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. About 34 million years ago the Earth’s long lasting tropical state in which most recent life forms evolved, shifted abruptly and irreversibly to a cooler state with ice caps.
This shift is 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 that is exposed. This causes further warming. Although such mechanisms are well known, it was difficult so far to determine whether these feedbacks were strong enough to cause tipping points.
A team of Dutch and German scientists around Marten Scheffer from Wageningen University and Hermann Held from the Potsdam Institute for Climate Impact Research has now analyzed the geological records of eight ancient events of abrupt climate change. These are the end of the greenhouse Earth, the end of the Younger Dryas, the Bølling-Alleröd-Transition, the desertification of North Africa and the ends of four glaciation periods.
In an article in the current online edition of the Proceedings of the National Academy of Sciences the researcher now report that sharp climatic shifts in the past were systematically preceded by subtle alterations in fluctuation patterns. These alterations are proven to be characteristic of systems approaching tipping points. This finding supports the theory that the sharp climatic shifts in the past have happened as the Earth system went over critical thresholds where self-catalyzing change pushed it further towards a contrasting state.
The demonstration of tipping points also has implications for the thinking about current climate change, the authors state. The well known projections by the Intergovernmental Panel on Climate Change 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 climate as a whole 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. However, for certain parts of the climate system the method may be readily applicable to predict future abrupt change.
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