Surface albedo as a first-order control on Mesoarchaean climate (PlaSim)

The radiative balance of the early Earth was governed by other boundary conditions than today, including a fainter Sun, elevated greenhouse gas concentrations, and a smaller land surface area. Although the role of atmospheric composition in sustaining habitable surface temperatures during the Mesoarchaean has been extensively investigated, especially to solve the faint young Sun paradox, the climatic impact of land position and distribution under varying albedo remains comparatively underexplored.

We therefore assess how variations in land-surface albedo, land fraction, and land distribution could have modulated Mesoarchaean climate states. Using the Planet Simulator (PlaSim), an intermediate-complexity climate model, we conducted 195 simulations spanning CO₂-equivalent forcing levels of 3–10 % (30,000–100,000 ppm). Land-surface albedo was varied between 0.15 and 0.30, land area between 8 % and 28 %, and idealised land distributions were prescribed, including diagonal, staggered, and mid-latitude configurations. Ocean albedo was held constant at 0.144 to isolate the climatic impact of continental reflectivity.

Across all simulations, global mean temperature responds strongly and non-linearly to both land fraction and land albedo. At low land albedo (0.15) and low to intermediate CO₂-equivalent forcing (3–5 %), increasing land area produces a slight warming trend, despite minimal differences between land and ocean reflectivity. This behaviour indicates that land–ocean contrasts in surface energy partitioning and effective heat capacity can modify global climate even when shortwave albedo contrasts are small. Sensitivity increases abruptly as land albedo rises from 0.20 to 0.25. Beyond this threshold, modest increases in land area result in pronounced global cooling, consistent with a regime shift in the radiative balance. This non-linear response is most prominent at low to intermediate CO₂-equivalent forcing and becomes progressively muted at higher forcing (10 %), where greenhouse effects dampen the temperature response to surface reflectivity changes. The pattern occurs across all land configurations but is amplified when landmasses occupy equatorial to mid-latitudes, where insolation is highest and albedo exerts maximum leverage, whereas high-latitude land has a comparatively weaker effect.

These findings highlight that land surface characteristics such as albedo and distribution were critical for early Earth’s climate. Even under strongly greenhouse-forced atmospheres, surface properties significantly altered the planetary energy budget. Recognising such sensitivities is essential for reconstructing Archaean climate states and assessing the potential for climatic stability under reduced solar luminosity.