Changes in solar insolation due to variability in Earth's orbit, inclination, and axial tilt, combined with ocean currents and plate tectonics changing the location of the continents.
Those changes took place over longer time-frames than the Industrial Age (mid 1800s to present) that is the time-frame of concern now.
It is only a man's vanity to believe that he can affect global temperatures one way or another. It is caused by ocean currents, volcanic activity, etc.
While those are certainly important, so is the composition of the atmosphere (which we are changing) and land surface cover. Some recent research indicates that land surface changes dating back to the beginning of agriculture, a couple thousand years ago, may have significantly affected climate.
Incidentaly, one of the largest contributors to greenhouse gas (methane) is termites. What does Mr Hansen sugest we do about that? Call Terminex?
What he suggests is that we don't have to worry about methane emissions right now, because they have declined so substantially in the past five years.