The proportion of an emission remaining in the atmosphere after a specified time is the "airborne fraction" (AF). The annual airborne fraction is the ratio of the atmospheric increase in a given year to that year's total emissions. The annual airborne fraction for  had been stable at 0.45 for the past six decades even as the emissions have been increasing. This means that the other 0.55 of emitted  is absorbed by the land and atmosphere carbon sinks within the first year of an emission. In the high-emission scenarios, the effectiveness of carbon sinks will be lower, increasing the atmospheric fraction of  even though the raw amount of emissions absorbed will be higher than in the present.