This textbook chapter describes the properties of greenhouse gases that are being added to the atmosphere: carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and ozone. Deforestation, industrialization and population growth are presented as reasons why greenhouse gases are increasing in the atmosphere. This is the third chapter in the unit, Climate Change, which addresses the question of how human activities are changing Earth's climate. The resource includes links to current news articles, and a suite of pre and post unit assessments. A teacher's guide supports classroom use. The resource is part of Global Systems Science (GSS), an interdisciplinary course for high school students that emphasizes how scientists from a wide variety of fields work together to understand significant problems of global impact.