Research into an historic episode of climate change, during which temperatures rose by 5C (9F), suggests that it was caused by a massive release of carbon dioxide and methane. The abrupt period of global warming, known as the Palaeocene-Eocene Thermal Maximum (PETM), caused temperatures to rise significantly, as 4.5 trillion tons of carbon were released into the Earth’s atmosphere. The reasons for the release have yet to be fully explained.

“The emissions that caused this past episode of global warming probably lasted 10,000 years,” James Zachos, Professor of Earth Sciences at the University of California, Santa Cruz, told the American Association for the Advancement of Science conference in St Louis. “By burning fossil fuels we are likely to emit the same amount over the next three centuries.”

Professor Zachos has begun to reconstruct the processes that led to the PETM event by studying deep-sea sediments, which record changes in ocean chemistry. These show that vast quantities of methane, a potent greenhouse gas, were suddenly released from beneath the ocean floor. Much of the methane then reacted with oxygen in the water to form carbon dioxide.

His research estimates that it then took about 100,000 years for carbon dioxide levels in the air and water to return to normal and bring global temperatures back down. These findings add to the evidence that temperature changes brought about by greenhouse gas emissions will take centuries to reverse.

“We set out to test the hypotheses put forward by a small group of geochemists who model the global carbon cycle, and our findings support their predictions,” Professor Zachos said. “It will take tens of thousands of years before atmospheric carbon dioxide comes down to pre-industrial levels. Even after humans stop burning fossil fuels, the effects will be long-lasting.”

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