Paleogeographic modulation of Phanerozoic climate by albedo compensation of interhemispheric land imbalances

Earth's climate during the Phanerozoic oscillated between intervals punctuated by a few ice ages (three in the Paleozoic and the current ice age in the Late Cenozoic, corresponding to a cumulative glacial occupancy of ~17% of the eon) to none documented in the intervening ~225-Myr-long interval encompassing the latest Paleozoic, entire Mesozoic, and early Cenozoic, here termed the ‘Greater Mesozoic’ glacial gap. In this review, we compile and synthesize published paleogeographic reconstructions from 530 Ma to the present to quantify the distribution of land and sea by hemisphere and zonal climate belts and reconstruct surface reflectivity through time. We show that hemispheric asymmetries in land area generated large imbalances in surface albedo that require compensating processes—most plausibly more clouds to increase albedo of the darker, more oceanic hemisphere, as occurs today—to maintain Earth's overall hemispheric radiative symmetry and heat balance. Intervals including glaciations, especially in the Paleozoic but also in the Late Cenozoic, are characterized by high compensated reflectivity and lower absorbed solar flux, calculated from compensated reflectivity and secular solar evolution, while low compensation values and higher absorbed solar flux mark the intervening ~225-Myrlong ‘Greater Mesozoic’ glacial gap. For example, the contrast between the pre-onset states of the Late Cenozoic and Late Paleozoic ice ages is ~14 W m− 2 , implying that, even assuming atmospheric damping, the effective climate conditioning remains on the order of CO₂ doubling (or halving) forcings. We therefore propose that glaciations were preferentially triggered under high compensated reflectivity preconditions, corresponding to low absorbed solar flux, which may have reduced the energetic threshold for CO₂-driven ice-sheet initiation, especially when coupled with enhanced CO₂ sink efficiency through continental weathering on equatorial landmasses, including arc–continent collision zones and topographically elevated large continental igneous provinces.