High levels of greenhouse gases could kill off life in the oceans in the same way as happened during prehistoric times, geologists warn.
Professor Martin Kennedy from the University of Adelaide and Professor Thomas Wagner from the UK's Newcastle University have been studying 'greenhouse oceans' which have been depleted of oxygen.
Using core samples from the ocean bed off western Africa, the geologists studied sediment from the Late Cretaceous Period - 85 million years ago - over a 400,000-year period. They found a significant amount of organic material buried within deoxygenated layers of the sediment, indicating a mass mortality in the oceans.
"We know that 'dead zones' are rapidly growing in size and number in seas and oceans across the globe," says Wagner. "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."
High levels of carbon dioxide in the atmosphere and rising temperatures appear to have led to a severe lack of oxygen, or hypoxia, in the water.
"What's alarming to us as scientists is that there were only very slight natural changes that resulted in the onset of hypoxia in the deep ocean," says Kennedy.
"This occurred relatively rapidly – in periods of hundreds of years, or possibly even less – not gradually over longer, geological time scales, suggesting that the Earth's oceans are in a much more delicate balance during greenhouse conditions than originally thought, and may respond in a more abrupt fashion to even subtle changes in temperature and CO2 levels."
Kennedy described the doubling of carbon dioxide in the atmosphere over the past 50 years as "like hitting our ecosystem with a sledge-hammer", compared to the very small changes in solar radiationwhich have triggered similar events in the past.
"This could have a catastrophic, profound impact on the sustainability of life in our oceans, which in turn is likely to impact on the sustainability of life for many land-based species, including humankind," he says.