The climate, as represented by the mean Northern Hemisphere temperature, has shown substantial changes within the past century. The temperature record is utilized as a means of elucidating the relative importance of anthropogenic CO2 increase, volcanic aerosols, and possible solar insolation variations in externally forcing climate changes. Solar luminosity variations, suggested by observed solar radius variations on an ≈ 80 yr time scale, allow a self-consistent explanation of the hemispheric temperature trends. Evidence for solar influences on the climate is also found on the shorter 11 and 22 yr time scales present in solar activity cycles.