The 145-Million-Year Galactic Cycle — and What Else Might Be Driving It

For anyone doing cycles research on geological timescales, one of the more provocative proposals of the last two decades is the idea that Earth's long-term climate carries the fingerprint of our solar system's orbit through the Milky Way. This post walks through the galactic cycle hypothesis, the evidence for it, the pushback it's received, and a couple of shorter geological cycles that seem to sit alongside it — including a quick reanalysis of a well-known Phanerozoic temperature curve that turned up some interesting harmonics.

The mechanism

The hypothesis, developed mainly by astrophysicist Nir Shaviv and geochemist Ján Veizer in the early-to-mid 2000s, starts from a simple observation: the solar system doesn't sit still relative to the galaxy. As it orbits the galactic centre, it periodically passes through the Milky Way's spiral arms, and spiral arms are where star formation — and supernova rates — run highest. More nearby supernovae means more galactic cosmic rays reaching the inner solar system.

The proposed climate link is cosmic-ray-driven cloud nucleation: cosmic rays ionize the troposphere, ionization promotes the formation of cloud condensation nuclei, and more low-altitude cloud cover means a cooler planet. On this view, spiral arm passages should show up in the geological record as recurring cold intervals — ice age epochs — spaced out by however long it takes the solar system to cross from one arm to the next.

The evidence, and the number

Shaviv and Veizer approached this from three independent directions and looked for agreement:

Galactic dynamics models predict a spiral-arm-crossing period of roughly 135 ± 25 million years. Separately, cosmic ray exposure ages measured from iron meteorites — which record the flux of cosmic rays over the meteorites' long residence time in space — show a periodicity of 143 ± 10 million years. And when the geological record of ice age epochs over the Phanerozoic is examined directly, the recurrence period comes out to 145 ± 7 million years, with the ice ages lagging the predicted cosmic ray peaks by 30-some million years — which the authors argue is consistent with the time it takes cooling to fully develop. Shaviv has also claimed that cosmic ray flux variation accounts for a large share of the variance in a reconstructed 500-million-year tropical ocean temperature record.

Three independent lines pointing to roughly the same ~140–145 Myr number is the heart of the argument, and it's a genuinely striking coincidence if it holds up.

It's contested, not settled

It's worth being upfront that this is a live scientific dispute, not an accepted result. The most direct pushback came from Rahmstorf, Archer, Ellis and coauthors in a 2004 rebuttal, "Cosmic Rays, Carbon Dioxide and Climate," which challenged both the statistical robustness of the correlation and the physical plausibility of the cloud-nucleation mechanism — cosmic-ray-cloud links have generally been hard to establish convincingly in atmospheric physics more broadly. Mainstream climate science has not adopted the galactic-cycle framing as an explanation for Phanerozoic climate swings. So the honest framing for a cycles-research audience is: an intriguing, quantitatively specific hypothesis, with real evidence behind it, that remains disputed rather than confirmed.

Shorter cycles that might be riding along

The ~145 Myr galactic cycle isn't the only periodicity that's been proposed in the same part of the geological record. Studies of Phanerozoic marine biodiversity — using the fossil record as a proxy for environmental and evolutionary turnover — have independently turned up spectral peaks in the 62–77 million year range, along with a shorter one around 27–31 million years, and longer ones near 125 and 333 million years. Unlike the galactic cycle proposal, the cause of these shorter cycles is genuinely unresolved: some researchers have floated astronomical drivers, but more recent work leans toward internal geological processes — mantle plume activity, tectonic cycles, sea-level change (glacio-eustasy), and volcanism — as the more likely explanation, and is explicit that there's currently no settled mechanism.

What's interesting is that 62–77 Myr sits close to half of 145 Myr, and 27–31 Myr sits close to a quarter of it. If there's any real coupling between a galactic-scale forcing and Earth's internal cycles — or if these are simply independent processes that happen to fall in a similar range — a harmonic-looking relationship is exactly what you'd expect to see show up in a spectral analysis of the geological record.

A quick reanalysis

That prompted a small test: digitizing a published Exxon-style Phanerozoic temperature curve (spanning roughly 500 million years to present) point by point, then running a least-squares amplitude spectrum across candidate cycle counts from 1.5 to 20 cycles over the full record.

The dominant low-frequency peak came out at a period of about 139 million years — close to, though not exactly, the 145 Myr figure above. Worth noting: at that end of the spectrum the record only spans about 3.6 cycles of that period, so the peak is intrinsically broad — the resolution limit at that frequency spans roughly 122 to 164 million years, and 145 sits comfortably inside it at about 94% of peak amplitude. In other words, this dataset can't really distinguish 139 from 145; they're the same broad feature.

More interesting were two better-resolved, shorter-period peaks: one at about 80.6 million years and another at about 38.5 million years — a ratio of roughly 2.09, close to a clean 2:1 harmonic. Both land within about 5–10% of the 62–77 Myr and 27–31 Myr bands reported in the biodiversity literature, and because they correspond to more cycles across the record, they're better resolved than the long-period peak — a more solid data point than the 139/145 comparison.

None of this is a rigorous test of causation — one digitized curve and a single spectral method is a sanity check, not a study. But it's a reasonable illustration of why these particular period ranges keep showing up across independent proxies, and why the galactic cycle idea, contested as it is, keeps drawing attention from people working on long-period cycles.

Further reading

• Shaviv, N. — "The Milky Way Galaxy's Spiral Arms and Ice-Age Epochs and the Cosmic Ray Connection" (sciencebits.com/ice-ages)
• Rahmstorf, S., Archer, D., Ellis, D.V., et al. (2004) — "Cosmic Rays, Carbon Dioxide and Climate: Rebuttal of Shaviv and Veizer" (pik-potsdam.de)
• "Timing and periodicity of Phanerozoic marine biodiversity and environmental change," Scientific Reports (nature.com/articles/s41598-019-42538-7)

Thanks to Claude for gathering facts.