The influences of natural forcings (i.e. solar radiation and volcanic aerosol) and anthropogenic forcings (i.e. greenhouse gases) on the tropical Pacific zonal sea-surface temperature (SST) gradient are investigated through numerical sensitivity experiments, forced by natural forcing, anthropogenic forcing, or full forcing (both natural and anthropogenic forcing) using the Community Earth System Model (CESM) over the past two millennia. Under full forcing, during the present warming period (PWP; 1901–1999), the Pacific SST shows a larger warming over the tropical Pacific region than the subtropical Pacific region. This pattern is composed of an El Niño-like SST gradient (due to the greenhouse gas forcing) and a La Niña-like SST gradient (due to natural forcing). Two sensitivity experiments, the PWP under greenhouse gas forcing and the medieval warming period (MWP; 751–1250) under natural forcing, were used to examine the mechanisms. The results showed that under the greenhouse gas forcing, the larger warming over the eastern tropical Pacific is induced by an increased surface net heat flux, which is mainly caused by short-wave radiation and long-wave radiation. Under natural forcing, a larger warming is induced over the western tropical Pacific by changes in the ocean vertical heat transportation, caused by surface wind anomalies.