How Climate Change Could Make Smog Harder to Control in North America and Europe

An MIT team led by Emmie Le Roy and Noelle Selin used detailed climate–air‐chemistry modeling to assess how global warming could affect efforts to reduce ground‐level ozone (“smog”). 

“Average climate isn’t the only thing that matters,” said Le Roy. “Extreme hot or stagnant episodes can drive high-ozone days that break air quality targets.”

Their findings, reported May 22, 2025, show that while cutting nitrogen oxides (NOₓ) is a key strategy for reducing ozone pollution, warming soils may emit more NOₓ naturally—offsetting the benefits of human emission reductions. Meanwhile, in Northeast Asia, cuts in industrial NOₓ emissions may become more effective under warming, making regional responses vary sharply. The related MIT report can be found here . 

In particular, eastern North America and Western Europe –where warmer soils emit more nitrogen oxides (NOₓ) – will see less ozone reduction from human NOₓ cuts under a warmer climate. (In other words, emissions of NOₓ from soils increase with warming, so reducing human NOₓ gives smaller ozone gains.) By contrast, northeast Asia exhibits the reverse pattern: industrial NOₓ emissions there produce more ozone per unit NOₓ, so cutting them yields larger ozone benefits even as climate warms.

In all cases the net effect is that future warming tends to raise baseline ozone levels, meaning that more aggressive emission cuts may be needed to meet air quality goals. The MIT news release emphasizes that these results “could help scientists and policymakers develop more effective strategies for improving air quality and human health” by highlighting where climate will blunt conventional pollution controls.

The researchers reached these conclusions by running a series of long-term simulations (80 model years per scenario) with a state-of-the-art Earth system model that couples meteorology and atmospheric chemistry. Unlike many earlier studies that use averaged weather fields, they explicitly simulated year-to year meteorological variability.

“Average climate isn’t the only thing that matters,” Le Roy notes –extreme hot or stagnant episodes can drive high-ozone days that break air quality targets. Their approach thus captures how heatwaves and other anomalies influence ozone. For example, they show that a single extreme‐high‐ozone day (even if anomalous) “could mean we don’t meet our air quality target and have negative human health impacts”.

In short, the model studies indicate that controlling smog in a warming world may require both deeper emissions cuts and accounting for climate variability. The team highlights the need to improve how biosphere–atmosphere interactions (like soil NOₓ emissions) are represented, since these natural fluxes become more important to ozone under warming. Selin adds that future work should expand this approach to other climate-driven factors (e.g. wildfires) because their high variability “really does affect the answer that you get” for ozone policy. 

The Climate Penalty: Why Smog Could Get Worse

Ground-level ozone (O₃) is a secondary pollutant formed by sunlight-driven chemistry. It arises when precursors – primarily nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) from vehicles, industry and other combustion – react in sunlight. (Methane, carbon monoxide and biogenic VOCs play a lesser role.) Because its formation is photochemical, ozone peaks in warm, sunny conditions. Indeed, ozone is often described as the major component of urban smog, and even household ozone generators can elevate local concentrations. Health authorities note that breathing excessive ozone causes respiratory distress, asthma attacks and lung damage. (By contrast, stratospheric ozone is beneficial by blocking UV; here we focus on the harmful “bad” tropospheric ozone). More details can be read here   

Because ozone chemistry is sensitive to temperature, humidity and atmospheric stability, climate change can markedly influence ground-level ozone. Hotter days accelerate the photochemical reactions and cause natural emissions of ozone precursors (like soil-emitted NOₓ and biogenic VOCs) to rise. For example, NASA notes that higher temperatures drive more frequent high-ozone days: “hot, sunny days can increase the amount of ozone at ground level” . Likewise, the European Environment Agency (EEA) reports that ozone is “very sensitive to meteorological and climatic variability” . In Europe, ozone levels vary strongly year-to-year: warm, dry summers and heatwaves tend to produce peak ozone spikes, whereas cloudier or cooler conditions suppress it. In fact, intense heatwaves consistently lead to the highest ozone values in monitoring records. The same pattern holds globally: the fraction of annual ozone that comes from extreme hot days is expected to rise under climate warming. This climate-driven “ozone penalty” – i.e., higher ozone caused by warming – is a well-known concept in air quality research. Climatologists have quantified this effect: for example, modeling studies project that an additional ~3 °C of warming above pre-industrial levels could raise summertime mean ozone by about 1–3 parts per billion (ppb) in highly polluted regions like northern India and eastern China. 

In cleaner or remote regions, however, the opposite can occur: increased water vapor and clouds in a warmer climate actually enhance ozone destruction, yielding a slight decrease in background ozone. In short, a warmer world tends to boost ozone formation in polluted or sunlit regions (the climate penalty), while reducing baseline ozone far from pollution sources (a climate benefit). These physical links between heat and ozone underscore why smog is especially severe during heatwaves and why climate change is expected to complicate ozone control measures. More details can be read here and here  

What the Models Show

The MIT team’s model revealed that:

Eastern North America and Western Europe: Soil NOₓ emissions increase under warming, making human NOₓ cuts less effective.

Northeast Asia: Industrial NOₓ produces more ozone per unit in warm climates, so emission cuts yield larger ozone reductions

Globally: Future warming will likely raise baseline ozone levels, meaning stronger pollution cuts will be needed to meet air quality goals.

Prof. Selin emphasized the need to better represent natural fluxes—like soil NOₓ and vegetation emissions—in future air quality models. She also noted the growing importance of including climate-driven events such as wildfires.

“These highly variable events really do affect the answer that you get when designing ozone policies,” said Selin.

Evidence from Global and Regional Studies

Several major studies echo the MIT findings:

  1. Global modeling (Zanis et al., 2022) – An ensemble of five state-of-the-art Earth System Models (CMIP6/AerChemMIP) isolated the effect of climate change (holding anthropogenic emissions constant) on ozone. Le Roy’s MIT news release cites this study, which found a net “climate benefit” (ozone decline) of about –0.96 ppb/°C on average (mainly from stronger H₂O-driven destruction) . However, the same study reported large regional contrasts: over remote clean areas ozone fell (up to –2 ppb/°C in the tropics), while near major pollution or biogenic-emission sources ozone rose by roughly 0.2–2 ppb/°C . In other words, climate warming makes ozone harder to reduce in polluted regions by increasing the efficiency of ozone production per emission unit. Overall, Zanis et al. concluded that precursor emissions remain the dominant factor for future ozone trends, but climate change introduces a significant regional “penalty” where pollutant levels are high. The study can be read here 
  2. IPCC Sixth Assessment (2021) – The IPCC’s Working Group I report that future surface ozone will largely be driven by precursor emissions (high confidence), with climate effects secondary. Nevertheless, its climate projections echo the climate-penalty concept: “a warmer climate is expected to reduce surface O₃ in regions remote from pollution sources (high confidence) but … increase it by a few parts per billion over polluted regions” (medium–high confidence). In other words, climate change alone produces only a modest global ozone rise or fall, but amplifies ozone in polluted areas. The report notes overall that, under any scenario with little climate mitigation, ozone pollution will tend to worsen across continents through 2100. This assessment aligns with earlier literature:even under strong future emission controls, a warmer world is projected to have higher ozone peaks (though robust cuts could offset most of the climate effect).The IPCC report can be reached here 
  3. Regional projections (Europe) – European model studies have quantified the climate impact on ozone peaks. The EEA cites work (Jacob & Winner 2009; Lin et al. 2020) showing that by mid-century, climate change alone could raise daily maximum ozone by up to 5 µg/m³ on hot summer days, and by ~8 µg/m³ by 2100, compared to pre-industrial .More details can be read here  In practice, however, these projected increases are expected to be smaller than the decreases from planned precursor reductions. Current observations in Europe illustrate the challenge: annual ozone has risen slightly over 2005–2019 even as peak values fell , and in 2020 only 19% of European ozone monitors met the EU target (8h mean ≤120 µg/m³) . The EEA notes that 64% of Europeans were exposed to ozone above the (older) WHO 100 µg/m³ guideline in 2003, and although that fraction fell to 9% by 2014, it has since leveled off (93–98% still exceed the WHO guideline in 2013–2020) . These data imply that climate variability (notably hot summers) plays a dominant role in ozone episodes in Europe
  4. Regional projections (China) – A 2025 study by Kang et al. used a high-resolution regional chemistry–climate model (GCAP 2.0) driven by CMIP6 projections to isolate climate impacts (assuming aggressive NOₓ/VOC cuts). It found that 2010–2045 climate change alone would increase summertime 8h-ozone by about 3–7% across China’s industrial regions. For example, the North China Plain saw ~4.7 ppb (7.3%) higher ozone under future climate. The rise was driven mainly by stronger chemical production due to higher temperatures and sunlight; Kang et al. note that meteorological changes (higher peak temperatures and solar radiation) explain 58–76% of the ozone increase. These results indicate that even under an optimistic carbon-neutral pathway, warming will hinder ozone abatement in East Asia – echoing the MIT finding for northeast Asia. The study can be read here  Similarly, recent analyses of Chinese city data show that compound heat-ozone events have surged: Su et al. (2025) report a 67% increase from 2003–2020 in population exposure to concurrent extreme heat and high ozone in Chinese cities. Northern regions (e.g., North China Plain) saw the strongest upward trends, driven by more frequent heatwaves amplifying ozone production. The study can be read here 
  5. Other global regions – Though less studied in detail, comparable effects are found elsewhere. For example, modeling in North America (Wu et al. 2008) showed that mid-21st century climate change could raise the 95th-percentile ozone (worst days) by ~10 ppb in the Midwest USA. The research can be reached here In India, high baseline ozone and rapid warming suggest a large climate penalty (IPCC/WMO note N. India as a future hotspot). Overall, the pattern is clear: model intercomparisons and observations consistently find that global warming increases ozone peaks in polluted areas while modestly reducing it in pristine areas. The related IPCC report can be found here . Any comprehensive assessment (e.g., the Tropospheric Ozone Assessment Report) emphasizes that future ozone distributions will reflect this interplay of emissions and climate. 

The Policy Message: Smarter, Deeper Cuts Are Needed

In summary, the MIT research and related literature suggest that achieving future air quality goals will require bolder and smarter policies. Emission cuts – especially of NOₓ and other ozone precursors – must be deeper in a warmer world. At the same time, air quality agencies must integrate climate considerations into monitoring and standards. Coordinated action on climate change and smog offers a co-benefit: curbing greenhouse gases and ozone precursors together can simultaneously slow global warming and clean up the air we breathe.

Reference:

https://news.mit.edu/2025/study-climate-change-may-make-it-harder-to-reduce-smog-0522#:~:text=Global%20warming%20will%20likely%20hinder,to%20a%20new%20MIT%20study

https://climate-adapt.eea.europa.eu/en/observatory/evidence/health-effects/ground-level-ozone#:~:text=late%20summer,EEA%2C%202022a

https://acp.copernicus.org/articles/25/5101/2025/#:~:text=ozone%20air%20quality%20due%20to,the%20USA%20show%20evidence%20of

https://www.giss.nasa.gov/pubs/abs/za03100n.html#:~:text=increases%20over%20regions%20close%20to,on%20surface%20ozone%20versus%20the

https://iopscience.iop.org/article/10.1088/1748-9326/ac4a34

https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FGD_Chapter06.pdf#:~:text=44%20Future%20air%20quality%20,by%20a%20few%20parts%20per

https://climate-adapt.eea.europa.eu/en/observatory/evidence/health-effects/ground-level-ozone#:~:text=Future%20climate%20change%20is%20expected,Figure%201

https://acp.copernicus.org/articles/25/3603/2025

https://www.nature.com/articles/s41612-025-00966-5

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2007JD008917

https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FGD_Chapter06.pdf#:~:text=46%20and%20climate%20change%20is,medium

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