State-Dependent Biophysical Processes Prevented a Late Paleozoic Snowball Earth

Tianyi Chu , Daniel John Lunt, Ros M. Death, Philip C. J. Donoghue, Junxuan Fan

Terrestrial vegetation modulates Earth’s climate through two distinct pathways—biogeochemically drawing down CO₂ through enhanced organic carbon burial ^{[1, 2]} and silicate weathering ^{[3, 4]}, and biophysically altering surface energy and water balances through potentially competing albedo and evapotranspiration changes ^{[5, 6]}. Although the biogeochemical pathway is widely invoked to explain the Late Paleozoic Ice Age (LPIA, ∼370–260 Ma) ^[7–10], the role of biophysical processes remains poorly constrained. Here we show that the net biophysical effect of vegetation over the last 380 million years has been a strongly state-dependent warming: modest under greenhouse conditions but dramatically amplified as temperature declines. During the LPIA, an absence of these biophysical processes would have caused up to 19 ℃ of additional cooling, pushing global mean temperature below 0 ℃ and driving sea ice equatorward of 30° latitude, crossing the critical threshold for initiation of a Snowball Earth episode. Our findings reveal and resolve a stabilising paradox: while vegetation expansion biogeochemically drove Earth into an icehouse through CO₂ drawdown, its biophysical presence prevented a collapse into Snowball Earth, thereby regulating the climate system over long timescales and maintaining Earth’s habitability.