Roughly 90 percent of international trade travels by sea, and with all that maritime traffic accounting for an estimated 3 percent to 4 percent of global greenhouse gas emissions, the shipping sector is under increasing pressure tocontribute to a more sustainable future for itself and the planet.

The push for shipping companies to reduce emissions and improve the energy efficiency of their fleets is coming in part from stricter environmental regulations, at both the national and international levels.

With this in mind, some of the industry’s leading companies formed the Sustainable Shipping Initiative in September. Working with nongovernmental organizations, the Forum for the Future and the WWF, the group — whose participants include Cargill, which operates a 300-vessel charter fleet; Daewoo Shipbuilding & Marine Engineering; Rio Tinto Marine, the shipping arm of the international mining giant; the marine insurer RSA; and the Greek tanker operator Tsakos Energy Navigation — says it hopes to address “what it really means for shipping to be sustainable and create a vision of industry in 2040 which is socially and environmentally responsible, resilient and profitable.”

Among the measures the group is considering, to help cut costs and reduce shipping’s environmental effects, is a shift to liquefied natural gas as the primary energy to power engines, Jaakko Eskola, a vice president of the Wartsila Group, said in an e-mail.

But such a shift is not without challenges in an industry that relies mostly on heavy fuel oil — still the least expensive and therefore most common energy source.

Lars Eikeland, executive vice president for marine business development and strategy at Rolls-Royce, who is based in Singapore, called L.N.G. “a marine fuel of the future.”

“The cost, as well as the improved efficiency it provides, makes it an attractive option for ship owners and operators, and the environmental benefits are obvious,” he said.

There are obstacles, however, to making L.N.G. a viable commercial marine fuel. One drawback is that it cannot be pumped into conventional fuel tanks, because it needs to be stored at minus 165 degrees Celsius, or minus 265 Fahrenheit, which requires tanks to be heavily insulated. L.N.G. therefore takes up much more volume than marine diesel oil or heavy fuel oil for the same amount of energy provided.

Since fuel tank size is limited, that means more frequent refueling. That, in turn, means L.N.G.-powered ships are mostly restricted to short routes.

The still-developing bunker station network in many parts of the world and the size of the fuel tanks have so far inhibited the uptake of L.N.G. on a wide scale, Mr. Eikeland said, but this is changing.

“As bunker networks grow, and as improvements in hull design and power and propulsion efficiency continue to grow, the application of L.N.G. as a fuel source for larger intercontinental vessels will become more viable,” Mr. Eikeland said.

Norway has been a leader in developing L.N.G.-fueled ships, among them a car ferry plying the fjords, the Glutra, which entered service in 2001.

According to Det Norske Veritas, DNV, one of the world’s largest ship classification companies, Norway now has about 20 L.N.G.-powered ships operating along its coastline, including four offshore supply vessels and three coast guard patrol boats.

“It took 10 years to move from zero to 20 vessels, but last year alone 15 more L.N.G.-fueled vessels were ordered,” a DNV spokesman, Per Wiggo Richardsen, said.

While L.N.G.-fueled ships presently on order are mostly destined for use in and around Norway, one high-speed ferry is being built in Tasmania by Incat shipyard, said Lars Petter Blikom, segment director for L.N.G. at DNV.

Last month, DNV upped the ante by publishing an ambitious proposal for a giant L.N.G.-fueled crude oil tanker.

Named Triality, the concept tanker would have two high-pressure dual-fuel slow-speed main engines, using L.N.G. as the main fuel and marine gas oil as a start-up pilot fuel. An advanced hull design eliminates the need to carry ballast water, providing additional fuel savings through the efficiency gain resulting from the lower deadweight tonnage.

DNV says that these specifications would add only 10 percent to 15 percent to the cost of a traditional very large crude carrier, while the ship would use 25 percent less energy, produce 80 percent less nitrogen oxide emissions and emit 34 percent less carbon dioxide.

The concept tanker could operate on long routes — in fact it would be capable of a global round trip, Mr. Richardsen said in an e-mail.