This report is the Second National Climate Assessment. It summarizes the science of climate change and the impacts of climate change on the United States, at present and in the future. It is largely based on results of USGCRP research, and integrates those results with related research from around the world. This report discusses climate-related impacts for various societal and environmental sectors and regions across the nation. It is an authoritative scientific report written in plain language, with the goal of better informing public and private decision making at all levels. The report can be explored interactively at nca2009.globalchange.gov.