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T4
Abrupt transitions in the East Asian Summer Monsoon over the penultimate glacial
cycle: Early warnings and missed alarms
Z. A. Thomas 1 *, F. Kwasniok 2 , P. M. Cox 2 , R. T. Jones 1 , T. M. Lenton 1 , C. S. M. Turney 3
1 College of Life and Environmental Sciences, University of Exeter, Exeter
2 College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter
3 Climate Change Research Unit, University of New South Wales, Sydney, Australia
Abrupt shifts in the East Asian Summer Monsoon (EASM) have been well documented by
palaeo-records throughout China. Speleothem δ 18 O records from Hulu cave (Wang et al.
2001) and Sanbao cave (Wang et al. 2008) in southeastern China demonstrate that longterm
histories of the EASM are dominated by abrupt and large magnitude monsoon shifts,
alternating from periods of high monsoon rainfall to weak monsoon rains. The drivers of
past and future variability are highly uncertain; future abrupt shifts of the EASM would
have immense impacts on the growing human societies that depend upon them. The
potential for detecting early warning signals of such changes would therefore be extremely
valuable.
It has been hypothesised that the EASM exhibits bistability with bifurcation-type tipping
points (Schewe et al. 2012). Here we test this hypothesis by looking for early warning
signals of past EASM bifurcations in speleothem records from Sanbao Cave spanning the
penultimate glacial cycle, a shorter record from Hulu cave, and in multiple simulations of a
model derived from the data.
We find a clear slowing down signal during the Weak Monsoon Interval (WMI) immediately
prior to Termination II in two regionally distinct speleothem records (Sanbao and Hulu).
However, we find no consistent evidence of early warning signals for abrupt monsoonal
shifts between 230 and 150 ka BP in these data. Interpreting these results with our dataderived
model suggests that Northern Hemisphere summer insolation (NHSI) forcing
generally occurs sufficiently fast that the potential well describing the stability of the
current state of the system does not flatten before abrupt transitions, causing ‘missed
alarms’. In marked contrast, the critical slowing down detected at Termination II,
associated with a ~6.5 ka lag after the NHSI maxima, suggests that different forcing
mechanism(s) dominated at the deglaciation.
Our results have significant implications for understanding the future behaviour of the
EASM under enhanced anthropogenic greenhouse gas and aerosol forcing, and question
whether there would be any early warning signals of possible future abrupt monsoon
transitions, especially if they could be caused by relatively rapid anthropogenic forcing.
Keywords tipping points; speleothem; monsoon; early warning signals
Wang, Y. J. et al. A high-resolution absolute-dated late Pleistocene Monsoon record from Hulu Cave,
China. Science (New York, N.Y.) 294, 2345–8 (2001).
Wang, Y. et al. Millennial- and orbital-scale changes in the East Asian monsoon over the past 224,000
years. Nature 451, 1090–3 (2008).
Schewe, J., Levermann, a. & Cheng, H. A critical humidity threshold for monsoon transitions. Climate of
the Past 8, 535–544 (2012).