A carbon debt occurs when an activity intended to reduce carbon dioxide (or other greenhouse gas) emissions initially increases emissions. This temporary increase in emissions is the carbon debt. Any carbon debt must be offset or ‘repaid’ before an emission reduction activity produces a net reduction in emissions. The archetypical example is the carbon dioxide emissions from land use change when land is cleared to produce biofuels. Although biofuels can be used to offset fossil fuels and reduce greenhouse gas emissions, there are no net emissions reductions until the carbon debt has been repaid – which may take decades or centuries. More generally, a carbon debt is incurred when initiating any renewable energy development that generates emissions from land use change or construction of equipment and facilities.