It is intended that the Bill will establish an economically credible emissions reduction pathway to 2050, by putting into statute medium and long-term targets. These targets already exist on a non-statutory basis. In addition, a system of carbon budgeting is proposed.

This means that for successive five year periods, starting with the period 2008-12, there will be a limit on total carbon dioxide emissions.

The Bill proposes that carbon budget periods be set at least three periods (i.e. for fifteen years) ahead. This approach provides for both certainty and flexibility in the system: emissions can vary between years provided the total over a five year period does not exceed the budget.

The series of five year carbon budgets will provide a trajectory from now to 2050, thereby providing a clear framework of expected emissions reductions over time.

http://www.defra.gov.uk/environment/climatechange/uk/legislation