It's a magic number we've been seeing more and more of lately throughout the media. There's even a new organization named after it: 350.org
The message: we have to get global CO2 emissions down from 390 parts per million (current level) to 350 parts per million in order to stop the accelerating cascade of climate destabilization-related impacts we're observing from pole to pole.
Where does this number come from and what does it all mean?
Treehugger has a nice and simple summary that will explain the science of 350 to you over lunch.
While you're at it, check out this piece from 3P on The Economics of 350. Their favorite quote from the report they summarize:
“The most important conclusion involves what we did not find,” the report states. “There are no reasonable studies that say that a 350 ppm stabilization target will destroy the economy; there are no studies that claim that it is desirable to wait before taking action on climate protection. On the contrary, there is strong, widespread endorsement for policies to promote energy conservation, development of new energy technologies, and price incentives and other economic measures that will redirect the world economy onto a low-carbon path to sustainability [...] Is a potential cost of 1 to 3% of world GDP a large or a small number? The answer depends on how seriously you take the risks of climate change.”