It never fails every 100,000 years, just when the glaciers have retreated and you've been enjoying 10,000 years of balmy weather, another ice age comes along. It's been that way for the past million years, with Earth's climate swinging between glacial and interglacial (warm) periods. Scientists generally ascribe long climate change cycles to wobbles in Earth's orbit, a theory first proposed in 1912 by Serbian astronomer Milutin Milankovitch. But the Milankovitch theory does not satisfactorily explain the 100,000-year ice age cycle of the past million years. Now Dartmouth College geochemist Mukul Sharma says he may have found a connection between Earth's climate and magnetic activity in the sun.

Sharma did some armchair detective work to build a picture of the sun's magnetic activity during the past 200,000 years. He focused on the solar modulation factor, a measurement of how much the sun influences high-energy galactic cosmic rays (atomic nuclei that rocket through the galaxy at nearly the speed of light) on their way toward Earth. "One can imagine the sun being a bar magnet, creating a magnetic field by throwing charged particles out in space:' says Sharma. This magnetic field, along with Earth's own, deflects cosmic rays.

If you want to investigate the solar modulation factor over many thousands of years, the place to start is at the bottom of the ocean. Marine sediment cores — columns drilled from the ocean floor in the same way you might use a straw to take ice cream samples from a milkshake — offer clues to conditions on Earth at the time the sediment layers were deposited. Using data from published papers, Sharma studied the sediment cores' levels of beryllium-10, a radioactive isotope that is created when galactic cosmic rays slam into Earth's atmosphere. Sharma used beryllium-10 levels to graph the sun's magnetic activity over the past 200,000 years.

Sharma found a pattern: The solar modulation factor rose and fell in a 100,000-year cycle, consistent with the ice age cycle on Earth. To confirm this correlation, Sharma looked at proportions of an oxygen isotope called oxygen-18, which rise and fall depending on the amount of Earth's water that is bound up in ice caps and glaciers. Sharma found that, on the 100,000-year time scale, Earth's average temperature and solar magnetic activity rise and fall in unison.

The Paper	Variations in solar magnetic activity during the last 200,000 years: is there a Sun-climate connection
The Journal	Earth and Planetary Science Letters, Vol 199
The Author	Mukul Sharma
The Gist	Magnetic activity in the sun could be messing with our climate
Before  Translation	The gross features of the solar modulation over the past 200 ka are: (1) the solar modulation over last 200 ka has ranged from ~0 to >2, (2) for only one period during the last 200 ka does the significantly significantly exceed I (between III and 125 ka), and (3) between 25 and 35 ka, and 175 and 190 ka the  ratio was close to zero.

Astronomers have proposed several hypotheses for the apparent link (Sharma says more definite proof would require "maybe a million years 'worth" of data) between the sun's magnetism and Earth's climate. Perhaps it's the sun's effect on cosmic rays that, in turn, affects cloud formation, or simply that a magnetically active sun puts out more energy.

If a 100,000-year ice age cycle seems remote from the here and now, consider the problem of global warming. Other studies suggest that solar magnetic cycles could playa role in more immediate climate changes. In order to know how humans are affecting the climate, we'll need to understand the sun's role too - and magnetism is an important part of that equation. - Paul Beck