Read here. Well, it's not only the sun. When solar activity increases (decreases), the cosmic ray energy impact on Earth's climate decreases (increases). The Greenland ice cores are revealing the evidence of changing cosmic ray intensity from the varying levels of Beryllium isotopes being identified. Based on this evidence, there appears to be a strong relationship between the ups/downs of cosmic energy versus the downs/ups of Earth's climate.
The IPCC dismisses solar/cosmic influences because their marching orders are to solely blame anthropogenic reasons, namely human CO2. This is how we got a corrupted science, Climategate, and the Copenhagen boondoggle. But now, even major science organizations are expanding their climate change reasons beyond the simple CO2 hypothesis. Recently the National Center of Atmospheric Research (NCAR) changed their tune and is now producing research that points to a large solar impact.