Millennial scale feedbacks determine the shape and rapidity of glacial termination

Millennial-scale climate variability of the Asian summer monsoon over the last 690,000 years: insights from cave records

The Asian summer monsoon (ASM) is an important component of the global climate system. Cave oxygen isotope (δ18O) records from the region have well characterized the ASM millennial-scale climate variability (MCV) over the last 640 ka, with especially detailed insights for the most recent 60 ka, but little is known about ASM variability beyond the U-Th dating limit of ∼640 ka. Furthermore, questions remain regarding the climatic significance of millennial-scale ASM variability recorded among various climate archives, particularly in the context of the "orbital-scale paradox". Here, we present high-resolution and U-Pb dated cave δ18O records from two Chinese caves spanning 690-600 ka BP (before present, where present = 1950 CE). These records reveal coupling between millennial-scale ASM weakening, North Atlantic cooling, and Antarctic warming, essentially mirroring the pattern observed during the last 640 ka, despite a potential change in the Atlantic meridional overturning circulation (AMOC) caused by a switch in the freshwater route to the North Atlantic at ∼640 ka BP. Comparisons of MCV amplitudes among different ASM proxies show remarkable disparities, suggesting that each proxy reflects different aspects of the ASM. In our records, declines in summer insolation repeatedly triggered millennial-scale weak ASM events near the mid-precession band, associated with AMOC weakening, rather than only at interglacial terminations. Additionally, our analysis highlights the critical roles of atmospheric CO2 and global ice volume conditions in shaping the ASM variability during ∼688-635 ka BP.

Simulated millennial-scale climate variability driven by a convection-advection oscillator

The last glacial period, between around 115 and 12 thousand years before present, exhibited strong millennial-scale climate variability. This includes abrupt transitions between cold and warm climates, known as Dansgaard-Oeschger (D-O) cycles. D-O cycles have been linked to switches in dynamical regimes of the Atlantic Overturning Meridional Circulation (AMOC), but the exact mechanisms behind abrupt climate changes and AMOC regime shifts remain poorly understood. This paper introduces the convection-advection oscillator mechanism to explain the millennial-scale oscillations observed in a set of HadCM3 general circulation model simulations forced with snapshots of deglacial meltwater history. The oscillator can be separated into two components acting on different time scales. The fast convection component responds to changes in vertical stratification in the North Atlantic by activating or deactivating deep water formation sites. The slow advection component regulates the accumulation and depletion of salinity in the North Atlantic. This oscillator mechanism is triggered under specific background conditions and freshwater release patterns. The freshwater perturbation causes an instability that triggers a global salt reorganisation, modifying the North Atlantic stratification. For a given forcing pattern, the system oscillates if the salt transport can lead to an alternating reactivation and deactivation of the AMOC. Otherwise, the climate settles in a warm or cold steady state. This mechanism expands existing theories of millennial-scale variability and provides a general framework for understanding abrupt climate change in general circulation models.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00382-025-07630-x.

Distinct roles for precession, obliquity, and eccentricity in Pleistocene 100-kyr glacial cycles

Identifying the specific roles of precession, obliquity, and eccentricity in glacial-interglacial transitions is hindered by imprecise age control. We circumvent this problem by focusing on the morphology of deglaciation and inception, which we show depends strongly on the relative phasing of precession versus obliquity. We demonstrate that although both parameters are important, precession has more influence on deglacial onset, whereas obliquity is more important for the attainment of peak interglacial conditions and glacial inception. We find that the set of precession peaks (minima) responsible for terminations since 0.9 million years ago is a subset of those peaks that begin (i.e., the precession parameter starts decreasing) while obliquity is increasing. Specifically, termination occurs with the first of these candidate peaks to occur after each eccentricity minimum. Thus, the gross morphology of 100-thousand-year (100-kyr) glacial cycles appears largely deterministic.

Global and regional temperature change over the past 4.5 million years

Much of our understanding of Cenozoic climate is based on the record of δ18O measured in benthic foraminifera. However, this measurement reflects a combined signal of global temperature and sea level, thus preventing a clear understanding of the interactions and feedbacks of the climate system in causing global temperature change. Our new reconstruction of temperature change over the past 4.5 million years includes two phases of long-term cooling, with the second phase of accelerated cooling during the Middle Pleistocene Transition (1.5 to 0.9 million years ago) being accompanied by a transition from dominant 41,000-year low-amplitude periodicity to dominant 100,000-year high-amplitude periodicity. Changes in the rates of long-term cooling and variability are consistent with changes in the carbon cycle driven initially by geologic processes, followed by additional changes in the Southern Ocean carbon cycle.

Multi-proxy constraints on Atlantic circulation dynamics since the last ice age

Uncertainties persist in the understanding of the Atlantic meridional overturning circulation and its response to external perturbations such as freshwater or radiative forcing. Abrupt reduction of the Atlantic circulation is considered a climate tipping point that may have been crossed when Earth's climate was propelled out of the last ice age. However, the evolution of the circulation since the Last Glacial Maximum (22-18 thousand years ago) remains insufficiently constrained due to model and proxy limitations. Here we leverage information from both a compilation of proxy records that track various aspects of the circulation and climate model simulations to constrain the Atlantic circulation over the past 20,000 years. We find a coherent picture of a shallow and weak Atlantic overturning circulation during the Last Glacial Maximum that reconciles apparently conflicting proxy evidence. Model-data comparison of the last deglaciation-starting from this new, multiple constrained glacial state-indicates a muted response during Heinrich Stadial 1 and that water mass geometry did not fully adjust to the strong reduction in overturning circulation during the comparably short Younger Dryas period. This demonstrates that the relationship between freshwater forcing and Atlantic overturning strength is strongly dependent on the climatic and oceanic background state.

Subglacial precipitates record Antarctic ice sheet response to late Pleistocene millennial climate cycles

Ice cores and offshore sedimentary records demonstrate enhanced ice loss along Antarctic coastal margins during millennial-scale warm intervals within the last glacial termination. However, the distal location and short temporal coverage of these records leads to uncertainty in both the spatial footprint of ice loss, and whether millennial-scale ice response occurs outside of glacial terminations. Here we present a >100kyr archive of periodic transitions in subglacial precipitate mineralogy that are synchronous with Late Pleistocene millennial-scale climate cycles. Geochemical and geochronologic data provide evidence for opal formation during cold periods via cryoconcentration of subglacial brine, and calcite formation during warm periods through the addition of subglacial meltwater originating from the ice sheet interior. These freeze-flush cycles represent cyclic changes in subglacial hydrologic-connectivity driven by ice sheet velocity fluctuations. Our findings imply that oscillating Southern Ocean temperatures drive a dynamic response in the Antarctic ice sheet on millennial timescales, regardless of the background climate state.

Piccione et al find evidence for Antarctic ice sheet instability driven by millennial cycles in Southern Ocean temperature, providing clues for the mechanisms that link climate change and rapid Antarctic ice loss events.

Rapid northern hemisphere ice sheet melting during the penultimate deglaciation

The rate and consequences of future high latitude ice sheet retreat remain a major concern given ongoing anthropogenic warming. Here, new precisely dated stalagmite data from NW Iberia provide the first direct, high-resolution records of periods of rapid melting of Northern Hemisphere ice sheets during the penultimate deglaciation. These records reveal the penultimate deglaciation initiated with rapid century-scale meltwater pulses which subsequently trigger abrupt coolings of air temperature in NW Iberia consistent with freshwater-induced AMOC slowdowns. 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.

Stalagmites from NW Iberia record the rapid demise of large ice sheets during the penultimate deglaciation, and reveal decadal-scale feedbacks between warming and ice melting.

Persistent influence of precession on northern ice sheet variability since the early Pleistocene

Prior to ~1 million years ago (Ma), variations in global ice volume were dominated by changes in obliquity; however, the role of precession remains unresolved. Using a record of North Atlantic ice rafting spanning the past 1.7 million years, we find that the onset of ice rafting within a given glacial cycle (reflecting ice sheet expansion) consistently occurred during times of decreasing obliquity whereas mass ice wasting (ablation) events were consistently tied to minima in precession. Furthermore, our results suggest that the ubiquitous association between precession-driven mass wasting events and glacial termination is a distinct feature of the mid to late Pleistocene. Before then (increasing), obliquity alone was sufficient to end a glacial cycle, before losing its dominant grip on deglaciation with the southward extension of Northern Hemisphere ice sheets since ~1 Ma.