The world's maritime industry — from ferries to freighters — is trying to navigate a once-in-a-century transition away from fossil fuels to new, cleaner means of propulsion.

Why it matters: International shipping is key to the world's economy, responsible for 90% of global trade. But the vessels burn about 4 million barrels of oil a day, accounting for almost 3% of the world's carbon emissions, and regulators are demanding they clean up their act.

The big picture: Oceans cover 70% of the planet, and their microscopic plant life provides 80% of the world's oxygen. Increased carbon emissions are warming the oceans, causing rising sea levels that threaten coastal communities while disrupting aquatic ecosystems, harming marine life.

Driving the news: The International Maritime Organization, a United Nations regulatory agency, in 2018 set tough new targets for greenhouse gas reduction.

• By 2050, shipping should aim to reduce emissions by 50% (compared to 2008 levels), with the goal of eventually phasing out all emissions.

Yes, but: Technologies to meet those ambitions don't yet exist in a form or scale that is commercially viable for trans-ocean shipping. And with most ships built to last 30 years, there's not much time left to find solutions.

• "We basically have five to seven years to figure out what the fuel of the future is for these giant vessels plying around the globe, and that has everybody with their hair on fire," says Joshua Berger, founder of Washington Maritime Blue, a strategic alliance focused on maritime sustainability.
• An added complication: trying to wrangle 174 countries and international shipping companies to agree on future technology standards.

Where it stands: Despite the recent introduction of low-sulfur fuel and other efforts, the growth of shipping has been outpacing efficiency improvements.

• At the current rate, the nonprofit International Council on Clean Transportation projects that 2050 emissions will be 130% higher than 2008 levels, per Reuters.

What's happening: The industry is urgently exploring several promising zero-carbon fuels and technologies, but each poses challenges that will require more R&D.

Electricity: A typical container ship would require the equivalent of 70,000 Tesla Model S 85 batteries to sail for a week across the ocean, according to the International Chamber of Shipping.

• Vessels like ferries and harbor tugs, however, make more sense for electrification because they stick close to shore and can be recharged fairly easily. Still, the grid must be able to handle the load.
• One model project: The state of Washington is deploying a fleet of 25 electric ferries in Puget Sound.

Liquified Natural Gas: It's relatively cheap and emits approximately 25% less carbon dioxide than conventional marine fuels. But because it is mostly methane, LNG has other climate problems and is not a permanent solution, marine experts say.

• The world's first LNG-powered containerships are being tested at the Jacksonville Port Authority in Florida.

Other options have shortcomings, too: Ammonia and hydrogen, for example, can be generated from renewable sources, but they're not as energy-dense as oil, meaning ships will consume up to five times as much fuel.

• Liquid biofuels hold promise, but they depend on available feedstocks, which may be insufficient for the scale of international shipping.

What's needed: An international solution, says David Hume, a marine energy manager with the Pacific Northwest National Lab, helping to coordinate a U.S. strategy.

• The industry is even proposing to tax itself on fuel in order to create a $5 billion R&D fund, overseen by the IMO, to accelerate innovation.

The bottom line: An industry whose ships were once powered by sails, steam and then oil needs to come up with another answer — fast.