Hans Hansen (1)

What does the number 350 mean?



350 is the most important number in the world—it's what scientists say is the safe upper limit for carbon dioxide in the atmosphere.



Two years ago, after leading climatologists observed rapid ice melt in the Arctic and other frightening signs of climate change, they issued a series of studies showing that the planet faced both human and natural disaster if atmospheric concentrations of CO2 remained above 350 parts per…