Decoupling Land Surface Effects from CO2 Effective Radiative Forcing in a Climate Model

Abstract Accurate estimation of effective radiative forcing (ERF) is essential for understanding climate responses to external forcing. A common approach involves simulations with fixed sea surface temperatures (SSTs) while allowing land temperatures to evolve freely. However, this setup can introduce land-induced adjustments that influence ERF estimates. Using a state-of-the-art climate model, we investigate the influence of land surface temperature changes in shaping ERF under an abrupt quadrupling of carbon dioxide (CO 2 ) (4xCO 2 ). We conduct a series of experiments where land temperatures and soil moisture are either interactive or prescribed to their climatological values. We find that fixing land temperatures and soil moisture increases 4xCO 2 ERF from 8.26 to 9.78 W m −2 , primarily due to reduced longwave cooling and cloud changes. Additional idealized experiments demonstrate a monotonic decrease in 4xCO 2 ERF but enhanced land precipitation with increasing land warming but fixed SSTs. These findings highlight the critical role of land processes in shaping radiative forcing estimates and provide insights into how land–sea warming contrasts influence convection, moisture transport, and precipitation patterns. Significance Statement Effective radiative forcing (ERF) is a fundamental metric for understanding how agents like carbon dioxide (CO 2 ) perturb Earth’s energy balance. A widely used method estimates CO 2 ERF by fixing sea surface temperatures while allowing land temperatures to respond in atmosphere–land coupled models. However, this approach involves land-induced responses, as land warming influences atmospheric energy exchanges in complex ways. Using an atmosphere–land coupled model, we show that suppressing land temperature changes amplifies the estimated CO 2 ERF. Idealized experiments further reveal that ERF decreases consistently as land warms. These findings underscore the importance of representing land surface processes when quantifying radiative forcing and highlight the critical role of land–atmosphere interactions in shaping climate response.