Climate change predictions backed by warm past evidence

A multinational research team at the University of Southampton has analysed new records showing the CO2 content of the Earth's atmosphere between 2.3 to 3.3 million years ago, over the Pliocene.

During the Pliocene, the Earth was around 2degree C warmer than it is today and atmospheric CO2 levels were around 350-400 parts per million (ppm), similar to the levels reached in recent years.

By studying the relationship between CO2 levels and climate change during a warmer period in Earth's history, the scientists have been able to estimate how the climate will respond to increasing levels of carbon dioxide, a parameter known as climate sensitivity, and also show how it can vary over the long term.

Co-author Gavin Foster said that today the Earth is still adjusting to the recent rapid rise of CO2 caused by human activities, whereas the longer-term Pliocene records document the full response of CO2-related warming.

Foster added that estimates of climate sensitivity lie well within the range of 1.5 to 4.5 degree C increase per CO2 doubling summarised in the latest IPCC report. This suggests that the research community has a sound understanding of what the climate will be like as we move toward a Pliocene-like warmer future caused by human greenhouse gas emissions.

Lead author Miguel Martinez-Boti said that their new records also reveal an important change at around 2.8 million years ago, when levels rapidly dropped to values of about 280 ppm, similar to those seen before the industrial revolution. This caused a dramatic global cooling that initiated the ice-age cycles that have dominated Earth's climate ever since.

Richard Pancost from the University of Bristol Cabot Institute added that when they account for the influence of the ice sheets, they confirm that the Earth's climate changed with a similar sensitivity to overall forcing during both warmer and colder climates.

The findings have been published in Nature.