The School of Earth and Atmospheric Sciences Presents Dr. Kaustubh Thirumalai, University of Arizona

Methane, Monsoons, and Milankovitch Modulation of Millennial-scale Climate Variability

Earth's orbital geometry exerts a profound influence on climate by regulating changes in incoming solar radiation. Superimposed on orbitally paced climate change, Pleistocene records reveal substantial millennial-scale variability characterized by abrupt changes and rapid swings. However, the extent to which orbital forcing modulates the amplitude and timing of these millennial variations is unclear.

Here I will present results where we isolate the magnitude of millennial-scale variability (MMV) in two well-dated records, both linked to precession cycles (19,000- and 23,000-year periodicity): composite Chinese speleothem δ¹⁸O, commonly interpreted as a proxy for Asian monsoon intensity, and atmospheric methane. At the millennial timescale (1,000–10,000 years), we find a fundamental decoupling wherein precession directly modulates the MMV of methane but not that of speleothem δ¹⁸O, which is shown to be strikingly similar to the MMV of Antarctic ice core δD. One explanation is that the MMV of methane responds to changes in midlatitude to high-latitude insolation, whereas speleothem δ¹⁸O is modulated by internal climate feedbacks.