The evidence that Earth’s climate was different in the past is overwhelming, but there is little agreement on what caused these changes.

One of the more exotic theories is that past changes in the earth’s climate have been caused by the sun’s gradual passage through the spiral arms of the galaxy. The thinking is that in regions of denser star populations, supernovas would have been more common, bathing the earth in cosmic rays more often. These cosmic rays would then have seeded the formation of clouds, which cools the planet.

There is even evidence to back this claim. It turns out that the sun seems to move between spiral arms every 140 million years or so, and a similar 140-million-year cycle also crops up in climate change on Earth.

Today, Adrian Melott at the University of Kansas and a couple of buddies put the idea to rest once and for all. They point out that in recent years, astronomers have mapped out the structure of our galaxy in increasing detail. How does this new information sit with the galactic theory of climate change?

Not well. Melott and co have compared the times of transit between regions of the new galactic map with changes in Earth’s climate and found that the 140-million-year correlations simply disappear.

You might say that this could be explained if the movement of the sun had not been regular during this period. But no, Melott says that the cycle cannot be made to match for any reasonable change in the motion of the sun through the galaxy.

So whatever caused the 140-million-year climate-change cycle on Earth, it wasn’t the sun’s passage through the galaxy.

Ref: arxiv.org/abs/0906.2777: Testing the Link Between Terrestrial Climate Change and Galactic Spiral Structure