A team of geologists from Newcastle University in the UK have discovered evidence that 'greenhouse oceans' occurred in prehistoric times, resulting in areas of ocean with little or no life due to low levels of oxygen in the water. Their research indicates that the planet could be on the brink of the same phenomenon re-occurring.

Professor Martin Kennedy and Professor Thomas Wagner studied sediment samples across a 400,000 year span from the sea floor off the coast of western Africa and discovered that around 85 million years ago, there were mass extinctions of marine life due to what is they have termed 'greenhouse oceans'. High levels of carbon dioxide in the water led to insufficient oxygen to sustain life in many species of marine life.

What they noted was layers of deoxygenated sediment from the Late Cretaceous period sandwiched between layers of sediment with visible evidence of life. Thomas Wagner commented on the findings:

''We know that 'dead zones' are rapidly growing in size and number in seas and oceans across the globe. These are areas of water that are lacking in oxygen and are suffering from increases of CO2, rising temperatures, nutrient run-off from agriculture and other factors.''

Worryingly, the onset of hypoxia resulted over a relatively short period of time, perhaps over a few hundred years, rather than millennia, and from only slight changes in the surrounding environment. Obviously, if the same mass extinctions occur again because of current global warming, it could have a massive effect on sustainability of life in our oceans, which in turn, could affect the life cycles of land-based species and, ultimately, humans.

There is some good news though. The scientists say that natural processes respond to greenhouse conditions to bring oxygen levels back to levels that can sustain life. Nutrients from soil arriving in the oceans collect and bury organic matter and, in doing so, remove CO2 from the environment.

Wagner said: ''This is nature's solution to the greenhouse effect and it could offer a possible solution for us. If we are able to learn more about this effect and its feedbacks, we may be able to manage it, and reduce the present rate of warming threatening our oceans.''