With rising temperatures, more frequent extreme weather events, and a host of effects on air quality, food supply, and infectious diseases, climate change is projected to have a major impact on human health and well-being. In many places, these effects are already being felt. In this part 1 of a 3-part series, Dr. John Balbus, senior advisor to the NIEHS Director on public health issues, provides an overview of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment report and the Third National Climate Assessment.