Cars and trucks aren't the only source of transportation-related carbon emissions.
The massive ships that ply the world's oceans are major sources of pollution as well, but generally receive less attention from regulators.
But one cruise line is now taking steps to reduce the environmental impact of its fleet.
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Carnival Corporation is planning seven cruise ships powered by liquefied natural gas (LNG).
It recently confirmed a deal with Shell to supply LNG for the first two of those ships, which are scheduled to be launched in 2019.
The ships will be operated under Carnival's Aida Cruises and Costa Cruises brands, on routes "visiting popular northwest European and Mediterranean ports," according to a company press release.
Cosat Atlantica cruise ship
Carnival began buying LNG from Shell earlier this year to power one of its ships while in port—which the company calls an industry first.
Prior to that, it also employed a barge that used LNG fuel to supply power to a different ship while it was docked.
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The two new ships expected to enter service in 2019 will have dual-fuel engines, but Carnival says they will only use LNG while in service.
That includes time both at sea and in port. In current cruise ships, the difference in emissions between the two can be significant.
While in port and close to some U.S. and European coasts, most cruise ships must use emissions-abatement equipment and specific grades of diesel fuel.
Princess Sapphire cruise ship
Large vessels burning massive amounts of diesel fuel can also have an adverse effect on the air quality in ports frequented by cruise ships.
MORE: EU proposes deep cuts to vehicle emissions, ignores ships, airplanes
While Carnival is making efforts to reduce ship emissions on its own, very little regulatory action has been taken to address the issue.
Earlier this year, both carmakers and the environmental group Transport & Environment lodged complaints about shipping not being included in European emissions standards.
Those protests were not met with any concrete plans from regulators.
[EDITOR'S NOTE: Green Car Reports thanks our tipster, who prefers to remain an International Man of Mystery.]