This report outlines how residents of cities are responsible for as much as 80% of global greenhouse gas emissions while at the same time facing significant impacts from climate change. It says that up to 80 percent of the expected $80 billion to $100 billion per year in climate change adaptation costs will likely be borne by urban areas. Nevertheless, says the report, climate change offers cities opportunities to alter course, implement smart policies, and develop sustainable communities. Well managed, dense cities are also shown to be the most important pre-requisite to mitigation of GHG emissions and overall sustainable development.