The findings “demand urgent action, through stronger multilateral cooperation, greater ambition and decisions grounded in the best available science”.
The WOA welcomed a UN treaty that came into force in January to protect and sustainably use marine life in international waters, saying it “marks a historic milestone for ocean stewardship and multilateral cooperation”.
UN Secretary-General Antonio Guterres said “we cannot keep treating the ocean as limitless”.
“We must build a new relationship with the ocean: Grounded in science. Framed by international law. And built on shared responsibility.”
The report, which mostly covers the period between 2018-2023, paints a stark picture of the state of the oceans.
Around 16% of the total increase in ocean heat content recorded since 1955 has occurred since 2018 alone, the assessment found.
The oceans have absorbed more than 90% of the excess heat and 30% of the carbon dioxide released into the atmosphere by the burning of fossil fuels.
As waters warm, they expand, helping drive sea-level rise alongside meltwater from glaciers and ice sheets.
The report said sea level continued to rise “at increasing rates”, more than doubling from less than 2mm a year before 2015 to 4.3mm a year in 2023.
While millimetres might seem small, they “multiply up really quickly”, said Ian Butler, an Australia-based marine ecologist and joint coordinator of the WOA’s group of experts.
Melting ice
The Arctic Ocean could become ice-free in Septembers by the middle of the century, with the earliest such conditions possible in the 2030s under all emissions scenarios, the report said.
Butler said: “We’re seriously looking at an ice-free Arctic Ocean for parts of the year within 10 or 20 years.”
The ice melt in the northern pole is also reshaping geopolitics, opening previously inaccessible shipping routes and heightening competition among major powers including the United States, Russia and China.
In the South Pole, Antarctic sea ice, which had gradually increased between 1979 and 2015, has “rapidly declined” since 2016.
Marine ecosystems
Climate change is also reshaping marine life, with some fish species moving to cooler or deeper waters to survive.
Butler said: “Some have no future at all because there’s nowhere for them to go.”
Coral reefs are among the most threatened ecosystems.
Repeated marine heatwaves and storms “leave little time for recovery and are pushing reefs towards collapse”, the report said.
Bleaching events since 2018 have caused widespread coral mortality, with the WOA warning that 90% of reefs could disappear if warming tops 1.5C above pre-industrial levels.
Plastic pollution and deep-sea mining
The report called for a reduction in the production of plastics – an issue at a standstill in international negotiations.
Each year, 52.1 million tonnes of plastic waste spills into the ocean, contributing to an estimated 24.4 trillion microplastic particles.
Microplastics are now known to affect more than 4000 marine species.
The report highlighted growing concerns about deep-sea mining and called for a coordinated international response.
While exploration for deep-sea mining is far advanced, no company or nation has started production on a commercial scale.
Critics fear it would smother marine life with waste and the noise of heavy machinery will disrupt oceanic migrations.
Trump ocean cuts
US President Donald Trump’s administration is set to remove hundreds of deep-sea scientific instruments used for a decade to monitor the effects of climate change on marine environments.
Butler said: “The deep ocean monitoring system is an extremely important part of our global monitoring and understanding of the ocean. The removal of it would leave a huge gap in our long-term ocean science.”
– AFP