To achieve this rate, the total concentration of all greenhouse gases would have to be held to a maximum of less than 550 ppm CO2 equivalent at mid-century. The current level of all emissions is roughly 430 ppm CO2 equivalent. If emissions of all greenhouse gases were to be held at 550 ppm CO2 equivalent, CO2 emissions alone would have to level off at 450 to 500 ppm. This compares to current levels of 360 ppm, and projections of over 600 ppm by late in the next century under a business as usual scenario. Complicating stabilization is the fact that many greenhouse gases remain in the atmosphere for a long time.
Carbon dioxide remains for a century or more.
Even if CO 2 emissions were kept at near 1994 levels, there would be a nearly constant rate of increase in atmospheric concentrations for at least two centuries, reaching about 500 ppm by the end of the 21st Century. This is the upper end of the safe level described above. Stabilizing concentrations of the other greenhouse gases also requires significant emission reductions. Like carbon dioxide, nitrous oxide remains in the atmosphere for decades to centuries. Stabilization near current levels would require a 50 percent reduction in anthropogenic sources immediately.