The overwhelming majority of climate scientists have repeatedly warned of a threshold in atmospheric concentrations of carbon dioxide beyond which a run-away warming effect could take place, with a large possibility of causing irreversible damage. With such a risk, a quantity instrument may be a better choice because the quantity of emissions may be capped with more certainty. However, this may not be true if this risk exists but cannot be attached to a known level of greenhouse gas (GHG) concentration or a known emission pathway.