350 parts per million (ppm) is the safe upper limit for carbon dioxide in the Earth's atmosphere. But the atmosphere already has 390 ppm and the number increases each year. Glaciers are melting. Mosquitoes are spreading. And more. October 24 is International Day of Climate Action. Don't sit around waiting for politicians and businesses to do something. You can do something. Sign up for an action by going to 350.org.