Even two years ago, I was in complete despair about our chances of fighting climate change. But something’s changed. It’s not the science, which has gotten steadily worse. It’s the first signs that the planet’s immune system–conscious citizens ready to make a difference–is finally kicking in. Bloggers, in this metaphor, are key antibodies–they recognize threats, and rally people to take the steps needed. So this year’s Blogger Action Day is, in a sense, a test: is the planet now wired together in a way that will let it act swiftly, nimbly, decisively against the great trouble we’ve ever faced?

In particular, we at 350.org need your help spreading the word about what’s quickly turned into the biggest day of global action on climate ever–and perhaps the most geographically widespread day of political action the planet has ever seen. On October 24–a week from Saturday–citizens will hold thousands of rallies and events and demonstrations in almost 170 nations to demand that our leaders take tougher action heading to Copenhagen.

It’s the first day like it ever devoted to a scientific data point, the number 350. As in 350 parts per million carbon dioxide, which scientists began telling us two years ago was the most we could safely have in the atmosphere. It’s a tough number, because we’re already past it, at 390 parts per million and rising. And it’s tough because to get back to it we’d need much stronger and quicker action than most of our leaders–and even some of our old-line environmental groups–support.