The challenge is huge, because shipping produces two per cent of global carbon emissions - about the same as the whole German economy. Plus, a report by the Royal Society says ammonia production currently creates 1.8 per cent of global CO2 emissions - the most of any chemical industry, BBC News reports.

Yet the authors of the report say new technology can create zero carbon ammonia. One way is by trapping the CO2 emissions created when ammonia is produced, and burying the CO2 in underground rocks. Another way of making so-called "green" ammonia is to use renewable energy, which doesn't create any CO2.

The big question is whether enough clean energy will be available to create ammonia in the coming decades. Ammonia is shipped in bulk as an essential base for chemicals, textiles, explosives, refrigerants and fertilisers. But when it is spread carelessly on fields it causes air and water pollution, and it can react with other chemicals to make greenhouse gases.

The Royal Society report notes that the near ubiquitous use of ammonia for agriculture has conveniently generated a global network of ports where the chemical is traded or stored.

That means the infrastructure for storing chilled ammonia as a shipping fuel already exists. In the US, for instance, one existing ammonia pipeline runs for 2,000 miles.

Ammonia engines can be developed from liquefied petroleum gas engines. Engine designer Man Energy Solutions is making a two-stroke ammonia-powered engine it hopes will be ready by 2024. The firm's spokesman, Peter Kirkeby, told the BBC: "We see a very big interest from the market in ammonia as a fuel - even though there are challenges.

"We expect the first ships fuelled with ammonia will be existing tankers that are already transporting ammonia for fertiliser. They know how to handle it.'

He said he expected ammonia to match the price of other alternative fuels such as liquefied petroleum gas, liquefied natural gas or methanol.

However the report shows how inefficient these fuels are compared with energy-dense diesel.

This means that ships powered by greener fuels need to set aside more valuable space for fuel storage. That's why the report's authors say hydrogen will be too bulky to fuel ocean-going ships.

And there's a further problem. Burning ammonia may not cause CO2 emissions, but it does create nitrogen oxides, which are also greenhouse gases. The report says technology will need to be developed to deal with this.

The Royal Society's lead author, Professor Bill David, told the BBC: "Ammonia is the only zero-carbon fuel that will get you across the oceans."