Climate Sensitivity in a Pre-Plant World: Why High CO₂ May Not Have Been Sufficient to Maintain a Paleozoic Hothouse

Despite evidence for generally elevated atmospheric CO₂ concentrations, the climate of the early Phanerozoic appears to have been neither uniformly warm nor stable. Proxy records, climate simulations, and paleogeographic reconstructions all carry large uncertainties, yet taken together they suggest that greenhouse forcing alone may not fully explain observed climatic variability, including intervals of pronounced cooling, such as the Hirnatian Glaciation. Understanding how early Phanerozoic climate responded to high CO₂ therefore requires explicit consideration of the boundary conditions under which greenhouse forcing operated.

Here, we examine the combined roles of paleogeography, land-surface properties, and reduced solar luminosity in shaping early Phanerozoic climate states. Using an intermediate-complexity Earth system model, we systematically explore climate sensitivity across a wide range of atmospheric CO₂ concentrations under pre-vegetation boundary conditions and early Paleozoic paleogeographic configurations. The experimental design focuses on how land–sea distribution, continental arrangement, and surface characteristics influence large-scale heat transport, cryospheric feedbacks, and the CO₂ levels required to maintain ice-free conditions.

Our working hypothesis is that early Phanerozoic climates were intrinsically biased toward cooler states relative to later, vegetated periods, due to higher surface albedo, altered hydrological cycling, and reduced incoming solar radiation. In such a climate system, maintaining temperate conditions may have required persistently high CO₂ concentrations, while gradual CO₂ drawdown could have positioned the system close to critical thresholds. Under these circumstances, comparatively small paleogeographic changes—such as shifts in continental connectivity or topographic relief—may have been sufficient to trigger short-lived glacial episodes, without invoking abrupt or extreme changes in greenhouse forcing.

By framing early Phanerozoic climate evolution as a problem of threshold behavior under uncertain boundary conditions, this work aims to clarify why high CO₂ and cooling are not necessarily incompatible. The results will help constrain which combinations of forcing and boundary conditions are physically plausible and guide more robust interpretations of proxy records and future paleoclimate modeling efforts.