<p>Pronounced model-observation discrepancies in the changes of tropical Pacific zonal sea surface temperature gradient during the satellite era imply systematic model deficiencies. However, the relatively short high-quality instrumental record hampers robustly determining the response of tropical Pacific sea surface temperature to greenhouse gas increases. By adopting paleoclimate proxy records along with a series of climate model simulations, here we show that the zonal gradient is likely to decrease under sustained strong forcing. Paleoclimate proxy records indicate an overall increase of the zonal gradient over time, which has been accompanied by global-mean cooling associated with decreasing carbon dioxide concentrations. Model simulations are found to broadly reproduce the distinct contrast between warmer high carbon dioxide climates and the opposite climates, albeit with large inter-model discrepancy. The qualitative agreement among paleoclimate proxy records and modeled representations therefore lends some important credence to the sign of model-projected future tropical Pacific mean state change.</p>