Global warming could accelerate in future

The rate at which the planet warms in response to the ongoing buildup of heat-trapping carbon dioxide gas may increase in the future, according to simulations of a comparable warm period over 50 million years ago.

Researchers used a climate model to successfully simulate – for the first time – the extreme warming of the Early Eocene Period, which is considered an analogue for Earth's future climate.

The recently published study found that the rate of warming increased dramatically as carbon dioxide levels rose, a finding with far-reaching implications for Earth's future climate.

Another way of stating this result is that the climate of the Early Eocene became increasingly sensitive to additional carbon dioxide as the planet warmed.

"We were surprised that the climate sensitivity increased as much as it did with increasing carbon dioxide levels," said Jiang Zhu, a postdoctoral researcher at the University of Michigan.

"It is a scary finding because it indicates that the temperature response to an increase in carbon dioxide in the future might be larger than the response to the same increase in CO2 now. This is not good news for us," Zhu said.

The researchers determined that the large increase in climate sensitivity they observed – which had not been seen in previous attempts to simulate the Early Eocene using similar amounts of carbon dioxide – is likely due to an improved representation of cloud processes in the climate model they used.

Global warming is expected to change the distribution and types of clouds in the Earth's atmosphere, and clouds can have both warming and cooling effects on the climate.

"Our findings highlight the role of small-scale cloud processes in determining large-scale climate changes and suggest a potential increase in climate sensitivity with future warming," said Christopher Poulsen from University of Michigan.

The Early Eocene -–roughly 48 million to 56 million years ago -- was the warmest period of the past 66 million years.

It began with the Paleocene-Eocene Thermal Maximum, which is known as the PETM, the most severe of several short, intensely warm events.

The Early Eocene was a time of elevated atmospheric carbon dioxide concentrations and surface temperatures at least 14 degrees Celsius warmer, on average, than today.

The difference between temperatures at the equator and the poles was much smaller.

Geological evidence suggests that atmospheric carbon dioxide levels reached 1,000 parts per million (ppm) in the Early Eocene, more than twice the present-day level of 412 ppm, researchers said.

If nothing is done to limit carbon emissions from the burning of fossil fuels, CO2 levels could once again reach 1,000 ppm by the year 2100, according to climate scientists, they said.

Until now, climate models have been unable to simulate the extreme surface warmth of the Early Eocene – including the sudden and dramatic temperature spikes of the PETM – by relying solely on atmospheric CO2 levels.

Unsubstantiated changes to the models were required to make the numbers work, said Poulsen, a professor at the University of Michigan.