Climate activists demonstrate for the oceans in Portugal

A long-delayed UN conference on how to restore the faltering health of global oceans kicks off in Lisbon Monday, with thousands of policymakers, experts and advocates on the case.

Humanity needs healthy oceans. They generate 50 percent of the oxygen we breathe and provide essential protein and nutrients to billions of people every day.

Covering more than two-thirds of Earth's surface, the seven seas have also softened the impact of climate change for life on land.

But at a terrible cost.

Absorbing around a quarter of CO2 pollution -- even as emissions increased by half over the last 60 years -- has turned sea water acidic, threatening aquatic food chains and the ocean's capacity to pull down carbon.

And soaking up more than 90 percent of the excess heat from global warming has spawned massive marine heatwaves that are killing off precious coral reefs and expanding dead zones bereft of oxygen.