350.org:

350.org are an international movement of ordinary people working to end the age of fossil fuels and build a world of community-led renewable energy for all.

The 350 in the name stands for 350 parts per million of carbon dioxide, which has been identified as the safe upper limit to avoid a climate tipping point. By the end of 2007, the year 350.org was founded, atmospheric CO2 had already exceeded this threshold, reaching 383 ppm of carbon dioxide and as of July 2022, the concentration had reached 421 ppm of carbon dioxide, a level 50% higher than pre-industrial levels.

Through online campaigns, grassroots organizing, mass public actions, and collaboration with an extensive network of partner groups and organizations, 350.org mobilized thousands of volunteer organizers in over 188 countries. It was one of the many organizers of the September 2019 Global Climate Strike, which evolved from the Fridays for Future movement.