Now Marten Scheffer of Wageningen University in The Netherlands and colleagues from the Potsdam Institute for Climate Impact Research, Germany, believe they have shown that such tipping points occurred in the past. And they may also have found a way of telling when the Earth is approaching a climate tipping point. By analysing geological records of eight rapid climate shifts in the past, the team discovered that a slow-down in fluctuations preceding the rapid shift acted as an early warning signal.

"We had been pursuing a theoretical prediction that there are universal indicators that should signal if a system is approaching a tipping point," Scheffer told environmentalresearchweb. "One prediction on which we focused is that subtle fluctuations in the state of the system should become slower before the shift; a phenomenon known as 'critical slowing down'. Climate seemed an obvious place to look for such signals, as we have long records of reconstructed dynamics as well as notoriously dramatic shifts."

After applying techniques such as smart filtering to the real historical climate data, the team found that they could pick up the theoretically predicted early warning signals. "This opens the fascinating and important possibility that we might predict shifts at tipping points in a wider range of systems, including ecosystems and social or financial systems," said Scheffer.

The researchers analysed the end of the greenhouse Earth period around 34 million years ago, the end of the Younger Dryas, the desertification of North Africa roughly 5000 years ago, the Bølling-Alleröd transition and the end of four glacial periods. All these climate shifts were preceded by a slow-down in climate fluctuations, although the strength of the effect varied. The end of the greenhouse Earth, the end of the Younger Dryas and the end of one of the glacial periods showed a particularly marked slowdown.

According to Scheffer, before this work tipping points had been hypothesized, but were not so easy to prove. "Speculations were based on the observation of sharp shifts and on the existence of mechanisms of positive feedback," he explained. "However, none of those by itself implies tipping points, so the research community was pretty divided on this issue."

A positive feedback mechanism helps create a tipping point by exaggerating the effect of a change. For example, in the Arctic if higher temperatures melt more sea ice that leads to a decrease in the surface reflectivity (albedo) of the Earth’s surface, causing the absorption of more energy from the sun and further warming.

"So far, support for the idea that tipping points can be the explanation for dramatic climatic shifts in the past has been based on models of specific mechanisms," write the researchers in their paper in PNAS. "Although compelling cases have been built, there is always considerable uncertainty as it is simply very difficult to prove what has been the mechanism behind such events in the far past."

In contrast, a pre-tipping point slowdown does not point to any specific mechanism but is instead a universal property of systems approaching a tipping point. "Therefore it represents an independent line of evidence, complementing model-based approaches suggesting that tipping points exist in the climate system," say the researchers.

Scheffer believes it would be unlikely that we don’t stumble across a climate tipping point in the future. "It is not clear when this will happen," he said. 'However, given that we demonstrated that tipping points in the climate do exist, our large impact on greenhouse gas concentrations, might obviously have significantly larger effects at some point than the ones predicted by the relatively linear predictions of the IPCC [Intergovernmental Panel on Climate Change]."

But predicting our current proximity to a tipping point isn’t that straightforward. The researchers' technique requires long time series that reveal how a system gradually approaches a tipping point, but data for recent anthropogenic climate change is only available for about 150 years or so.

"The time series we have is therefore too short to draw conclusions if it comes to slow aspects of the climate such as ice cap dynamics and ocean current dynamics," said Scheffer. “Nonetheless, we are now moving to analyse faster 'subsystems' such as regional climate systems to see if we can predict events such as persistent droughts that are also thought to have a component of positive feedback causing threshold behaviour."

One limitation of the analysis is that slowing down will only occur if the system is moving gradually towards a threshold. So transitions caused by a sudden large disturbance without a preceding gradual loss of resilience will not be announced by slowing down, say the researchers. "Nonetheless, in view of our current inability to predict potential abrupt climate shifts, having slowing-down as a clue for detecting whether parts of the climate system may be approaching a threshold is a marked step forward in projecting future climatic changes," they add.