Temperature observations from the Northern Hemisphere reveal a warming since 1861 which is larger in winter than in summer. Possible explanations for a decline in seasonal spread are discussed using the Earth system model CLIMBER-2. Simulations forced by natural and anthropogenic factors (Milankovitch forcing, solar variability, volcanism, atmospheric CO2 concentration, deforestation) generate specific seasonal responses. While the Milankovitch forcing increased the millennial seasonal spread, and solar variability and volcanism proved ancillary in reducing the spread on the centennial timescale, the anthropogenic factors appear the primary agents to attenuate the seasonal spread. The climatic effect of the anthropogenic factors is amplified by seasonally varying feedbacks related to the albedo of changing sea-ice and snow cover.