In the next few decades, fish in huge swaths of the ocean could be struggling to breathe

One of the most talked-about consequences of climate change is ocean acidification, which particularly threatens creatures that build shells.

But there's another big problem in the ocean's chemistry that's beginning to get out of control: oxygen.

Even though they live underwater, fish breathe oxygen just like we do. Their gills pull dissolved oxygen out of the water.

But the warmer the ocean's water gets, the less dissolved oxygen it can hold. It's basically the underwater equivalent of a human panting in the thin air on a mountaintop.

There are naturally some parts of the ocean that have less oxygen in them. But as climate change gets worse, a lot of areas aren't going to be able to hold as much oxygen as they have in the past — and the fish that live there won't be able to breathe. Each of these low-oxygen pockets will expand, spreading both horizontally and up towards the surface.

A recent study found that within the next 15 to 25 years, whole swaths of the ocean will have noticeably less oxygen than they would without a changing climate.

Oxygen levels are highest at the surface, since the ocean gets its oxygen from mixing with the air and from plants that use sunlight to grow.

That means the deeper parts of the ocean are in the most danger. Some traditionally deep dwelling ocean creatures have been seen much closer to the surface than usual, forced into new zones where they can breathe.

It's hard to pin down oxygen levels precisely, and modeling what will happen gets complicated fast thanks to other factors at play. But none of the stats are comforting.

One prediction has deepwater oxygen levels plummeting as much as 20-40% in the next century. Some regions off the coast of California have lost a third of their oxygen in the past 25 years. For more than a decade, water off the Pacific Northwest has lost so much oxygen that National Geographic has called them a "lifeless wasteland."

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