Scientists have found the world's first large-scale area of acidified water in the open ocean in the seas of the western Arctic.
The say the area of acidified water in the Canada Basin has expanded northwards by about 500 kilometres since the mid-90s.
It's also increased in depth by about 100 metres.
They report the area is now six times more acidic than it used to be.
Their study suggests the change is related to disappearing sea ice, which lets more acidic water from the Pacific flow north and increases the exposure of Arctic water to the air.
Ocean acidification is one of the effects of higher levels of carbon dioxide in the atmosphere.
Scientists say it's already starting to affect animals from plankton to shellfish.