Tropospheric Ozone Assessment Report II: Past and present tropospheric ozone using satellite observations

Satellite observations enable global assessments of ozone in the free troposphere, yet uncertainties in earlier records limited the first Tropospheric Ozone Assessment Report's (TOAR-I) confidence in regional distributions and, especially, decadal trends. Here, we leverage longer and improved satellite records, additional datasets and new harmonization approaches to provide a second, more confident, TOAR assessment of present-day and longer-term changes. We quantify the 2018-2022 climatology and seasonal cycle at 1∘×1∘ resolution and evaluate trends in a combined tropospheric column record since 2005. Summertime maxima exceed 50 DU over the Middle East, and annual-mean values exceed 40 DU from East Asia to the Mediterranean. The present-day global tropospheric ozone burden is 336±6 Tg. Decadal ozone changes are pronouncedly nonlinear: increases of 2-4.5% per decade during 2005-2019 occur over 20% of the globe (high confidence), most notably across Asia, followed by widespread declines after 2020, producing a net decrease since 2005 across 70% of the globe. Dynamical and episodic drivers, such as El Niño-Southern Oscillation, extreme wildfires, and the COVID-19 pandemic, strongly influenced variability, underscoring the sensitivity of tropospheric ozone to both natural and anthropogenic influences and the critical need for sustained, coordinated satellite monitoring. This article is part of the theme issue 'Tropospheric Ozone Assessment Report II'.