Atmospheric CO₂ over 800,000 years — with photosynthesis thresholds

Photosynthesis reference levels (plant physiology, not claims)

• ~150 ppm (red dashed) — C3 photosynthesis severe impairment threshold. Below ~150–200 ppm most C3 plants cannot sustain net photosynthesis. Source: Ward et al. 2005, PNAS; Sage 1995; Wikipedia “CO₂ in Earth’s atmosphere” citing peer-reviewed literature.
• ~50 ppm (orange dashed) — C3 CO₂ compensation point. Below this, respiration exceeds photosynthesis — net carbon loss even in full sunlight. Affects ~85% of plant species (wheat, rice, most trees). Source: Standard plant physiology; Taiz & Zeiger, Plant Physiology.
• ~5 ppm (green dashed) — C4 plant CO₂ compensation point. C4 plants (corn, sugarcane, many grasses) evolved a CO₂-concentrating mechanism ~7–5 Ma ago partly in response to declining CO₂. They can sustain photosynthesis at near-zero atmospheric CO₂. Source: Osborne & Beerling 2006, Phil. Trans. R. Soc. B.
• Ice-core minimum ~180 ppm — The glacial troughs sit in the C3 stress zone (150–200 ppm): plants were severely stressed but still above the shutdown floor.