Scientists have warned that climate change has sent the health of oceans "spiralling downward" faster than previously thought.
A key international assessment of climate change last week revealed the oceans are absorbing much of the warming and unprecedented levels of carbon dioxide caused by human activity such as burning fossil fuels.
Now experts are warning the impact of rising greenhouse gases combined with a range of other problems is far graver than previously thought.
Oceans are being hit by decreasing oxygen levels caused by climate change and increased nutrient run-off from agriculture and sewage, and are becoming more acidic as more carbon dioxide dissolves into the sea, both of which harm wildlife.
Warming, as the oceans absorb much of the extra energy being trapped by greenhouse gases, is set reduce seasonal sea ice and lead to changes to sea layers, which will also cause lowering of oxygen levels.
Warming will also lead to increased venting of methane, a potent greenhouse gas, from the Arctic seabed, experts from the International Programme on the State of the Ocean (IPSO) and the International Union for Conservation of Nature (IUCN) said.
The "deadly trio" of warming, de-oxygenation and acidification is seriously affecting how productive and efficient the ocean is, with impacts throughout the chain of marine life, the scientists said.
In addition, continued overfishing is damaging the resilience of the oceans, and despite improvements in some areas, fisheries management is failing to halt the decline in key species and prevent harm to important marine ecosystems.
Last year the UN's Food and Agriculture Organisation (FAO) found that 70% of world fish populations were unsustainably exploited, and almost a third of those (30%) had collapsed to less than 10% of unfished levels.
The scientists called for urgent action to limit temperature rises to less than 2C above pre-industrial levels.
They warned that current targets for carbon emission reductions are not enough to ensure coral reefs on which humans and wildlife depend survive increasingly acidic oceans.
Potential knock-on effects of climate change in the oceans such as methane releases from the melting permafrost and coral dieback could lead to worse consequences for humans and nature than presently expected.
The scientists also called for effective management of fishing, which favoured small-scale fisheries, taking steps such as eliminating subsidies that back more fishing vessels than fisheries can support, and banning the most destructive gear.
And there needs to be "fit for purpose" global systems for governing the high seas, the experts said.
Oxford University professor Alex Rogers, scientific director of IPSO, said: "The health of the ocean is spiralling downwards far more rapidly than we had thought.
"We are seeing greater change, happening faster, and the effects are more imminent than previously anticipated.
"The situation should be of the gravest concern to everyone, since everyone will be affected by changes in the ability of the ocean to support life on Earth."
The IUCN's professor Dan Laffoley said: "What these latest reports make absolutely clear is that deferring action will increase costs in the future and lead to even greater, perhaps irreversible, losses.
"The UN climate report confirmed that the ocean is bearing a brunt of human-induced changes to our planet. These findings give us more cause for alarm - but also a road map for action. We must use it."
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