Global bottled water giants are ready to shift to aluminum cans as replacement for plastic that pollutes the world's seas. Sound like a slam-dunk for the environment? Not entirely.

Aluminum cans might indeed mean less ocean waste, but they come with their own eco-price: The production of each can pumps about twice as much carbon into the atmosphere as each plastic bottle.

The shift, previously unannounced, comes as multinational rivals like Coca-Cola, Pepsi and Nestle are also launching some canned versions of water brands.

The beverage industry has been scrambling to react to public anger over scenes of huge piles of plastic waste contaminating oceans, pledging to step up recycling efforts.

However it's not black and white on the green front. By increasing recycling via cans, companies could fall back in efforts to reduce their carbon footprints, illustrating the tough juggling act they can face to keep environmentally conscious investors, campaigners and consumers on-side.

"That's the dilemma you're going to have to choose between," said Ruben Griffioen, sustainability manager of packaging materials at Heineken, adding the company was trying to reduce both plastic waste and emissions.

Aluminum cans are a complex case:

Recycling plastic is more complex, leads to degradation and has lower reuse rates than aluminum – so aluminum has been heralded as a greener alternative. Cans have on average 68 percent recycled content compared to just three percent for plastic in the United States, Environmental Protection Agency data shows.

But when all types of metal are averaged out, however, cans still account for about double the greenhouse gases of plastic bottles, according to analysts.

At aluminum's most polluting level, a 330 ml can is responsible for 1,300 grams of carbon dioxide emissions, roughly equating to the emissions produced by driving a car seven to eight km.

A plastic bottle of the same size, made from the polyethylene terephthalate (PET) plastic typically used, accounts for up to 330 grams.

In addition, aluminum has a larger footprint in production because of the vast power needed in the smelting process. But, in a further example of the complexities of environmental impact, the overall carbon equation becomes more muddied when other issues such as logistics are taken into account.

Because aluminum is lightweight and cans make efficient use of space, less transport is usually needed than for plastics or glass, while less power is also needed to chill drinks in cans – particularly useful in tropical climes.

(All photos via VCG)

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