Past events of abrupt climatic change were preceded by characteristic symptoms
In the Earth’s history, periods of relatively stable climate have often been interrupted by sharp transitions to a contrasting state. For instance, glaciation periods typically ended suddenly, and about 34 million years ago the Earth’s long lasting tropical state in which most life forms evolved, shifted abruptly and irreversibly to a cooler state with ice caps, a shift known as the "Greenhouse - Icehouse Transition". Scientists long suspect that such sharp transitions might be related to tipping points where positive feedback mechanisms lead to self-propelling change. An example of such a mechanism is the ice-albedo feedback.
If ice caps melt, more sunlight is absorbed by the darker surface, causing further warming. Although such mechanisms were known, it was difficult so-far to determine whether these feedbacks might be strong enough to cause tipping points. A team of Dutch and German scientists has now shown that sharp climatic shifts in the past were systematically preceded by subtle alterations in fluctuation patterns that are proven to be characteristic of systems approaching tipping points. These findings imply that the sharp climatic shifts in the past indeed appear to have happened as the Earth system went over critical thresholds where self-catalyzing changed pushed it further towards a contrasting state.
The demonstration of tipping points also has implications for the thinking about current climate change. We know that the stability of the climate over the past 10,000 years is quite exceptional in the light of the turbulent dynamics at longer time scales. The current human induced increase in greenhouse gases may well put an end to this stable period. The well known IPCC predictions are based on the assumption of rather linear change. Although some feedbacks in the Earth system may dampen change, the new results imply that we should also consider the possibility that the climate will cross a tipping point after which changes will be amplified. Whether the climate is now approaching a tipping point is difficult to judge with the new techniques. This is because human influence is simply too fast to generate data records long enough for the detection method.
Publication
Slowing down as an early warning signal for abrupt climate change.
Vasilis Dakos, Marten Scheffer, Egbert H. van Nes, Victor Brovkin, Vladimir Petoukhov, and Hermann Held. PNAS early online publication, September 2008