2026 News & Events

An international team of scientists has released the second Tropospheric Ozone Assessment Report (TOAR-II), providing an updated global assessment of ozone pollution, its changing distribution, and its impacts on human health, vegetation, and climate.

While ozone in the stratosphere forms a protective layer that shields us from harmful UV radiation, ozone in the troposphere, the atmospheric layer closest to Earth's surface, acts as both a greenhouse gas and a harmful air pollutant. Near the surface, elevated ground-level ozone concentrations threaten human health and vegetation, and ozone present at higher altitudes in the troposphere has a warming effect on global climate.

The report, coordinated through the International Global Atmospheric Chemistry (IGAC) project, builds on the first Tropospheric Ozone Assessment Report (TOAR-I), completed in 2019. Dr. Owen Cooper of NOAA's Chemical Sciences Laboratory served as a co-chair of the TOAR IGAC activity (2014-2022) and guided the writing of both assessment reports.

A major advancement of TOAR-I was the creation of the world's largest, open-access database of surface measurements of ozone and related pollutants, which was used to assess the long-term trends in global ozone levels and calculate over 30 metrics used for health and vegetation exposure studies.

The TOAR-II report updates trends of tropospheric ozone abundances and ozone metrics, in addition to addressing underlying drivers of ozone trends such as changing precursor emissions. The new assessment examines developments from 2020 to 2026, bringing together findings from 58 review and original research papers and seven comprehensive assessment studies.

Ozone pollution continues to impact human health

The assessment identifies South Asia, eastern China, and parts of the Middle East and North Africa as regions with the highest ground-level ozone concentrations during 2016-2020.

Between 2000 and 2020, ground-level ozone decreased most in southeastern North America and parts of eastern Europe. In contrast, concentrations increased most rapidly in South Asia and sub-Saharan Africa.

The global health implications are substantial. Studies reviewed in TOAR-II estimate that long-term ozone exposure contributes to approximately 500,000 premature deaths annually from chronic obstructive pulmonary disease (COPD). The assessment also finds that ozone-attributable mortality rates increased over the past two decades in 14 of 21 world regions examined.

Widespread risks for crops and forests

Ozone pollution also poses a significant threat to agricultural production and natural ecosystems.

Using improved methods to evaluate ozone exposure and uptake by vegetation, researchers estimate that ozone damage is occurring at well over 60% of TOAR monitoring sites worldwide for sensitive crop and forest species.

The newer ozone-flux-based measures indicate greater potential for vegetation damage and higher risks under present-day conditions than traditional concentration-based measures.

The findings reinforce the importance of reducing surface ozone pollution, which could help protect staple crop production and improve the ability of forest ecosystems to absorb and store carbon.

Ozone's influence on climate is changing

TOAR-II also provides an improved understanding of the global tropospheric ozone burden and its relationship with climate change. The increase in tropospheric ozone since the pre-industrial era (approximately 40% since the year 1750) is the third most important greenhouse gas contributing to recent climate change, after carbon dioxide and methane.

By harmonizing 15 satellite data records, researchers estimated the present-day global tropospheric ozone burden at 336 million metric tons, with an uncertainty of 6 million tons.

Satellite observations show ozone increases of 2% to 4.5% per decade between 2005 and 2019 across approximately 20% of Earth's surface, particularly in Asia. Widespread, but temporary, declines followed in 2020, influenced by changes in emissions during the COVID-19 pandemic.

The assessment also finds that the redistribution of ozone precursor emissions toward the equator has contributed significantly to rising global ozone levels, leading to greater ozone accumulation in tropical and subtropical regions.

First assessment of ozone transport from the stratosphere

One major advancement in TOAR-II is the first dedicated assessment of stratosphere-troposphere exchange (STE), the process through which ozone moves between the stratosphere and troposphere.

STE is an important natural source of ozone in the troposphere and can influence ground-level concentrations under certain atmospheric conditions. The new assessment estimates that approximately 380 million metric tons of ozone are transported through this process globally each year.

Researchers found that STE declined by approximately 2.7% per decade during the late 20th century, largely driven by ozone depletion in the stratosphere.

However, that trend is expected to reverse. Future projections indicate that STE could increase by approximately 4.7% per decade as the stratospheric ozone layer recovers and global temperatures rise, driving changes in stratospheric circulation.

The projected increase means that the contribution of stratosphere-troposphere exchange to tropospheric ozone is expected to become more significant in the coming decades.

Improving global ozone monitoring remains a priority

Despite significant advances in satellite observations, atmospheric modeling and data collection, TOAR-II identifies substantial gaps in global ozone monitoring. Southeast Asia and the Southern Hemisphere, particularly Africa and Australia/Oceania, remain underrepresented in available observations.

The report emphasizes the need for expanded monitoring networks, improved satellite measurements and better understanding of how wildfires, extreme weather, and changing emissions influence ozone. Together, TOAR-II's findings underscore that tropospheric ozone remains a complex global challenge, shaped by pollution emissions, atmospheric chemistry, and a changing climate.

The findings will help guide the next phase of the international assessment, TOAR-III, scheduled for 2027-2030.