Tropospheric ozone changes, radiative forcing and attribution to emissions in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP)

Abstract

Ozone (O₃) from 17 atmospheric chemistry models taking part in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) has been used to calculate tropospheric ozone radiative forcings (RFs). All models applied a common set of anthropogenic emissions, which are better constrained for the present-day than the past. Future anthropogenic emissions follow the four Representative Concentration Pathway (RCP) scenarios, which define a relatively narrow range of possible air pollution emissions. We calculate a value for the pre-industrial (1750) to present-day (2010) tropospheric ozone RF of 410 mW m⁻². The model range of pre-industrial to present-day changes in O₃ produces a spread (±1 standard deviation) in RFs of ±17%. Three different radiation schemes were used – we find differences in RFs between schemes (for the same ozone fields) of ±10%. Applying two different tropopause definitions gives differences in RFs of ±3%. Given additional (unquantified) uncertainties associated with emissions, climate-chemistry interactions and land-use change, we estimate an overall uncertainty of ±30% for the tropospheric ozone RF. Experiments carried out by a subset of six models attribute tropospheric ozone RF to increased emissions of methane (44±12%), nitrogen oxides (31 ± 9%), carbon monoxide (15 ± 3%) and non-methane volatile organic compounds (9 ± 2%); earlier studies attributed more of the tropospheric ozone RF to methane and less to nitrogen oxides. Normalising RFs to changes in tropospheric column ozone, we find a global mean normalised RF of 42 mW m⁻² DU⁻¹, a value similar to previous work. Using normalised RFs and future tropospheric column ozone projections we calculate future tropospheric ozone RFs (mW m⁻²; relative to 1750) for the four future scenarios (RCP2.6, RCP4.5, RCP6.0 and RCP8.5) of 350, 420, 370 and 460 (in 2030), and 200, 300, 280 and 600 (in 2100). Models show some coherent responses of ozone to climate change: decreases in the tropical lower troposphere, associated with increases in water vapour; and increases in the sub-tropical to mid-latitude upper troposphere, associated with increases in lightning and stratosphere-to-troposphere transport. Climate change has relatively small impacts on global mean tropospheric ozone RF

Tropical tropospheric ozone columns from nadir retrievals of GOME-1/ERS-2, SCIAMACHY/Envisat, and GOME-2/MetOp-A (1996–2012)

“…In the UT, ozone is an important greenhouse gas with an estimated global-averaged radiative forcing of 0.40±0.20 W m −2 (Stevenson et al, 2013;IPCC, 2013), and it contributes greatly to the oxidation efficiency of the atmosphere. Ozone is the primary tropospheric source of hydroxyl (OH) and peroxy (RO x ) radicals, which affect the lifetime of other greenhouse gases such as methane CH 4 (Crutzen, 1974;Shindell et al, 2009;Cooper et al, 2014).…”

Key drivers of ozone change and its radiative forcing over the 21st century

“…We calculate a value of f of 1.43 which is at the mid-upper end of the literature range (1.19-1.53) (Prather et al, 2001;Stevenson et al, 2013;Voulgarakis et al, 2013) and within 7 % of the observationally constrained best estimate of 1.34 (Holmes et al, 2013). 10…”

“…Other modelling studies focused on the radiative effects of mainly tropospheric ozone changes under future emission scenarios in a non processed-based fashion (e.g. Gauss et al, 2003;Stevenson et al, 2013). One study has recently identified the indirect tropospheric and stratospheric ozone RF between 2000 and 2100 due to individual perturbations (Banerjee et al, 2017) Here we aim to narrow this gap by assessing how key factors drive net ozone radiative forcing, and provide a gauge of the uncertainty arising from non-linearities and long-term feedbacks.…”

“…Climate feedbacks, 25 land-use change, natural ozone precursor emissions, and future changes in the structure of the tropopause (Wilcox et al, 2012) may introduce at least an additional ±20 % uncertainty . Following Stevenson et al (2013), we assume that the above individual uncertainties are independent and combine them to estimate an overall uncertainty of ±30 %, which represents the 95 % confidence 30 interval. Note that Skeie et al (2011) from an independent analysis estimated the same overall uncertainty.…”

“…Prather et al, 2011), although the chosen definition may affect the resulting analysis (e.g. Wild, 2007;Stevenson et al, 2013;. 10 Stratospheric ozone is expected to recover to pre-industrial levels during the 21st century due to the implementation of the Montreal Protocol and its Amendments and Adjustments (WMO, 2014), as ODSs concentrations slowly decrease in the atmosphere (e.g.…”