Tomorrow is International Day of Climate Action. A site called 350.org, backed by some of the biggest names in the environmentalism movement, is calling for us to participate in one of the day’s 4,300 events in 170 countries aimed at raising awareness about climate change. 350.org says it will be “the most widespread day of political action in history.” It might be cool to be a part of that, so I’ll list some of the activities planned in Captivate markets in case anyone wants to join in. But first: Why is it called “350.org”?

They got the “350” from the concept, which I first heard of through James Hansen, that to maintain life as we know it atmospheric CO2 levels cannot exceed 350 parts per million for any extended time. Unfortunately, we’re already beyond that level and pushing closer to 400 ppm every day. So in order to get us back down, Hansen and others say, we have to take pretty drastic steps, such as cutting out coal use and replanting vast swaths of forest.

While the International Day of Climate Action isn’t about getting us to immediately stop using coal, it is about making people think about the ramifications of their actions, even the simplest ones, like flicking on a light switch (which draws electricity from your local power plant, which is probably powered by coal).

So take part. It’s a simple action, it’ll give you something fun to do on a Saturday, and it just may, when all is said and done, help save the planet. Check here for all U.S. events, or simply peruse these:

New York City
The 350.org organizing team will gather in Times Square, where the giant video screens will display “350” representations from around the world.

Philadelphia
In what will be a common theme Saturday, participants on Independence Mall will form a giant “350,” as well as hear from speakers such as city green czarina Katherine Gayewski and “environmental heavyweight” Ray Anderson.

San Francisco
The Justin Herman Plaza/Ferry Building is the site for the San Fran gathering, where cyclists completing a 350km route will rally with surfers, artists, poets and activists.

Seattle
You know you’ve always wanted to take part in one of those “Thriller” dance-a-thons. Now’s your chance. Admittedly, the link to climate is pretty weak here, but, hey, it’s probably gonna be fun.

Washington, D.C.A march to Lafayette Park will be followed by participants forming a giant "circle of hope" across from the White House. This "O" will serve as third in a series of 3, 5, and 0 photos taken from around the world.