Mitigation Option

Net Implementation Cost

Potential Emission Mitigation (t CO2 equivalent per year)

Low stratospheric soot

Low

8 billion to 25 billion

Low stratospheric dust, aircraft delivery

Low

8 billion to 80 billion

Stratospheric dust (guns or balloon lift)

Low

4 trillion or amount desired

Cloud stimulated by provision of cloud condensation nuclei

Low

4 trillion or amount desired

Stimulation of ocean biomass with iron

Low to moderate

7 billion or amount desired

Stratospheric bubbles (multiple balloons)

Low to moderate

4 trillion or amount desired

Space mirrors

Low to moderate

4 trillion or amount desired

Atmospheric CFC removal

Unknown

Unknown

NOTE: The feasibility and possible side-effects of these geoengineering options are poorly understood. Their possible effects on the climate system and its chemistry need considerably more study and research. They should not be implemented without careful assessment of their direct and indirect consequences.

Cost-effectiveness estimates are categorized as either savings (for less than 0), low (0 to $9/t CO2 equivalent), moderate ($10 to $99/t CO2 equivalent), or high (>$100/t CO2 equivalent). Potential emission savings (which in some cases include not only the annual emissions, but also changes in atmospheric concentrations already in the atmosphere—stock) for the geoengineering options are also shown. These options do not reduce the flow of emissions into the atmosphere but rather alter the amount of warming resulting from those emissions. Mitigation options are placed in order of cost-effectiveness.

The CO2-equivalent reductions are determined by calculating the equivalent reduction in radiative forcing.

Here and throughout this report, tons are metric.