Résumé
There is a great interest concerning recent occurrences of tipping points in the climate system and great concern about those that could occur in the near future as a result of anthropogenic forcing. A lot of attention has been devoted to the study of past Dansgaard-Oeschger events, abrupt warmings of about 12°C on a time-scale of about 50 yrs that occurred during the last glacial period. Great effort is also dedicated to understanding the Atlantic Meridionnal overturning circulation and the Amazon forest dieback, which are already entering an unstable regime leading to tipping behavior.
Instead, here we focus on the study of critical transitions in the SE Asian Monsoon that have occurred in the past 3.6 Myrs by a novel combination of advanced statistical tools (KS-test, recurrence quantification analysis). The SE Asian Monsoon is characterized by variations in the grain size with the occurrence of coarse material characterizing a strong winter monsoon mechanism with grains transported from the Chinese northern deserts by strong winds generated by the Siberian High located northward. By contrast intervals of fine grain size characterized periods during which the summer monsoon was rather reinforced. We have analyzed high-resolution grain-size datasets derived from Chinese loess sequences, i.e. the CHILOMOS and the LGS640 datasets, that we compare with the Chinese composite speleothem d18O records that arguably provide one the best representation of the Earth’s climate in the last 650 kyrs. Although visually observed rapid grain-size variations were previously interpreted as representing millennial-scale variations, our statistical analysis shows that both winter and summer monsoons co-varied at glacial-interglacial to millennial timescales. Analyzing a third dataset, i.e., MQSG, our statistical analysis shows that both winter and summer monsoon variations reflect a three-stage evolution of increasing intensity: (1) from 3.6 Ma to 2.6 Ma, (2) from 2.6 Ma to 1.2 Ma, and (3) from 1.2 Ma to present, with the winter monsoon strength increasing over these3 main steps.
List of tables
Table 1 KS test of the NGRIP and Hulu cave d18O for the last climate cycle. Comparison of the dates of abrupt warmings/moistening (left) and cooling/drying transitions (right). Labels for NGRIP according to Rasmussen et al. (2014) and for Hulu cave according to Wang et al. (2008).
Table 2 KS test of the Chinese speleothem d18O and of the CHILOMOS grain-size composite for the last two climate cycles. Comparison of the dates of abrupt moistening (left) and drying transitions (right). Labels for the moistening transitions in the Chinese speleothem are from Wang et al. (2008) and in CHILOMOS from Yang and Ding (2014). The labels of the drying in CHILOMOS are from the present study
Table 3 RQA of the Chinese speleothem d18O and of the CHILOMOS grain-size composite for the last two climate cycles. Dates of the minima are identified by the RR prominence, shown together with the equivalent transitions detected by the KS method. For easier reading, the dates have been re-ordered from younger to older. The most significant minima are highlighted in yellow. The original ranking is given in Suppl. Tab 1.
Table 4 KS test of the Chinese speleothem d18O and of the LGS640 dataset for the last 640 Myrs. Comparison of the dates of abrupt moistening and drying transitions in both records. The dates found in both records are highlighted in red and in blue for drying or moistening events respectively.
Table 5 RQA of the LGS640 grain-size composite for the last seven climate cycles. Dates of the minima are identified by the RR prominence. The most significant minima (RR prominence >0.5) are highlighted in yellow.
Table 6 KS test and RQA of MGSQ grain dataset for the last 3.6 Myrs. In this analysis the moistening and drying transitions is labeled as warming and cooling. On the left, KS results with the corresponding marine isotope stage (MIS) boundaries. On the right, RQA results with minima ordered according their prominence value. Dates with RR>0.6 are highlighted in yellow.
Table Supp.1. Abrupt transitions over the past 130 kyrs BP from the NGRIP, Chinese Speleothem and CHILOMOS records. Identification of the common abrupt warmings or moistenings on the left, and abrupt coolings or dryings on the right. Differences between the highest and lowest transition dates. Indication of the NGRIP and Chinese interstadials and stadials (GI-GS and A-SA respectively).
Table Supp.2. RQA of the 250 kyrs Chinese speleothem and CHILOMOS records ranked according the RR prominence, the chronology. Indication of the time difference between the identified transitions.
Table Supp.3. Comparison of the KS-test results from the LGS 640 and the Chinese speleothem over the past 650 kyrs. Indication of the Marine isotope stratigraphy and the number of cool and warm transitions and the percentage of drying events per climate cycle