All at sea Is it ever possible to take an environmentally friendly cruise?

Toxic, filthy and cheap, the sludge-like substance known as heavy fuel oil has powered the shipping industry since the 1960s.

- Karen McVeigh

What is perhaps less well known is that it is still used to power more than half of cruise ships, making what many choose as an alternative to flying one of the most environmentally damaging ways to travel.

Under pressure from environmentalists and new regulations, the industry is adopting new technologies and energy-saving designs, and studying alternative fuels.

But can cruises, especially at the rate the market is growing, ever be considered a "sustainable" way to travel? And, with a target date of net zero by 2050, are they decarbonising fast enough?

While the cruise sector forms just 2% of the shipping industry, the boats are growing in size and number. As a result, they need to do more than they are doing now towards sustainability, says Inesa Ulichina, sustainable shipping officer at Transport & Environment (T&E) campaign group.

"Cruise ships spend, proportionately, more time in port than other ships," she says, "particularly in pristine and natural locations where the impact of pollution is greater."

Critics say the cruising boom - ships have soared in number from 21 in the 1970s to 515 today and the world's biggest have doubled in size since 2000 - dwarfs decarbonisation efforts by individual lines and vessels.

A study commissioned by the high level panel for a sustainable ocean economy concluded that the 2% annual improvements in carbon intensity reported by two leading cruise lines in recent years was "cancelled out many times over" by an expected 6% to 7% rise in passenger traffic.

"If we look at Europe, the worst emitting ships are cruise ships or large ferries," says Ulichina.