NASA

Amara_Graps_at_Stanford says "People may be interested in knowing that the variation in the sun's energy output has far more impact on our climate than the tiny increases of various chemicals. Eg. doubling the amount of CO2 in our atmosphere has the effect (on our climate) as increasing the solar irradiance by 0.1% more or less."

The IPCC TAR view is that forcing from solar variations is considerably smaller than forcing from greenhouse gases. They believe that measured as a difference from 1750, GHG forcing is estimated as 1.4 W/m2 compared to 0.3 W/m2 from solar. (from wikipedia)

342 w m^-2 is the critical number for maintaining the current temperature. That's the average insolation of the Earth. The instantaneous value is 1367, or 4 times the average. Lucky for us the 11 year variation (sunspot cycle) in solar output seems to be around 1 watt/meter² out of the 1366 or 0.07%. Carbon 14 data suggests we are approaching historic highs in solar output. The Carbon-14 data correlates as well to the temperature increases since 1840 as does the CO² data.