Abstract
Radiative forcing (RF) time series for total ozone from 1850 up to the present day are calculated based on historical simulations of ozone from 10 climate models contributing to the Coupled Model Intercomparison Project Phase 6 (CMIP6). In addition, RF is calculated for ozone fields prepared as an input for CMIP6 models without chemistry schemes and from a chemical transport model simulation. A radiative kernel for ozone is constructed and used to derive the RF. The ozone RF in 2010 (2005–2014) relative to 1850 is 0.35 W m−2 [0.08–0.61] (5–95% uncertainty range) based on models with both tropospheric and stratospheric chemistry. One of these models has a negative present-day total ozone RF. Excluding this model, the present-day ozone RF increases to 0.39 W m−2 [0.27–0.51] (5–95% uncertainty range). The rest of the models have RF close to or stronger than the RF time series assessed by the Intergovernmental Panel on Climate Change in the fifth assessment report with the primary driver likely being the new precursor emissions used in CMIP6. The rapid adjustments beyond stratospheric temperature are estimated to be weak and thus the RF is a good measure of effective radiative forcing.
Highlights
Tropospheric ozone has increased since pre-industrial times primarily due to photochemical reactions involving components emitted to the atmosphere by anthropogenic activities1
As Coupled Model Intercomparison Project Phase 6 (CMIP6) model runs do not provide direct means of calculating time-varying radiative forcing (RF) due to anthropogenic changes in ozone, we calculate ozone forcing in this study based on the CMIP6 simulations over the historical period for those models that include tropospheric and/or stratospheric chemistry, using a radiative kernel
This study includes RF estimates using the OsloCTM3, a chemical transport model (CTM) driven by the same emission inventory as used in CMIP6, and RF estimates based on the ozone fields from input4MIPs prepared for CMIP6
Summary
Tropospheric ozone has increased since pre-industrial times primarily due to photochemical reactions involving components emitted to the atmosphere by anthropogenic activities. Tropospheric ozone has increased since pre-industrial times primarily due to photochemical reactions involving components emitted to the atmosphere by anthropogenic activities1 Such emissions include methane (CH4), nitrogen oxides (NOx), carbon monoxide (CO), and non-methane volatile organic components (NMVOCs), collectively referred to as ozone precursors. Ozone is a greenhouse gas and considered as a “near-term climate forcer” due to its short global mean lifetime of ~23 days in the troposphere for the present day. Ozone is a greenhouse gas and considered as a “near-term climate forcer” due to its short global mean lifetime of ~23 days in the troposphere for the present day4 It is not well-mixed in the atmosphere and that challenges the estimation of the historical development of ozone in the atmosphere. Due to insufficient long-term observations, the time series of the historical development of ozone and its radiative forcing (RF) since pre-industrial times must be estimated using models
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