| The Ice Age hypothesis of Milankovitch suggests that the glacial-interglacial cycles are mainly forced by insolation.Thus,the intervals characterized by similar insolation conditions to the current interglacial may be used to infer the timing of the next "natural”glacial inception.There has been extensively debate about how appropriate MIS 11(Marine Isotope Stage 11;ca 420-360ka)is as an analogue for the Holocene(MIS 1)and the near future,despite the obvious similarities in insolation characteristics between the two interglacials.The different alignment strategies in the literature in terms of the similar insolation,the obliquity signal and regional climates have led to different interpretations on the future glacial inception.Cave records may have the potential to provide a new alignment strategy between MIS11 and MIS1.Here,we present stalagmite ?18O and ?13C profiles from Luoshui Cave(31°35'N,111°14'E),Hubei Province,where a subtropical monsoon climate domains.Precisely 230Th dated high-resolution cave records may bring new insights into orbital-and millennial-scale climate changes during MIS 11.Age models of stalagmites are based on linear interpolation,and some are further modified by tuning to records from Sanbao Cave.The records cover MIS 12 to 10 and MIS 4 to 1,which provide a good opportunity to compare climate changes between MIS 11 and MIS 1 based on records from the same cave.Cave monitoring may provide an important reference to interpret cave records from the monsoon region of China.3-year monitoring data from Yongxing Cave(?300 km away from Luoshui Cave)suggest that stalagmite ?18O values from the monsoon regions of China mainly inherit ?18O signals of summer rainwater,which are inturn affected by the Asian monsoon(AM)circulation over broad regions.Thus,the stalagmite ?18O can be used as a proxy of the AM intensity.At interannual-decadal scales,the stalagmite ?18O is primarily controlled by El Nino-Southern Oscillation(ENSO).The stalagmite ?13C from Luoshui Cave is a proxy reflecting biogenic CO2 supply from the soil,which is intum influenced by biomass at the site.The ?13C records presented here indicate that the orbital-millennial changes of biomass around Luoshui Cave are primary controlled by temperature in the Nothern hemisphere.These results provide a foundation for interpreting ?18O and ?13C records from Luoshui Cave.Our records show a sequence of millennial-scale abrupt AM changes from MIS 12 to 10.We can identify 4 weak AM events during MIS 12 and 7 weak AM events from MIS 11 to 10.Within dating uncertainty,these events are individually correlated to low-temperature intervals evidenced by marine sediment records from North Atlantic,suggesting the strong coupling of millennial-scale changes between the AM and the North Atlantic climates.We can also identify equivalents of these events in the Antarctic ice records based on the bipolar seesaw mechanism and the dynamic linkage between the AM and the Southern Hemisphere climates.The strongly coupled millennial-scale changes between the AM and the polar region climates imply that stalagmite archives can be used as tools to synchronize the records from hemispheres.In addition,our data suggest that the millennial-scale variability during the last glacial is likely reproducible from MIS 11 to 10 with a similar frequency.Compared to the last deglacial(Termination-I[T-I]),the B A-YD(B(?)lling/Aller(?)d-Younger Dryas)sequence cannot be observed during T-V.However,similar orbital-scale changes are observed in?13C records during MIS 11c and MIS 1,which may bring new insights into the comparison between the two interglacials.Whether the late-Holocene CO2 increase,reaching?280 ppm before the industrial revolution,was natural in origin seems to be the key to predict the natural length of current interglacial.Simulations suggest that under 280 ppm of CO2 current interglacial will last for 50 thousand years even in the absence of human perturbations.However,if the "early anthropogenic hypothesis" is right,the CO2 concentration would be only?240 ppm before the industrial revolution.In this case,the end of the current interglacial would have occurred or will occur within the next several thousand years.For MIS 11c,its end occurred under?280 ppm of CO2,which may imply that CO2 is not a factor determining the natural length of the current interglacial.The persistence of interglacial conditions(e.g.,MIS 11c)over two insolation peaks is caused by the special phasing of precession and obliquity.During MIS 11c,obliquity is out-of-phase with insolation,and strengthened meridional overturning circulation inhibits significant ice-sheet build-up after the first insolation peak and prolongs interglacial conditions.Because similar phasing of precession and obliquity to MIS 11c is not observed during current interglacial,MIS 1 will only cover a single insolation peak in the absence of human perturbations.In addition,each of interglacials during the past 800 ka is always shorter than an obliquity cycle,and glacial inception always takes place when obliquity is decreasing.We are currently at a decreasing limb of obliquity curve,and thus the current interglacial will end within the next 10 ka without human perturbations based on the empirical evidence.The bipolar seesaw of millennial-scale changes is a characteristic feature of a glacial condition,and thus the first major reactivation of the bipolar seesaw would probably constitute an indication that the transition to a glacial state had already taken place.After MIS 11c,the first weak monsoon event,occurred at?392 ka,has a similar amplitude(?2‰)to Heinrich events during the last glacial,which are typical bipolar seesaw events.In addition,the amplitude of its counterpart in the Antarctic ice cores is also similar to that of Antarctic warming events in the last glacial.Therefore,this weak monsoon event can be seen as the first bipolar seesaw event after MIS 11c.At the last glacial inception,sea level started falling?3 ka before the onset of significant bipolar-seesaw variability.Thus,we can determine that glacial inception after MIS11c occurred at?395 ka.Northern Hemisphere summer insolation(NHSI)is the pacemaker of millennial-scale events in a glacial state,and the increasing insolation at 65°N in July always causes(a)millennial-scale event(s).The insolation over MIS 11 and 10 shows a strong linear correlation with the insolation signal over the recent past and the future.Thus,the first major reactivation of the bipolar seesaw during MIS 11 may imply timing of glacial inception in the future.Based on character of NHSI,millennial-scale events and regional climates,we proposed a new alignment strategy between MIS 11 and MIS 1.This alignment strategy suggests that the end of the current interglacial would occur at?3 ka in the future without human perturbations. |
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