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Peter Evans this week investigates the effect greenhouse gases are having on our oceans. Recent research shows that oceans will become more acidic and that will threaten many species, from plankton to whales, as well as coral reefs, with extinction - not in a thousand years but possibly within the next century.

Our oceans are doubly besieged. The same pollution that is heating the world's oceans through global warming is also altering their chemical balance. Until recently, few scientists worried about the effect of carbon dioxide - a major greenhouse gas - on the Earth's oceans. Oceans 'mop up' most of the carbon dioxide in the atmosphere and this, it's been thought, has in fact helped to slow down the greenhouse effect - helping to reduce the impact of carbon dioxide on global warming. There have even been plans, discussed in a previous Frontiers programme, to sequester carbon dioxide in the oceans - to keep it out of harms way. But recent research has shown that carbon dioxide is having a potentially devastating affect on our seas - and in particular on the acidity of the water.

A study published last year by Livermore National Laboratory in the United States suggested that the projected increases in carbon dioxide in the atmosphere may drive ocean pH values, the scale for measuring acidity, to change more rapidly than at any time over the last 25 million years. According to one researcher, to find the same sort of acid ocean levels in Earth history, you have to look back to mass extinction events such as those which are thought to have wiped out the dinosaurs.

Currently, the impact of this rising ocean acidity on marine life is largely unknown. Only small scale experiments have been done but these all point to the same conclusion - that altering the PH level of the oceans (decreasing it and thus making the ocean a bit more acidic) has a detrimental effect on corals and sea creatures with hard shells.

Increased acidity may also directly affect the growth and reproduction rates of larger organisms such as fish, as well as affecting the plankton populations which they rely on for food, and have potentially disastrous consequences for marine food webs all the way up to whales.