Book excerpt: Global warming is a complicated problem. Gas Trees and Car Turds is a fun, fast read about the carbon cycle: trees are made of air and water, electricity is made from coal that is made from trees, gasoline is made from plankton, and all of these things are related to each other and to our climate through carbon dioxide. The book makes carbon dioxide, an invisible odorless gas responsible for global warming and plant growth, into something that can be imagined and understood by children.