Urban trees can affect climate change through the direct removal, or sequestration, of carbon dioxide�the dominant greenhouse gas�from the atmosphere. Trees act as a sink for carbon dioxide by fixing carbon during photosynthesis and storing carbon as biomass. Forest Service scientists quantified carbon storage and sequestration by urban trees in the United States to assess the magnitude and role of urban forests in relation to climate change. Total tree carbon storage in U.S. urban areas (2005) is estimated at 643 million metric tons ($50.5 billion value) with annual carbon sequestration estimated at 25.6 million tons ($2.0 billion value). A better understanding and accounting of urban ecosystems can be used to develop management plans and national policies that can significantly improve environmental quality and human health across the nation.