The scientific evidence is unequivocal. The Earth is warming due to the dramatic increase of concentrations of heat-trapping gases in the atmosphere caused by human activities, especially the burning of fossil fuels. As more greenhouse gases are released into the atmosphere, the Earth will continue to warm in the decades and centuries ahead. The impacts of warming can already be observed throughout the United States, from rising sea levels to droughts, wildfires and extreme rainfall. Climate change is already affecting ecosystems, freshwater supplies, and public health around the world. Although some climate change in the coming decades is now unavoidable, much worse impacts can be avoided and our efforts to adapt made easier by substantially reducing the amount of heat-trapping gases released into the atmosphere. Preparing for the impacts that are unavoidable will make our economy, natural systems and society more resilient and minimize the greatest costs of climate change.