According to 350.org, it's—you guessed it!—350. This is because scientists say that the safe upper limit for carbon dioxide levels in the atmosphere is 350 parts per million.
350.org is organizing The International Day of Climate Action on October 24. Check out the website for ideas on how to get involved, whether it means spreading the word among family and friends, organizing an event, or raising awarness through the media.
Other ways to take part on this day: search the "action map" to see what others are doing around the world, or hone your photography skills. That's right—a hot number like this has to be pretty photogenic...so if you find a way to incorporate the number 350 in an iconic spot in your community, be sure to take a photo and upload to the website.
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