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Past tipping points where the climate changed abruptly

Measurements of climatic parameters, such as temperature, often show ups and downs and it is difficult to tell if these variations mark significant climate changes or whether they reflect normal ‘noise’ in the system. A team of statisticians has now developed a method to test whether abrupt changes in climate data from the past reflect times when environmental tipping points have been reached.

The researchers from CNRS (France), UCLA, and Columbia University used an augmented Kolmogorov-Smirnov (KS) test that has been successfully applied to data from other, inherently noisy systems, such as finance and signal processing.

The method compares two samples taken before and after the potential transition point to test whether they come from the same continuous distribution. If not, the transition point is identified as a significant, abrupt change that is indicative of a true climatic shift.

They used data about changes in the ratios of two isotopes of oxygen (oxygen-16 and oxygen-18) from Greenland ice cores, as well as data from mineral isotopes found in speleothems (cave mineral deposits such as stalactites and stalagmites). Since the proportion of oxygen-16 and oxygen-18 in ice change depending on temperature, the scientists were able to map the changes in the ice cores to changes in climate. In a similar way, changes in mineral isotopes in speleothems also reflect changes in temperature.

In the past, such matching was done by visual inspection, which could be very time-consuming, and tended to be subjective. The statistical method used in the case of this study is less prone to errors and does not rely on human judgement to decide whether a jump in the recorded parameter (for example, oxygen isotope proportions) marks a significant climatic transition or not. 

“We applied our method to two paleoclimate records of the last climate cycle, a Greenland ice core and a speleothem composite record from China,” said author Witold Bagniewski.

The researchers found that the statistical analysis identified changes in the oxygen isotope proportions in ice cores that coincided with the start or end of periods of cooler climate in Greenland’s past. 

“Many of the abrupt transitions in the Greenland ice core record correspond to shifts between a warmer climate, known as Greenland Interstadials (GIs), and a colder climate, the Greenland Stadials (GSs),” said Bagniewski. 

The existence of these two climate states, GI and GS, is an example of a bistable climate system, in which two distinct states are both stable. The climate may jump abruptly from one to the other after crossing a tipping point.

The authors of the study state that their statistical method correctly detected abrupt changes in climate in both sets of data. They hypothesize that these abrupt and significant transitions may have been caused by the climate parameters crossing a tipping point – where change has reached a point at which the entire system can no longer maintain a stable state and thus enters a period of dramatic change. 

“Our methodology is very effective in correctly detecting abrupt transitions in climate records,” said Bagniewski. “Its wider application may help reconstruct the chronology of Earth’s climatic events.”

In addition to helping identify these events in the climatic history of the Earth, this new methodology will also enhance our understanding of the potential consequences of the tipping points we may encounter in the near future, as the warming climate leads to instability in the Earth’s physical systems and ecosystems.

The study is published by the American Institute of Physics in Chaos: An Interdisciplinary Journal of Nonlinear Science.

By Alison Bosman, Staff Writer

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