#350ppm refers to 350 parts per million CO2—the safe upper limit for atmospheric carbon according to climate scientist James Hansen, becoming rallying number for climate movement and name of major activist organization.
Scientific Basis
Hansen’s 2008 research concluded 350 ppm was maximum safe CO2 level to preserve stable climate. Atmospheric CO2 had already reached 385 ppm (2008) and continued rising past 400 ppm (2013), 410 ppm (2019). The number crystallized climate emergency: humanity had already exceeded safe boundaries.
350.org Formation
Bill McKibben founded 350.org (2008) using the number as organizing principle. The organization coordinated global climate actions: 2009 International Day of Climate Action (5,200 events, 181 countries), fossil fuel divestment campaign, anti-pipeline protests, and People’s Climate March (2014, 400,000 NYC).
Simple Message
“350” provided memorable, science-based target contrasting with vague policy goals. Activists carried “350” signs globally, making arcane atmospheric chemistry accessible. The number unified diverse climate actions under common framework: return to 350 ppm required immediate fossil fuel phase-out and carbon drawdown.
Movement Evolution
As atmospheric CO2 passed 410 ppm (2019), 350 became aspiration rather than achievable target—recovering to 350 ppm required not just emissions reduction but massive carbon removal. 350.org evolved focus from “return to 350” to broader climate justice, Indigenous rights, and fossil fuel resistance.
Symbolic Power
The hashtag demonstrated power of concrete, science-based framing. “350” was more tangible than “2°C warming” or “carbon neutral by 2050.” It gave global climate movement unified rallying cry, translating complex climate science into activist demand.