Constraining effective radiative forcing and adjustments driving past warm intervals

Understanding the drivers of past warming climates is fundamental to reconstructing Earth’s climate history and refining projections of future climate change. Cenozoic warming intervals, including the mid-Pliocene (3–3.3 Ma), mid-Miocene (11.6–16 Ma), and early Eocene (47–56 Ma), provide analogs for present-day warming, featuring similar boundary conditions but substantially warmer climates. These intervals illustrate how variations in CO₂, ice sheets, vegetation, geography, and topography influenced global climates through radiative forcing and feedback mechanisms, offering essential insights into the climate dynamics following an intermediate warming pathway.

Despite extensive research on these intervals, the role of radiative forcings from changing boundary conditions remains poorly constrained. Here, focusing on the mid-Pliocene, we leverage three generations of the Community Earth System Model (CCSM4, CESM1.2, CESM2) to quantify radiative forcing and decompose the contributions of CO₂, vegetation and ice sheets, and topography and geography. Using published CESM radiative kernels, we diagnose radiative adjustments in atmospheric temperature, water vapor, surface albedo, and cloud properties, with a particular focus on cloud forcing and its interactions due to their critical role in modulating radiative adjustments and subsequent feedbacks.

Effective radiative forcing (ERF) is calculated as the difference in net top-of-atmosphere radiative fluxes between pre-industrial control and warming interval simulations, with prescribed sea surface temperatures specific to each interval. The simulations incorporate CO₂ levels, ice and vegetation, and geographic and topographic conditions representative of each period.

Our results show that CO₂ contributes approximately 60% of total forcing, with the strongest impacts in the tropics and Arctic. Cloud-related adjustments exhibit significant variability, with the net cloud adjustment even reversing its sign in with different radiative perturbations--emphasizing the complex interplay between radiative adjustments and the role of clouds in shaping climate responses across Cenozoic warming intervals.

We suggest that constraining radiative forcing from different perturbations of the past warm intervals is essential for understanding and decomposing drivers of past climate warmth and reducing inter-model variability in climate simulations.