Tropospheric Ozone Assessment Report II: ozone interactions with climate change: drivers and long-term trends

This Tropospheric Ozone Assessment Report Phase-II (TOAR-II) study has assessed the current state of knowledge regarding ozone's important role as a short-lived climate forcer (SLCF), as well as the influence of climate change on global tropospheric ozone, relying on new studies published within the past five years. Models estimate that the tropospheric ozone burden has increased substantially since the pre-industrial era, supported by available measurements, contributing to a radiative climate forcing from total column ozone of 0.50 (0.25 to 0.75) W m-2 for 1750-2024, making ozone the third most important anthropogenic greenhouse gas (GHG). Since 1990, surface, airborne and satellite measurements, as well as models, have suggested a continued rise in the tropospheric burden, although ground-level ozone is decreasing in some regions. This is driven, in part, by the continued rise of methane, despite estimates that global anthropogenic NOx emissions may have peaked recently. Climate change influences ozone through a variety of physical, biological and chemical processes. Increasing temperatures are expected to reduce the global average surface ozone owing to increased water vapour, but increase ozone in polluted regions. Future surface and free tropospheric ozone concentrations will be determined by a complex interplay of emission changes, as well as by climate change and land-biosphere feedbacks. This article is part of the theme issue 'Tropospheric Ozone Assessment Report II'.