"As a key input into the Fourth National Climate Assessment (NCA4), the U.S. Global Change Research Program (USGCRP) oversaw the production of this special, stand-alone report of the state of science relating to climate change and its physical impacts. This report is designed to be an authoritative assessment of the science of climate change, with a focus on the United States, to serve as the foundation for efforts to assess climate-related risks and inform decision-making about responses. In accordance with this purpose, it does not include an assessment of literature on climate change mitigation, adaptation, economic valuation, or societal responses, nor does it include policy recommendations. The Climate Science Special Report (CSSR) serves several purposes for NCA4, including providing 1) an updated detailed analysis of the findings of how climate change is affecting weather and climate across the United States; 2) an executive summary that will be used as the basis for the science summary of NCA4; and 3) foundational information and projections for climate change, including extremes, to improve ?end-to-end” consistency in sectoral, regional, and resilience analyses for NCA4. As an assessment and analysis of the science, this report provides important input to the development of NCA4 and its primary focus on the human welfare, societal, economic and environmental elements of climate change..."