Energy-saving houses, solar factories, rainwater collection, more than 400 kilometres of cycling paths and even a zero-emissions hotel: these are just some of the ways the city of Freiburg, in south-western Germany, is leading the world in sustainable city-building.
![The cover of the book, The Story of CO2, "Big Ideas for a Small Molecule and profile images of the two authors.](https://www.artsci.utoronto.ca/sites/default/files/styles/ne_3btn_widget/public/book-ozin-ghoussoub-lead.jpg?itok=Qx4AcVHk "The cover of the book, The Story of CO2, "Big Ideas for a Small Molecule and profile images of the two authors.")
