Methane, Monsoons, and Modulation of Millennial‐Scale Climate

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 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 δ2H. 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. Key Points Millennial fluctuations in atmospheric methane are directly modulated by precession, whereas those in composite Chinese speleothem δ18O are not Millennial variations in the speleothem composite and Antarctic ice‐core δ2H are similarly modulated and have increased since ∼430 ka Boreal sources of methane are hypothesized to modulate millennial‐scale variability via sensitivity to insolation