Tag: “amount effect”

Ancient Stalagmites Reveal When the Global Monsoon Breaks Its Seesaw

A new 450,000- year speleothem (stalagmite) record from the Peruvian Andes shows that the South American Summer Monsoon (SASM) and Asian monsoon were mostly out of phase (a “seesaw”), but occasionally synchronized, both strengthening or both weakening, especially during glacial terminations and warm intervals.

This research relates to climate change primarily by showing how the global monsoon system has responded to rapid shifts in planetary boundary conditions in the past, conditions that are analogous in some ways to those changing today.

Infographic titled 'Ancient Stalagmites Reveal When the Global Monsoon Breaks Its Seesaw' illustrating isotopic archives of rainfall, atmosphere, and climate connections, featuring a diagram of stalagmite layers, a world map highlighting South America and Asia, and details on monsoon patterns and growth rates over 120,000 years.
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A new 450,000- year speleothem (stalagmite) record from the Peruvian Andes shows that the South American Summer Monsoon (SASM) and Asian monsoon were mostly out of phase (a “seesaw”), but occasionally synchronized, both strengthening or both weakening, especially during glacial terminations and warm intervals.

The study, published in Nature as “Interhemispheric monsoon dynamics over the past 450,000 years” (Olson, E. J. et al., 2026), is based on oxygen -isotope (δ¹⁸O) data from Andean cave stalagmites (including sites such as Huagapo and Pacupahuain in the Junín region of the Amazon Basin and Andes).

These records track monsoon intensity via the “amount effect”, in which heavier rainfall produces lower δ¹⁸O values. Uranium -thorium dating provided precise ages. Researchers combined the cave data with a nearby Lake Junín glacial/sediment record to isolate a regional SASM signal from local effects.

Key findings

  • Dominant pattern (seesaw): For most of the past ~450,000 years (spanning multiple glacial–interglacial cycles), SASM and Asian monsoon strength varied in antiphase. When one strengthened, the other weakened. This matches Milankovitch orbital forcing (especially precession, ~21,000- year cycles), which shifts seasonal solar insolation between hemispheres and supports the idea of a coherent “global monsoon” system.⁠
  • Exceptions (synchrony): About ten intervals exceeded a synchronicity threshold (identified via running Pearson correlations and mean comparisons of the records). These include glacial terminations (abrupt ends of ice ages) and some warm interglacial and interstadial periods. Synchronous drying occurred during terminations and several warm intervals; synchronous wet conditions were rarer (mainly in the penultimate glacial, MIS 6). In these windows the monsoons moved together rather than opposing each other.⁠
  • Drivers of the exceptions: These events coincide with rapidly changing boundary conditions, rising greenhouse- gas levels, shifting cross- equatorial thermal gradients, anomalous Atlantic and Pacific sea- surface temperatures, ice- sheet collapse, and disruptions to ocean circulation (e.g., Atlantic Meridional Overturning Circulation). Model comparisons support links between high- latitude ocean changes and tropical rainfall reorganization.

Broader implications

The global monsoon system supplies freshwater to more than 2.7 billion people. The record shows it is not rigidly locked to orbital cycles alone; it can reorganize for millennia when ice sheets, greenhouse gases, and ocean temperatures change quickly.

Synchronous droughts across hemispheres would strain water availability on both sides of the equator. The findings provide a long- term natural experiment relevant to understanding monsoon responses under rapid modern forcing.⁠

Related coverage and summaries appear in Nature research briefings, Springer Nature Communities (“Behind the Paper”), Scienmag, and similar outlets; the primary paper is in Nature (doi: 10.1038/s41586-026-11166-3). Data are archived at NOAA NCEI.

Topographic map of a mountainous region in South America showing elevation changes, with a highlighted lake labeled 'd' and marked locations 'a', 'b', and 'c'. An inset map indicates the region's location within South America.
Location of caves where speleothems were recovered. Credit: Nature (2026). DOI: 10.1038/s41586-026-11166-3

This research relates to climate change primarily by showing how the global monsoon system has responded to rapid shifts in planetary boundary conditions in the past, conditions that are analogous in some ways to those changing today.

Core relevance

The 450,000- year speleothem record demonstrates that the South American Summer Monsoon (SASM) and Asian monsoon are usually out of phase (a “seesaw” driven largely by orbital- Milankovitch insolation differences).

However, they occasionally synchronize, both strengthening or, more concerningly, both weakening, during intervals of rapid change.

These synchronicity events (about ten identified) cluster around glacial terminations and warm interglacial/interstadial periods.⁠

Key drivers of those exceptions include:

  • Rising greenhouse- gas concentrations
  • Changes in cross- equatorial thermal gradients
  • Anomalous sea- surface temperatures in the Atlantic and Pacific
  • Disruptions to ocean circulation (including Atlantic Meridional Overturning Circulation, or AMOC, collapse or weakening)

In short, when ice sheets, carbon dioxide levels, ocean temperatures, and circulation shift quickly, the global monsoon can reorganize for millennia rather than simply following the slow orbital “clock”. Synchronous drying across both hemispheres would reduce freshwater availability in major monsoon regions at the same time.

Link to present- day and future climate change

  • Greenhouse gases and rapid forcing: Past synchronous events occurred when greenhouse gases rose (though more slowly than today). Modern CO₂ is increasing far faster than during most of the last 450,000 years. The paper notes that the monsoon is not rigidly locked to orbital cycles; it can flip modes when boundary conditions change rapidly, exactly the situation under (anthropogenic) warming.
  • Ocean circulation risks: Model comparisons (including AMOC “hosing” experiments) link high- latitude ocean changes to tropical rainfall reorganization. Ongoing observations and projections of AMOC weakening under climate change therefore raise the possibility of monsoon disruptions that are more simultaneous and less “seesaw- like.”
  • Water security: Monsoons supply freshwater to more than 2.7 billion people. Historical evidence of multi- millennial synchronous droughts underscores the risk that future reorganization could strain water resources across both hemispheres rather than allowing one region’s gains to offset another’s losses.
  • Not a direct prediction: The study is a paleoclimate reconstruction, not a future forecast. Orbital forcing today is different from glacial terminations, and human- driven change includes additional factors (aerosols, land- use change, etc.). Still, it supplies a long- term natural experiment showing the monsoon system’s sensitivity to fast shifts in ice, greenhouse gases, SSTs, and ocean circulation, the same variables that climate models project will continue changing this century.

In essence, the stalagmite record strengthens the scientific case that rapid climate change can push the global monsoon beyond its usual interhemispheric balance, with potentially widespread impacts on rainfall patterns and water availability. It does not claim that today’s warming will automatically produce the exact same synchronicity events, but it shows that such reorganization is physically possible and has occurred under analogous (if slower) past conditions.

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Interhemispheric monsoon dynamics over the past 450,000 years

Abstract (key points)

The global monsoon system supplies freshwater to much of Earth’s biota and human population. Milankovitch orbital variations in solar insolation are expected to drive long- term changes in the monsoon, and internal factors (ice cover, sea- surface temperatures) can also influence it. Southern Hemisphere monsoon records spanning multiple glacial– interglacial cycles have been scarce.

This study presents a 450,000- year record of South American Summer Monsoon (SASM) variability based on speleothem (stalagmite) oxygen- isotope (δ¹⁸O) data from the Amazon Basin (Peruvian Andes caves).

  • SASM variability and the Asian monsoon have been strikingly out of phase for most of this interval (a classic interhemispheric “seesaw”).
  • Occasional synchronous behaviour occurs in regions affected by both monsoons.
  • A synchronicity threshold is exceeded during ten events, including glacial terminations.
  • Comparison with palaeoclimate forcings indicates that greenhouse- gas concentrations, cross- equatorial thermal gradients, and Atlantic and Pacific sea- surface temperatures probably influence these synchronous intervals.

The record shows that the global monsoon can reorganize itself, especially during periods of rapidly changing planetary boundary conditions.

Additional details from the paper and related coverage

  • Data & methods: Speleothem δ¹⁸O records (validated by modern cave monitoring) combined with a nearby Lake Junín glacial record to separate local vs. regional signals. Uranium-thorium dating provides precise chronology. Compared with established Asian monsoon speleothem records (e.g., Cheng et al.).
  • Figures (from the page): Location of caves; assessment of local vs. regional SASM signals; comparisons with other records; synchronicity events vs. climate forcings; Pearson correlations between Andean and Asian records; tropical precipitation maps and modelled response to AMOC collapse.
  • Data & code availability: Data at NOAA NCEI (doi:10.25921/ecnf-7908); analysis code on CodeOcean; related model data on Zenodo.

The full text is behind a paywall/subscription. Summaries and “Behind the Paper” discussions are available via Nature research briefings and Springer Nature Communities.

Journal: Nature, 658, pages 943–950 (2026)

DOI: 10.1038/s41586-026-11166-3

Published: 14 October 2026

Authors: E. J. Olson,
D. P. Gillikin,
D. Parmenter,
S. Burns,
A. Verheyden,
O. Miyawaki,
M. Vuille,
L. F. Piccirillo,
R. Castro,
G. Yurivilca,
R. L. Edwards &
D. T. Rodbell

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