that. New design standards are already lowering harmful emissions. Zero-carbon fuels are becoming available.

Last week’s agreement was a good start toward adoption of such changes. As with many such agreements, the implementation is everything.