The development and distribution of modern arid subtropical regions (drylands) worldwide are thought to have been brought about by global cooling marking the dawn of the Quaternary (~2.7 Ma). This coincides with global shifts in ocean circulation patterns, the intensification of the Walker-Hadley circulation, declining atmospheric carbon dioxide, the initiation of glacial–interglacial cycles and the intensification of Northern Hemisphere glaciation, the combination of which has lead to the current global distribution of non-polar deserts at ~30° latitude. Today, drylands represent Earth’s largest terrestrial biome, covering ~46% of global land surfaces and supporting a global population of ~3 billion people. Climate models suggest that future warming, caused by anthropogenic greenhouse gas emissions, is predicted to modify the global distribution of rainfall, resulting in further drying and expansion of drylands. This is contradictory to the geological, suggesting warmer global temperatures coincide with humid periods in the Sahara-Arabian desert arid zone, for example, the African Humid Period (15-5 ka).

Speleothems preserved in arid-zone caves are particularly useful archives for accurately and precisely identifying past humid periods, as they require a minimum of 300 mm precipitation, pedogenesis and vegetation cover to form. Here we present new data from speleothems during the Pleistocene from the Arabian hyperarid zone. Speleothem evidence suggests that during the Pleistocene, the Sahara-Arabian desert has experienced numerous intermittent humid phases, typically occurring with periods of low global ice-volume and warmer global temperatures. This is together with abundant archeological evidence of human settlements during humid phases. We further explore tropical push-pull mechanisms driving heating of the deep tropics and subsequent expansion of the tropical zone and synchronicity of humid phases regionally. These results have significant implications for understanding the drivers of dryland aridity in non-polar deserts globally.

The interpretation of the iconic Chinese speleothem d¹⁸O records remains debated, in part because the records contain muted glacial-interglacial variability, distinctly different from the majority of other proxy records from the Asian monsoon region. Here, we extended a speleothem record from central Myanmar, now covering a large portion of the past 40,000 years. When comparing with other cave records from locations along the trajectory of the Indian summer monsoon, our record from central Myanmar confirms our previous observations, that is, a larger d¹⁸O gradient along the moisture trajectory during the glacial time relative to today. We also performed triple oxygen isotope (¹⁶O-¹⁷O-¹⁸O) analysis on the speleothems. We found that ¹⁷O-excess (or ∆¹⁷O) of monsoon precipitation during the last glacial maximum (LGM) is ~26 ± 2 per meg, substantially higher than the value of ~15 ± 5 per meg in modern times. The decrease of 10 per meg in ∆¹⁷O from the LGM to Holocene probably indicates an increase in relative humidity (RH) of monsoon moisture. Thus, during the glacial time, there existed a significant drop in RH, corresponding to a stronger continental re-evaporation and possibly suppressed plant transpiration. Both our speleothem d¹⁸O and ∆¹⁷O results therefore support our hypothesised mechanism of the moisture transport pathway effect in addressing the d¹⁸O variability in Chinese speleothem records.

Asian monsoon speleothem carbonate δ¹⁸O responds strongly to massive iceberg discharge events (Heinrich events: HE) in the North Atlantic, which were associated with a slow down or full collapse of the Atlantic Meridional Overturning Circulation. Thus far, the climate mechanism that links the AMOC strength to speleothem carbonate δ¹⁸O is not fully understood, because speleothem carbonate δ¹⁸O is controlled by parent water δ¹⁸O, cave air temperature and potential kinetic isotope effects. We use a unique approach by combining three novel proxies (fluid inclusion isotopes, TEX86 and dual clumped isotopes) to quantitatively deconvolve the speleothem carbonate δ¹⁸O signal from southwest Chinese speleothems that cover the Penultimate deglaciation (i.e. Termination II) and multiple North Atlantic meltwater pulses.

The record (145-125 ka) consists of two speleothems that partly overlap in time, and all proxies replicate well. Fluid inclusion isotopes plot on or close to the meteoric water line, suggesting that it is representative for the isotope composition of fossil rainfall, whereas our dual clumped isotope data (D47, D48)show that the speleothem calcite precipitated indistinguishable from oxygen isotope equilibrium. TEX86 temperature data and dual clumped temperature data from HE11 and MIS5 are within uncertainty.

TEX86 temperature data follows the same trend as carbonate δ¹⁸O, with temperatures varying between 15.8°C during HE11 and 20.4°C during MIS5e. Cave air temperature thus accounts for 1 permille δ¹⁸O variability. The carbonate δ¹⁸O shows an amplitude of about 7 permille across this time interval, so 6 permille can be attributed to parent water δ¹⁸O changes. Interestingly, the fluid inclusion isotopes only show about 4-5 permille variation, and do not show the same trend as cave air temperature or carbonate δ¹⁸O. We suggest that this is related to a bias towards fluid inclusion rich summer layers in the speleothems, whereas carbonate δ¹⁸O represents an annual weighted mean. The fluid inclusion isotope composition is thus interpreted as an Asian summer monsoon intensity signal.

On millennial timescales the Penultimate deglaciation is associated with several smaller meltwater pulses and a large one peaking at 133 ka. The carbonate δ¹⁸O and TEX86 temperatures show a response to all of these, but the fluid inclusion isotope data only increased dramatically at the peak of the large meltwater pulse during HE11. Our deconvolution of the carbonate δ¹⁸O signal, thus suggests that the Asian summer monsoon intensity responds less sensitive to meltwater pulses compared to cave air temperature and carbonate δ¹⁸O.

The intertropical convergence zone (ITCZ) plays a key role in regulating tropical hydroclimate and global water cycle through its latitudinal movement, contraction-expansion, and rainfall intensity change. The long-term variability of the ITCZ, for instance, its changes in strength, position, and width, along with the corresponding driving mechanisms, however, remains ambiguous. Here we incorporate a new speleothem oxygen isotope (δ¹⁸O) record from southwestern Sulawesi, Indonesia with existing speleothem δ¹⁸O records from the Maritime Continent. The spatial distribution of the absolute speleothem δ¹⁸O allows us to constrain the latitudinal movements of the mean ITCZ within 3°during the past 30,000 years. Moreover, using the δ¹⁸O gradients between inside and outside of the mean ITCZ and between central and marginal ITCZ, we are able to separately reconstruct the latitudinal movements and width changes of the ITCZ. We find that the ITCZ gets more intensified and also expanded during the northward migration from the LGM to the Holocene, in contrast with the "deep tropics squeeze“ observed during the present day. We also find intensified ITCZ and El Niño Southern Oscillation (ENSO) coupling during the Holocene, which likely also persist and bring profound tropical hydroclimate changes in the future.

Speleothems from coastal caves in NW Iberia provide very high resolution records of the interplay of rapid meltwater addition and regional cooling accompanying instability in Northern Hemisphere ice sheets. Through a comparison of marine and speleothem records on independent chronology spanning TI, we find that the δ¹⁸O of NW Iberia speleothem archive (NISA) is a robust recorder of the δ¹⁸Oseawater of the surface eastern North Atlantic, the main moisture source for precipitation in this region. Additionally, the δ¹³C_NISA is highly correlated with marine SST records because temperature is a dominant control over soil pCO₂ and therefore dripwater δ¹³C. We show how these proxies can be used to provide direct, high-resolution records of periods of rapid melting of Northern Hemisphere ice sheets during the penultimate deglaciation, including centennial scale meltwater pulses whose duration is constrained by counting of annual fluorescent bands. At the multidecadal to centennial scale, we assess the relationship between freshening events and antecedent temperature change, as well as the temperature feedbacks to century-scale freshening events consistent with AMOC slowdown. Across TII, we find the first of these AMOC slowdowns, 600 year duration, was shorter than Heinrich 1 of the last deglaciation. Although similar insolation forcing initiated the last two deglaciations, the more rapid and sustained rate of freshening in the eastern North Atlantic penultimate deglaciation likely reflects a larger volume of ice stored in the marine-based Eurasian Ice sheet during the penultimate glacial in contrast to the land-based ice sheet on North America as during the last glacial. Across long term orbital timescales, we illustrate the potential to deconvolve vegetation/soil drivers of speleothem δ¹³C from in cave degassing and prior calcite precipitation effects, to attain more robust and reproducible estimates of temperature evolution.