Comparing radiative forcing measures for ozone

Ozone is the third most important greenhouse gas, contributing a forcing of 0.47 W/m2 over the historical period. All assessments of ozone forcing so far have used the stratospheric-temperature adjusted radiative forcing (SARF) calculated by offline radiative transfer models. The two most recent IPCC reports have recommended the use of effective radiative forcing (ERF) as the preferred measure of forcing, but no calculations have been available.

For the first time we calculate the future ozone online ERF from six Earth system models and compare this to the SARF calculations. The future ozone calculations are for the SSP3-7.0 scenario for the year 2050. Only the ozone changes (and any consequent impacts on meteorology) are included in the radiative forcing calculations. We find an ERF of 0.27+/- 0.09 Wm-2  and an ozone column increase of 12 DU. Approximately half of the forcing change comes from ozone recovery following the decline in halocarbons.

By decomposing the radiative forcing into instantaneous (IRF), stratospheric-temperature adjusted (SARF) and effective (ERF) radiative forcing we gain insights into the adjustment processes causing the differences between the radiative forcing measures. The ERF is typically larger than the SARF. This is mostly due to positive non-cloud adjustments through increased water vapour (particularly in the stratosphere) and decreased surface albedo. Reductions in high and mid-level cloud increase the short-wave forcing, but decrease the long-wave forcing. The adjustments to the forcing depend on the altitude of the ozone change, with adjustments to ozone changes following reductions in ozone-depleting substances being more strongly positive than those following increases in ozone precursors.