Youth and environmental groups participated last October 24 in the largest global day of climate action to urge world and local leaders to take fast and effective action on global warming. This global campaign is focusing the attention on the number 350, because according to the latest scientific data, 350 parts per million CO2 (carbon dioxide) is the safe upper limit for the atmosphere. However, the current CO2 concentration is 390 parts per million. It is therefore necessary for the world to cut down on CO2 concentrations soon before we reach the level where the climate crisis will be irreversible.