Why Ice Ages Return Every 100,000 Years

Around 125,000 years ago, hippos swam in the Thames where Trafalgar Square now stands. Then the northern summers weakened, the snow stopped melting, and the ice came back, on a schedule set by Earth's orbit. CHAPTERS 00:00 — A Circle That Moves 05:43 — Hippos Under Trafalgar Square 12:14 — The Museum Keeper Who Read the Planets 20:09 — Leaning Toward the Sun 26:32 — The Wandering Pole Star 32:34 — A Letter on the Equinox 38:10 — The Snow That Stayed 43:38 — How Ice Makes More Ice 48:56 — The Ocean Holds Its Breath 55:34 — When the Sahara Turned Green 1:01:06 — Under Three Kilometres of Ice 1:09:46 — Survivors on the Move 1:18:42 — Faster Than It Came 1:27:03 — What the Trawl Net Brought Up 1:34:50 — A Line Cut in 1731 1:42:34 — Eighteen Years Too Late 1:50:22 — The Clock in Devils Hole 1:57:57 — When a Day Lasted Seventeen Hours 2:05:33 — Waiting for a Cold Summer 2:12:13 — The Weakest Beat SOURCES (selected) Croll, J. (1875). Climate and Time Milankovitch, M. (1941). Kanon der Erdbestrahlung (English edition 1969) Hays, Imbrie & Shackleton (1976). Variations in the Earth's Orbit: Pacemaker of the Ice Ages. Science EPICA community members (2004). Eight glacial cycles from an Antarctic ice core. Nature Laskar et al. (2004). Astronomy & Astrophysics Lisiecki & Raymo (2005). Paleoceanography Abe-Ouchi et al. (2013). Nature Lambeck et al. (2014). PNAS Winograd et al. (1992); Moseley et al. (2016). Science Lantink et al. (2022). PNAS Ganopolski, Winkelmann & Schellnhuber (2016). Nature Barker et al. (2025). Science