Ozone (O3) is an optically active gas that plays a key role in the Earth's atmosphere. It absorbs and emits long-wave (LW) infrared radiation and absorbs short-wave (SW) solar radiation in the ultraviolet and visible spectrum. Although 90% of ozone is is located in the stratosphere, changes in tropospheric ozone have a greater impact on radiative forcing, according to long-term findings. The new study aims to assess three different measures of radiative forcing (instantaneous: IRF; stratospheric temperature-adjusted: SARF; effective: ERF) for future ozone changes, while separating the effects of changes in ozone precursors and ozone-depleting substances (ODS).

Radiation exposure as a key metric

Radiative forcing is a useful metric in climate science that provides an initial estimate of the potential climatic significance of different forcing mechanisms.

• Immediate Radiation Force (IRF) is the simplest definition, representing the change in radiative fluxes caused by a disturbance in the composition of the atmosphere without any other changes.
• Stratospheric temperature-adjusted radiative forcing (SARF) involves the adjustment of temperatures in the stratosphere that occurs over a period of months in response to changes in radiative heating.
• Effective radiative forcing (ERF) is defined as the energy imbalance at the top of the atmosphere (TOA) when the change in global mean surface temperature is zero. This definition implicitly includes consequential changes in the climate system, such as changes in atmospheric temperatures, water vapor, cloud cover, and surface albedo, which are independent of the change in global mean surface temperature. The study highlights that ERF is a more appropriate metric for diagnosing the climate effects of changes in stratospheric ozone.

Key findings for the SSP3-7.0 scenario (2015-2050)

The study focused on the Shared Socioeconomic Pathway 3-7.0 (SSP3-7.0) scenario, which assumes low levels of air pollution emission controls. It found robust increases in ozone due to future increases in ozone precursors and decreases in ozone-depleting substances (ODS).

• Ozone increase: The global average of total column ozone (TCO) is expected to increase by 12.2 ± 5.2 DU in the period from 2015 to 2050. Of this 4.3 ± 1.0 DU (approximately 39 %) is due to an increase in tropospheric ozone (TrCO). The largest increases in TrCO are expected over the Middle East, India and Southeast Asia.
• Radiation exposure: This leads to an increase in radiation exposure between 2015 and 2050 by 0.268 ± 0.084 W m−2 ERFThis increase makes ozone second largest contributor to future warming by 2050 in this scenario.
  • Approximately half of this increase is due to stratospheric ozone recovery and half to tropospheric ozone precursors.
  • The ODS contribution to the EDF is 0.156 ± 0.071 W m−2, while the contribution of tropospheric ozone precursors is 0.136 ± 0.035 W m−2This result suggests that the efficiency of the radiative forcing of ODS changes is almost three times higher than assumed in previous assessments.

Climate system modifications

The study also analysed the "quick fixes" that are included in the EDF:

• Cloud changes: It has been found that the increase in ozone leads to decreasing cloud cover, especially in the upper troposphere. This causes an overall negative adjustment of the radiative forcing (positive in the shortwave region, negative in the longwave region). However, the overall cloud effects largely cancel each other out.
• Changes in water vapor and albedo: The adjustments related to changes in water vapor and albedo are positive. It was observed robust decrease in surface albedo, which translates into a positive contribution of SW to the EDF.

Comparison of methodologies

The study compared radiative forcing calculations from global atmosphere/earth system models (online methods) and using radiative cores (offline methods). Although online ERFs and SARFs are similar for the combined effects of ozone precursors and ODS, this is not the case when they are considered separately. For example, the SARF calculated by the core is two times lower than the ERF for the ozone radiative forcing from ODS changes, highlighting the need to compare offline radiative transfer models with comprehensive Earth system models.

Conclusions for political decision-making

This one study provides important information for policymaking because it focuses on future changes. It suggests that the increase in radiative forcing expected from ODS reduction commitments could offset most of the climate benefits of reducing their direct greenhouse effect. Caution is therefore needed when interpreting and comparing radiative forcing calculations for ozone, especially when considering the separate effects of ozone precursors and ODS. JRi