LAUSR

Abstract A simulation of Earth's climate system through geological time was produced with a view to estimating the sun's continuously habitable zone (CHZ). The model consists of four boxes (air, land, ocean, and rock) and parameterizes the carbonate-silicate cycle as a five-step process, comprising volcanism, dissolution of airborne carbon dioxide in seawater, continental weathering, neritic deposition of carbonates on the sea floor, and seafloor weathering. Weathering, surface albedo, and planetary albedo are affected by the growth of continents over time, and by changes in cloud cover with planetary rotation rate. Radiative transfer is handled with a semigray method based on conditions on preindustrial Earth and present-day Mars. The model includes a process for recovering from Snowball Earth events, involving changing surface albedo due to volcanic soot deposition. The simulation produces a runaway greenhouse effect inside 0.982 AU and “limit cycling” between temperate and freezing temperatures outside 1.18 AU. Because continental weathering is less reactive to temperature than previously thought, the results indicate that Earth is more vulnerable to catastrophic climate runaways at the outer CHZ boundary than foreseen earlier. The narrow spacing ratio (R ​= ​1.20) implies that habitable planets will be less frequent than previously expected.