Home to most of earth's biodiversity and a vital buffer against climate change, the oceans have only recently been taken seriously by decision-makers. They were integrated into United Nations climate talks for the first time in 2021, while a landmark "high seas treaty" to protect marine health was finally agreed in March after two decades of negotiations.
"There cannot be a healthy planet without a healthy ocean," says Peter Thomson, the UN's special oceans envoy, who equates climate change and "ocean change" as essentially one and the same thing.
The oceans are the world's most important carbon sink, capable of absorbing half of the CO2 emitted into the atmosphere while also generating half of the oxygen we breathe.
However, their health is declining at an alarming rate.
Traditionally addressed as separate issues by UN processes, ocean health, climate change and biodiversity are increasingly being seen as inseparable issues - an achievement that this year's World Oceans Day, on 8 June, is seeking to emphasise.
To ensure that ocean-based action is ratcheted to the top of the agenda, the UN says it's joined forces with governments, scientists, indigenous leaders, civil society, NGOs and leaders from the private sector.
The theme "Planet Ocean: Tides are Changing" appears to ring true, despite studies consistently warning of dangerous tipping points being reached (acidification, sea-level rise and warming) and of melting polar ice disrupting the circulation of ocean currents with potentially disastrous consequences.
The tides are changing in two significant ways: the climate emergency has reached an unprecedented scale, and the role of the oceans as a recognised controller of climate has emerged as an imperative talking point, with ocean-based action progressively being included in national climate commitments.
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At a forum in South Korea last month, environmental experts said the ocean held greater importance for the survival of humanity than most people realised, warning that continued failure to pay attention to this crucial ecosystem would have irreversible consequences for humankind.
"The ocean is the life support system of our planet," says marine biologist Boris Worm from Canada's Dalhousie University. "For the longest time, we did not feel we had a large impact on the high seas. But that notion has changed with expansion of deep sea fishing, mining, plastic pollution and climate change."
A UN Conference on Trade and Development report estimates that 3 billion people depend on oceans for their livelihoods. It valued ocean-based industries, which employ some 40 million people, at $2.5 trillion annually.
Lawless and fragile
But because nearly two-thirds of the ocean lies outside national boundaries - what is called the "high seas" - it has remained largely lawless and unprotected, even in the face of international efforts to safeguard and promote land-based biodiversity.
This changed in March when an historic treaty to conserve 30 percent of the world's oceans by 2030 - a deal that had been held up for years amid disagreements over funding and fishing rights - was signed by 193 countries at the UN in New York.
Today only about 1 percent of the high seas is protected, but when the treaty takes effect, it will allow for the creation of marine protected areas in international waters.
It's a legally binding agreement that will take force when at least 60 countries have passed the legislation in their own countries.
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When the deal was announced, Greenpeace's Laura Meller called it "a historic day for conservation and a sign that in a divided world, protecting nature and people can triumph over geopolitics".
The last international agreement on ocean protection, the UN Convention on the Law of the Sea, was signed 40 years ago, in 1982, with the treaty talks being described as "one of the most important international negotiations that no one has ever heard of".
Despite humanity's utter reliance on it, the ocean has been receiving "a fragment of our attention and resources", the UN warns on its website promoting World Oceans Day, adding that we know more about the vastness of the universe than we do about the deep blue.
Only a small portion of the earth's waters has been explored, the UN reminds us, so how can we justify destroying what we have yet to discover?