Background
Dansgaard–Oeschger (DO) warm events saw high-latitude Northern Hemisphere temperatures rise by 5–16 °C, closely linked to strengthening of the Atlantic Meridional Overturning Circulation (AMOC), and are widely used as an analogue for future AMOC change. Existing studies suggest AMOC strengthening during DO events occurred within decades, but this rests on ice-core δ18O records, a signal influenced by both temperature and moisture source. Speleothem δ18O from India and southwest China responds directly to AMOC-driven changes, offering an independent test – but existing records lacked the chronology and annual-layer constraints needed to quantify transition rates.
New results
Two annually laminated speleothems from Cherrapunji Cave (northeast India) and Shuixi Cave (southwest China), together with published records from Zhangjia Cave, yielded high-resolution δ18O series spanning DO-1 to DO-8. The transitions consistently lasted longer than 100 years, averaging about 180 years – far longer than the decadal-scale shifts in Greenland ice-core δ18O records.
"The speleothems used in this study have clear annual laminae, which is essential," said Dr Xiyu Dong, first author. "U–Th dating gives us precise absolute ages, but it cannot give reliable relative ages. The annual laminae let us precisely constrain how long each transition lasted, and compare that directly with the Greenland ice-core records."
Modelling reveals a two-stage process: the retreat of sea ice in the northeastern North Atlantic triggered a rapid adjustment of regional atmospheric circulation from stadial to interstadial conditions, causing the sharp rise in Greenland δ18O. Subsequently, the AMOC strengthened further and sea ice in the northwestern North Atlantic continued to retreat, but atmospheric circulation modes and moisture transport pathways did not change significantly, so Greenland δ18O showed no further marked fluctuation. The simulations show that Asian monsoon speleothem δ18O records capture the full AMOC strengthening process.
"This work clarifies the relationship between Asian speleothem δ18O, Greenland ice-core δ18O and the AMOC, which is an important step toward understanding abrupt climate change", said prof. Hai Cheng.
"This work shows that natural AMOC transitions were not as fast as researchers previously referred from ice-core records. Therefore, when we talk to the public about the risk of AMOC tipping in the future, we need to be clearer about how long that transition would take. That matters just as much as the climate impacts it could cause", Xiyu Dong added.
"The study demonstrates the value of combining high-quality paleoclimate records with process-based modelling: annually laminated speleothem records provide essential observational constraints for quantifying the rates of abrupt climate transitions, while isotope-enabled Earth system models provide the theoretical framework for interpreting the underlying dynamics", Xu Zhang said.
The authors also emphasize that more high-quality speleothem records of DO events, reconstructions of North Atlantic sea ice, and climate models that integrate ice-sheet processes will be essential for further understanding abrupt climate change.
