The report also warned that this could upset delicate ecological balances by favoring sealife tolerant of low oxygen, like microbes and jellyfish. Even "basic processes" on Earth like the cycling of nitrogen and phosphorous could suffer, the IUCN said.

Nutrient pollution from farming and other sources is also a problem on coastlines, according to the IUCN.

The future could prove rough even if humans take corrective action. The researchers estimated that the ocean would lose 3 to 4 percent of its oxygen worldwide by the year 2100 if nothing changes, with more severe losses in "mid-to-high" latitudes. The IUCN still expects losses if political leaders take the "immediate and substantial" actions the group recommends -- it's just that they won't be as severe.

The findings contribute to an existing bleak picture of the future if humans don't limit their effect on climate change. At the same time, they illustrate a very practical reason to reduce emissions even if you're unconcerned about rising sea levels or other land-based problems. An imbalance underwater could hurt food supplies and create a knock-on effect where species lose their food sources or, without enough predators, flourish at unsustainable rates.