The global oceans are at their hottest temperature since records began, and have been warming by approximately 0.1 degree Celsius per decade. That might not sound like a great deal, but the extra heat contributes to thermal expansion of the water, causing sea levels to rise .
Carbon dioxide emissions lead not only to global warming, but also create enormous shifts in ocean chemistry. This has devastating effects on marine plants and animals.
The oceans are the lungs of the Earth. So as we pump carbon dioxide into the atmosphere, the ocean absorbs about a third of all our emissions. This is both good and bad, as it means that global warming progresses at a slower rate because less carbon goes into the atmosphere, but it also leads to acidification of the sea water.
When carbon dioxide dissolves in water it forms carbonic acid, just like in a carbonated soft drink. The oceans are already more acidic than they have been for the past several hundred thousand years, and 30% more acidic than in 1750 .
Mounting evidence warning of the imminent demise of coral reefs is perhaps the strongest signal yet that the planet is on the brink of an environmentally-lead mass extinction, for this appears to have been what happened in the planet’s history.
Reefs are likely to be the first major planetary-scale ecosystem to collapse in the face of climate changes now in progress. This raises the question: will this collapse be restricted to reefs or does it have wider implications?
It is already clear that, although mass bleaching is a reef phenomenon, the effects of ocean acidification will directly impact all species that depend on carbon: not only corals, all marine life that relies on carbonates for their skeletons and shells. This includes fish, which are particularly vulnerable during early stages in their life-cycle and also the ecosystem of the Southern Ocean, which is dependent on krill.
If ocean systems become contaminated by rising carbonic acidity and other pollutants, they become less effective at producing oxygen. In addition, as the seas warm, their ability to store carbon weakens. Historically the oceans have stored CO2, absorbing more than half of our total greenhouse gas emissions, but their ability to hold carbon dioxide has weakened as a result of pollution and ozone depletion . Warmer oceans also lead to increased evaporation of water vapour, a potent greenhouse gas. These factors combine to make ocean systems positive feedback loops, contributing to further global warming.
Most worrying of all is the massive concentration of methane stored in sea beds. As acid levels rise, the possibility arises that methane, the worst greenhouse gas, will be released from the oceans into the atmosphere. The last time methane stores escaped from the oceans, global temperatures rose by 10 degrees Celsius, causing massive ecological devastation around the world .
While research on these issues is still in its infancy, the enormity of the threat is terrifyingly real.
 Ian Ammison, et. al., 2011, ‘The Copenhagen Diagnosis: Updating the world on the latest climate science’, Elsevier, p. 47.
 Scott Doney, ‘Effects of Climate Change and Ocean Acidification on Living Marine Resources’, Woods Hole Oceanographic Institution, 10 May 2007,http://www.whoi.edu/page.do?pid=8916&tid=282&cid=27206; Ian Ammison, et. al., 2011, ‘The Copenhagen Diagnosis: Updating the world on the latest climate science’, Elsevier, p. 49.
 J Vernon, ‘A Reef in Time: the Great Barrier Reef from beginning to end’, Australian Institute of Marine Sciences, 2008.
 Scott Doney, ‘Effects of Climate Change and Ocean Acidification on Living Marine Resources’, Woods Hole Oceanographic Institution, 10 May 2007,http://www.whoi.edu/page.do?pid=8916&tid=282&cid=27206.
 Ian Ammison, et. al., ’The Copenhagen Diagnosis: Updating the world on the latest climate science’, Elsevier, 2011, p. 2.