CO 2 ‐driven changes to climate have occurred during many epochs of Earth's history when the solar insolation, atmospheric CO 2 concentration, and surface temperature of the planet were all significantly different than today. Each of these aspects affects the implied radiative forcings, climate feedbacks, and resultant changes in global mean surface temperature. Here we use a three‐dimensional climate system model to study the effects of increasing CO 2 on Earth's climate, across many orders of magnitude of variation, and under solar inputs relevant for paleo, present, and future Earth scenarios. We find that the change in global mean surface temperature from doubling CO 2 (i.e., the equilibrium climate sensitivity) may vary between 2.6 and 21.6 K over the course of Earth's history. In agreement with previous studies, we find that the adjusted radiative forcing from doubling CO 2 increases at high concentrations up to about 1.5 bars partial pressure, generally resulting in larger changes in the surface temperature. We also find that the cloud albedo feedback causes an abrupt transition in climate for warming atmospheres that depends both on the mean surface temperature and the total solar insolation. Climate sensitivity to atmospheric CO 2 has probably varied considerably across Earth's history.