{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"b3b372a0cdb60d0170fd383a9e575dc65a68935d15752de42ad5103a6049d8ee","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["3e9cbdf630d49b1ee93097d6ee819abe66fefe5e824f82d1d5d4303feb56bc32","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"cde0f5adefbc0867ca2fb142bd89a2aaee7ace655d48122e2f9ac9b4b04acae6","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"earth.paleoclimate-puzzles.faint-young-sun","kind":"phenomenon","literature_status":"contested","n":"2","parents":[],"plain":"Four billion years ago the Sun gave about 25 percent less light, enough to freeze today's Earth solid, yet rocks show liquid water. Something, such as extra greenhouse gases or a darker surface, must have made up the difference.","posed_since":"1972","precise":"For Archean Earth (3.8 to 2.5 Gyr ago) with solar constant $S \\sim 0.75 S_{0}$ at 3.8 Gyr ago rising to $\\sim 0.82 S_{0}$ at 2.5 Gyr ago ($S_{0} = 1.36 \\times 10^{6}\\ \\mathrm{erg}\\ \\mathrm{cm}^{-2}\\ \\mathrm{s}^{-1}$), identify the combination of greenhouse gases ($\\mathrm{CO_2}$, $\\mathrm{CH_4}$, $\\mathrm{N_2}$ pressure broadening, $\\mathrm{H_2}$), cloud and surface albedo changes that yields global mean surface temperature above freezing while satisfying geochemical limits on pCO2 from paleosols and marine carbonates. An answer is a mechanism with an atmospheric composition consistent with all proxy bounds in a 3D climate model.","problem_ref":"3e9cbdf630d49b1ee93097d6ee819abe66fefe5e824f82d1d5d4303feb56bc32","references":"","settled_by":"Proxy-consistent Archean pCO2 and pCH4 reconstructions that, in a 3D climate model with clouds, give above-freezing global temperatures without ad hoc assumptions.","status_note":"","title":"How did early Earth stay unfrozen under a fainter young 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