, designed by CHI & Partners, invites users to register to show their support for the climate change campaign.
To sign up, users are asked to upload a photo of their most serious face.
Friends of the Earth said that the uploaded photos would be used to petition the government for more funding for councils, to enable them to implement green technologies, insulate homes and improve public transport.
Users who register are asked to show how serious they are about a selection of topics ranging from renewable energy to stinky cheese.
They are then able to organise themselves into "tribes" based on shared levels of seriousness.
Christian Graham, Friends of the Earth's publishing and new media manager, said: "The Get Serious About CO2 website is a key part of our work to reach out to a new generation of activists.
"We're hoping the interactivity of the site will draw people more deeply into our work and establish a community of people serious about councils cutting CO2, so we can exert the pressure we need to tackle climate change at a local level."
Users are encouraged to return to the site regularly to track the growth and evolution of the movement.