One of the greatest causes of global climate change is the human emissions of greenhouse gases, specifically carbon dioxide (CO2). These emissions are released into the atmosphere, but much of it gets absorbed into the world's oceans.

A new study, published in the Proceedings of the National Academy of Sciences, looked at prehistoric ocean sediment and found a disturbing trend. Periods of high CO2 concentrations have historically coincided with mass extinctions of marine organisms.

British and Australian scientists examined ocean sediment samples taken off the coast of west Africa. They dated the samples to about 85 million years ago, a period during the late Cretaceous, and a time of high atmospheric CO2 levels. In these layers, they found a significantly high amount of organic matter buried within de-oxygenated sediment.

This indicates that a mass die-off occurred which was caused by lower oxygen levels in the water. As CO2 in the atmosphere rises, global ocean temperatures also rise. Higher ocean temperatures cause a lessened capacity for dissolved oxygen. This ocean condition is known as hypoxia and is the cause of dead zones.

Dead zones can be found can be found all over the world, most notoriously in the Gulf of Mexico. A dead zone exists at the mouth of the mighty Mississippi River which huge drainage area covers the heart of United States agriculture. The dead zone that occurs there is caused by high-nutrient runoff and is roughly the size of the state of New Jersey.

However, unlike today, the hypoxia of 85 million years ago did not come from the mouths of rivers, but from the atmosphere. In that way, the resulting dead zones covered the whole globe. Marine wildlife had literally nowhere to go. The researchers showed that the extinctions occurred over extremely short periods (geologically-speaking) of only hundreds of years or less. Furthermore, they were caused by only modest changes in atmospheric CO2 levels and ocean oxygen levels

This alarming finding puts greater emphasis on halting climate change, to the degree that we can. Current high atmospheric CO2 levels pose a grave threat to marine life. According to the NASA Earth Observatory, dead zones have expanded tremendously in the past half-century. Martin Kennedy from the University of Adelaide in Australia said, "Earth's oceans are in a much more delicate balance during greenhouse conditions than originally thought."