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

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.

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.

Mid-Pleistocene climate transition triggered by Antarctic Ice Sheet growth

Despite extensive investigation, the nature and causes of the Mid-Pleistocene Transition remain enigmatic. In this work, we assess its linkage to asynchronous development of bipolar ice sheets by synthesizing Pleistocene mid- to high-latitude proxy records linked to hemispheric ice sheet evolution. Our results indicate substantial growth of the Antarctic Ice Sheets (AISs) at 2.0 to 1.25 million years ago, preceding the rapid expansion of Northern Hemisphere Ice Sheets after ~1.25 million years ago. Proxy-model comparisons suggest that AIS and associated Southern Ocean sea ice expansion can induce northern high-latitude cooling and enhanced moisture transport to the Northern Hemisphere, thus triggering the Mid-Pleistocene Transition. The dynamic processes involved are crucial for assessing modern global warming that is already inducing asynchronous bipolar melting of ice sheets.

Extreme glacial cooling likely led to hominin depopulation of Europe in the Early Pleistocene

The oldest known hominin remains in Europe [~1.5 to ~1.1 million years ago (Ma)] have been recovered from Iberia, where paleoenvironmental reconstructions have indicated warm and wet interglacials and mild glacials, supporting the view that once established, hominin populations persisted continuously. We report analyses of marine and terrestrial proxies from a deep-sea core on the Portugese margin that show the presence of pronounced millennial-scale climate variability during a glacial period ~1.154 to ~1.123 Ma, culminating in a terminal stadial cooling comparable to the most extreme events of the last 400,000 years. Climate envelope-model simulations reveal a drastic decrease in early hominin habitat suitability around the Mediterranean during the terminal stadial. We suggest that these extreme conditions led to the depopulation of Europe, perhaps lasting for several successive glacial-interglacial cycles.

Milankovitch theory and monsoon

The widely accepted “Milankovitch theory” explains insolation-induced waxing and waning of the ice sheets and their effect on the global climate on orbital timescales. In the past half century, however, the theory has often come under scrutiny, especially regarding its “100-ka problem.” Another drawback, but the one that has received less attention, is the “monsoon problem,” which pertains to the exclusion of monsoon dynamics in classic Milankovitch theory even though the monsoon prevails over the vast low-latitude (∼30° N to ∼30° S) region that covers half of the Earth’s surface and receives the bulk of solar radiation. In this review, we discuss the major issues with the current form of Milankovitch theory and the progress made at the research forefront. We suggest shifting the emphasis from the ultimate outcomes of the ice volume to the causal relationship between changes in northern high-latitude insolation and ice age termination events (or ice sheet melting rate) to help reconcile the classic “100-ka problem.” We discuss the discrepancies associated with the characterization of monsoon dynamics, particularly the so-called “sea-land precession-phase paradox” and the “Chinese 100-ka problem.” We suggest that many of these discrepancies are superficial and can be resolved by applying a holistic “monsoon system science” approach. Finally, we propose blending the conventional Kutzbach orbital monsoon hypothesis, which calls for summer insolation forcing of monsoons, with Milankovitch theory to formulate a combined “Milankovitch-Kutzbach hypothesis” that can potentially explain the dual nature of orbital hydrodynamics of the ice sheet and monsoon systems, as well as their interplays and respective relationships with the northern high-latitude insolation and inter-tropical insolation differential.

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Orbital-scale climate variations of Earth are dictated by ice sheet and monsoon

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Views of “monsoon system science” reinforce the Kutzbach monsoon hypothesis

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A unified Milankovitch-Kutzbach hypothesis better explains the orbital dual nature