The sun, volcanoes, ocean currents... fine fine. Certainly these influence climactic variations on Earth. But a most important, and perhaps most influential, factor is the local interstellar medium.

Bottom line: space is not empty. As the sun (and attending planets) moves through the interstellar medium, the energy from the sun (as felt on the surface of the Earth) is attenuated.

The results may be vastly more significant than volcanic eruptions, ocean currents, and solar cycle variations combined.

Click HERE

and

Click HERE.

http://antwrp.gsfc.nasa.gov/apod/ap020210.html