The effects of natural and anthropogenic forcings (solar activity, volcanism, atmospheric CO2 concentration, deforestation) on climate changes are estimated with the Earth system model of intermediate complexity, CLIMBER-2, for the past millennium. Simulated surface air temperatures for the Northern Hemisphere from the combined forcing correlate reasonably well with paleoclimatic data (r = 0.70). The largest negative anomalies occur when insolation minima coincide with volcanic eruptions. Anthropogenic forcings impose additional climate changes after 1850. The increasing warming from increasing CO2 concentrations is attenuated by the cooling effect from deforestation. Results from differently combined forcings suggest that the relatively cool climate in the second half of 19th century is largely attributable to cooling from deforestation.