ABSTRACT
Greenhouse gas concentrations in the atmosphere must be stabilized at a level somewhere between 450 and 500ppm if these extreme risks to the US and the rest of the world from climate change are to be avoided. To achieve this, within this decade carbon dioxide emissions must begin falling by about 3 per cent per year and do so over the next 50 years. By then, emissions must be about 80 per cent below current levels. This crucial trajectory requires a sharp break with our past dependence on fossil fuels.
