The health of the oceans is "spiralling downward" faster than previously thought as a result of climate change, pollution and over-fishing, scientists have warned.

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 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, will lead to changes to sea layers, which will also cause lowering of oxygen levels, experts from the International Programme on the State of the Ocean and the International Union for Conservation of Nature said.