Vegetation feedbacks during drought exacerbate ozone air pollution extremes in Europe

Abstract

Reducing surface ozone to meet the European Union’s target for human health has proven challenging despite stringent controls on ozone precursor emissions over recent decades. The most extreme ozone pollution episodes are linked to heatwaves and droughts, which are increasing in frequency and intensity over Europe, with severe impacts on natural and human systems. Here, we use observations and Earth system model simulations for the period 1960–2018 to show that ecosystem–atmosphere interactions, especially reduced ozone removal by water-stressed vegetation, exacerbate ozone air pollution over Europe. These vegetation feedbacks worsen peak ozone episodes during European mega-droughts, such as the 2003 event, offsetting much of the air quality improvements gained from regional emissions controls. As the frequency of hot and dry summers is expected to increase over the coming decades, this climate penalty could be severe and therefore needs to be considered when designing clean air policy in the European Union.

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Fig. 1: Correlations between ozone air quality and temperature in Europe.
Fig. 2: Changes in European ozone air quality.
Fig. 3: Reduced ozone removal by forests under drought stress.
Fig. 4: Declining ozone removal by water-stressed vegetation in a warming climate.
Fig. 5: Reduced uptake by plants worsens ozone air pollution extremes.
Fig. 6: Ecosystem–atmosphere interactions exacerbate climate penalty on ozone extremes.

Data availability

Ozone flux measurements, the ozone climate penalty factors derived from observations and model simulations generated in this study are archived at a public data repository at NOAA GFDL (ftp://data1.gfdl.noaa.gov/users/Meiyun.Lin/Nature2020/). Ozone deposition velocities from LM4.0 are archived at ftp://data1.gfdl.noaa.gov/users/Meiyun.Lin/GBC2019/GFDL‐LM4/. Source data for Figs. 1–6 and Extended Data Figs. 1–10 are provided with the paper.

Code availability

The computer code for the standard versions of GFDL’s atmospheric and land models is publicly available at https://www.gfdl.noaa.gov/atmospheric-model/. Other codes used in this study are available from the corresponding author upon reasonable request.

Change history

  • 17 June 2020

    An amendment to this paper has been published and can be accessed via a link at the top of the paper.

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Acknowledgements

This report was prepared by M. Lin under awards NA14OAR4320106 and NA18OAR4320123 from the National Oceanic and Atmospheric Administration (NOAA), US Department of Commerce. The statements, findings, conclusions and recommendations are those of the authors and do not necessarily reflect the views of NOAA. We thank GFDL internal reviewers, K. Dixon and J. Krasting, for constructive comments, which have helped to strengthen the article.

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Authors

Contributions

M.L. conceived this study, performed the model experiments and analysis, and wrote the article. M.L., L.W.H. and E.S. designed the model experiments. Y.X. performed the ozone–temperature regression analysis under the supervision of M.L. F.P., M.L., S.M. and E.S. developed the dry deposition scheme. A.F., G.G. and K.P. provided ozone flux measurements. D.K. provided surface ozone measurements at Hohenpeissenberg. All authors contributed to discussions and edited the manuscript.

Corresponding author

Correspondence to Meiyun Lin.

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The authors declare no competing interests.

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Peer review information Nature Climate Change thanks Elena McDonald-Buller and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Ozone-temperature relationships.

Scatter plots of observed June-August mean MDA8 ozone anomalies (relative to 19802000) at Hohenpeissenberg and Zugspitze and observed June-August mean Tmax anomalies averaged over 42°–53°N and 0°–15°E, with linear regression fits using the Ordinary Least Squares (OLS, blue) and Reduced Major Axis (RMA, red) methods, respectively. The OLS regression slopes are reported in Fig. 1 in the main article. Source data

Extended Data Fig. 2 Trends in ozone precursor emissions.

a, b, Trends in anthropogenic emissions of carbon monoxide and non-methane volatile organic compounds (NMVOCs) in Europe (40–60N; 10W–25E) from the CMIP6 historical dataset used by the model. c, Observed trends in global average methane mixing ratios used by the model. d, Model estimated trends in biogenic isoprene emissions over Europe (40°–60°N;10°W–25°E). Source data

Extended Data Fig. 3 Surface ozone trends.

Maps of the 1990–2015 trends in daily MDA8 ozone for the 95th and 50th percentiles in July and August from observations (top) and IAVDEPV simulations (bottom). Results are shown for EMEP sites with at least 20 years of data, with larger circles indicating sites with significant ozone trends (p < 0.05). The percentage of sites with significant trends are reported at the top-left corner of each graph. Source data

Extended Data Fig. 4 Ozone pollution during the 2015 and 2018 heatwaves.

(Top) Maps of observed daily maximum temperature anomalies in June-August of 2015 and 2018 relative to the base period 19611990, with dots indicating area in drought (SPEI06 <−1). (Middle) The annual 4th highest MDA8 ozone concentrations from all available observations gridded at 0.5° resolution, with values above 70 ppb implying an exceedance of the health limit set by the U.S. Environmental Protection Agency. (Bottom) The annual 26th highest MDA8 ozone concentrations, with values above 60 ppb implying an exceedance of the health limit set by the European Union.

Extended Data Fig. 5 Evolution of drought events.

The Standardized Precipitation-Evapotranspiration Index (SPEI) integrated over the preceding 6 months, 2 months, and 1 month for August 2003, July 1994 and July 2006.

Extended Data Fig. 6 Land use.

a, Fraction of the four land use categories in each grid cell averaged over 20002015: Natural forests (lands undisturbed by human activities), secondary vegetation (lands harvested at least once, including managed forests and abandoned cropland and pasture), croplands, and pastures. b, Changes in 20002015 relative to the 1960s. The box denotes the area used for averaging in Extended Data Fig. 7.

Extended Data Fig. 7 Declining ozone removal by vegetation due to stomatal closure under soil drying as opposed to land use changes.

a, b, Evolution of land use over western Europe (5°W–25°E and 40°–55°N): total land areas and area-weighted leaf area indices for natural forests (dark green), secondary vegetation (green), croplands (orange), and pastures (blue). c, Evolution of June-August mean daytime ozone deposition velocities for the four land use types (area-weighted). d, Total (solid green lines) and stomatal (dashed green lines) ozone deposition velocities averaged over natural and secondary vegetation land areas. The vertical bars show the percentage of land areas in drought (SPEI06<−1; right axis). Source data

Extended Data Fig. 8 Climate-driven trends in surface ozone over Europe.

Maps of the 19792014 and 19902014 trends in the 95th and 50th percentile MDA8 ozone concentrations for July (a) and August (b), simulated by the IAVDEPV_FIXEM experiment with anthropogenic emissions held constant at 1980 levels. Stippling denotes areas where the change is statistically significant at the 95% confidence level (p < 0.05).

Extended Data Fig. 9 Observed trends in hot extremes over Europe.

Maps of the 1979–2019 trends in the frequency of warm days (that is, those above the 90th percentile for the base period 1961–1990) in July (a) and August (b), respectively, obtained from the Global Land-Based Datasets for Monitoring Climate Extremes (Methods). Stippling denotes areas where the change is statistically significant (p < 0.05).

Extended Data Fig. 10 Drivers of decadal mean ozone trends in Europe.

Changes in decadal mean ozone levels during spring (March-May) and summer (June-August) from 19791989 to 20012010 as inferred from surface observations at Hohenpeissenberg (985 m altitude, MDA8 values), from alpine observations at Zugspitze (2962 m altitude, 24-hour mean), and from 1990–2000 to 2001–2010 at 52 EMEP sites over 40°N–55°N with continuous observations (MDA8 values). For observations, both changes in decadal mean (grey bars) and median (circles) values are shown, with the error bars indicating the range of the mean change at the 95% confidence level. Model results are shown for the BASE (total; red bars) and IAVDEPV_FIXEM (climate-driven trends; green bars) experiments and the contributions from changes in Asian anthropogenic emissions (purple bars), global methane concentrations (cyan bars), and wildfire emissions (yellow bars). For comparisons with free tropospheric observations at the Zugspitze, model results are sampled at 700 hPa. Source data

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Lin, M., Horowitz, L.W., Xie, Y. et al. Vegetation feedbacks during drought exacerbate ozone air pollution extremes in Europe. Nat. Clim. Chang. 10, 444–451 (2020). https://doi.org/10.1038/s41558-020-0743-y

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