A wise old man was on his daily walk along the beach. At a distance he noticed a little boy reaching down to pick something up and throwing it into the ocean, then tirelessly repeating it over and over again.
The old man curiously moved closer to find out what he was up to. The boy was picking up starfish left stranded by the ebbing tide, and tossing them back into the water, one by one, as if trying to save them all from dying and also from the hungry seagulls.
convinced of the futility of the task the boy had taken upon himself, the old man went up to him and said, "Son, hundreds of starfish dry up on this beach everyday. What you're doing simply won't make a difference."
Clearly hearing what the old man had said, the little boy bent down, picked up another starfish and threw it as far as he could, back into the sea. Then he looked at the old man, smiled and politely said,
350 ppm carbon dioxide is the most we can have in the atmosphere if we wish to retain the planet similar to the one on which civilisation developed and to which life on Earth is adapted....
in other words, 350 parts per million is the safe upper limit for carbon dioxide in our atmosphere. it is the number humanity needs to get back to as soon as possible to avoid runaway climate change...