Alan AtKisson writes on worldchanging.com:
On the 350 Campaign
Thanks to that discussion, I finally understood more clearly the rationale behind the 350 campaign. The science was already clear enough: a level of 350 parts per million carbon dioxide in Earth's atmosphere appears, from the paleoclimatic evidence, to be the limit for maintaining the kind of global climate regime that gave rise to human civilization. We are already over 385 and rising. Meanwhile, most political goal-setting for stabilizing CO2 has hovered in the 450-550 range. Jim Hansen has led a growing chorus of scientific voices in saying that those numbers are too high, and that 350 is -- from a natural systems perspective -- the necessary goal, regardless of what has previously been considered as politically and economically feasible.
But the logic behind 350 as a campaign was still a bit fuzzy to me, until McKibben explained it. First, the number: it turns out that Arabic numerals are one of the few symbols that can travel into virtually any language and culture and be understood. People may not understand "ppm" (parts per million, which means that 350 can also be interpreted as 0.035 percent of the gases in our atmosphere), but they can understand the number anyway -- just as they understand that they have to keep their cholesterol under a certain number, even if they don't really know what cholesterol is. So 350 makes a certain universal outreach possible, in a way that anyone can respond to.
Second is the quality of response. According to McKibben, they were first worried that people would react to the news that we are already past the limit with despair. Instead, he says, it seems to be strangely empowering. It makes it clearer that the changes we are talking about must happen now, and must be big, just as a worrying medical report on your cholesterol level jolts you into taking action now to avoid the near-certainty of a heart attack or stroke. Number like 450 send an ambiguous message about slowing down slowly or "stopping in time." It's hard to get excited about that. But 350 says simply, "turn back." And as centuries of religious teaching might suggest, calls to repent now ("repent" means "turn back") are more galvanizing than calls to slow down eventually.
Third, 350 makes a number of very creative responses possible: churches ring their bells 350 times. A farmer in Cameroon plants 350 trees. The number can easily achieve iconic status and be interpreted, both symbolically and practically, in myriad ways -- which is precisely what's happening. After a rollicking discussion about whether people should be marching in the streets or doing home energy conversions, McKibben says that he's come to the conclusion that symbolic actions are, now, the most practical ones, because they have the potential spark the political actions that will drive large-scale systems change.