Historical and future changes in air pollutants from CMIP6 models

Turnock, Steven T. ; Allen, Robert J. ; Andrews, Martin ; Bauer, Susanne E. ; Deushi, Makoto ; Emmons, Louisa ; Good, Peter ; Horowitz, Larry ; John, Jasmin G. ; Michou, Martine ; Nabat, Pierre ; Naik, Vaishali ; Neubauer, David ; O'Connor, Fiona M. ; Olivié, Dirk ; Oshima, Naga ; Schulz, Michael ; Sellar, Alistair ; Shim, Sungbo ; Takemura, Toshihiko ; Tilmes, Simone ; Tsigaridis, Kostas ; Wu, Tongwen ; Zhang, Jie

Année de publication
2020
Résumé
<p align=justify>Poor air quality is currently responsible for large impacts on human health across the world. In addition, the air pollutants ozone (<span class="inline-formula">O<sub>3</sub></span>) and particulate matter less than 2.5 <span class="inline-formula">µm</span> in diameter (<span class="inline-formula">PM<sub>2.5</sub></span>) are also radiatively active in the atmosphere and can influence Earth's climate. It is important to understand the effect of air quality and climate mitigation measures over the historical period and in different future scenarios to ascertain any impacts from air pollutants on both climate and human health. The Coupled Model Intercomparison Project Phase 6 (CMIP6) presents an opportunity to analyse the change in air pollutants simulated by the current generation of climate and Earth system models that include a representation of chemistry and aerosols (particulate matter). The shared socio-economic pathways (SSPs) used within CMIP6 encompass a wide range of trajectories in precursor emissions and climate change, allowing for an improved analysis of future changes to air pollutants. Firstly, we conduct an evaluation of the available CMIP6 models against surface observations of <span class="inline-formula">O<sub>3</sub></span> and <span class="inline-formula">PM<sub>2.5</sub></span>. CMIP6 models consistently overestimate observed surface <span class="inline-formula">O<sub>3</sub></span> concentrations across most regions and in most seasons by up to 16 <span class="inline-formula">ppb</span>, with a large diversity in simulated values over Northern Hemisphere continental regions. Conversely, observed surface <span class="inline-formula">PM<sub>2.5</sub></span> concentrations are consistently underestimated in CMIP6 models by up to 10 <span class="inline-formula">µg m<sup>-3</sup></span>, particularly for the Northern Hemisphere winter months, with the largest model diversity near natural emission source regions. The biases in CMIP6 models when compared to observations of <span class="inline-formula">O<sub>3</sub></span> and <span class="inline-formula">PM<sub>2.5</sub></span> are similar to those found in previous studies. Over the historical period (1850-2014) large increases in both surface <span class="inline-formula">O<sub>3</sub></span> and <span class="inline-formula">PM<sub>2.5</sub></span> are simulated by the CMIP6 models across all regions, particularly over the mid to late 20th century, when anthropogenic emissions increase markedly. Large regional historical changes are simulated for both pollutants across East and South Asia with an annual mean increase of up to 40 <span class="inline-formula">ppb</span> for <span class="inline-formula">O<sub>3</sub></span> and 12 <span class="inline-formula">µg m<sup>-3</sup></span> for <span class="inline-formula">PM<sub>2.5</sub></span>. In future scenarios containing strong air quality and climate mitigation measures (ssp126), annual mean concentrations of air pollutants are substantially reduced across all regions by up to 15 <span class="inline-formula">ppb</span> for <span class="inline-formula">O<sub>3</sub></span> and 12 <span class="inline-formula">µg m<sup>-3</sup></span> for <span class="inline-formula">PM<sub>2.5</sub></span>. However, for scenarios that encompass weak action on mitigating climate and reducing air pollutant emissions (ssp370), annual mean increases in both surface <span class="inline-formula">O<sub>3</sub></span> (up 10 <span class="inline-formula">ppb</span>) and <span class="inline-formula">PM<sub>2.5</sub></span> (up to 8 <span class="inline-formula">µg m<sup>-3</sup></span>) are simulated across most regions, although, for regions like North America and Europe small reductions in <span class="inline-formula">PM<sub>2.5</sub></span> are simulated due to the regional reduction in precursor emissions in this scenario. A comparison of simulated regional changes in both surface <span class="inline-formula">O<sub>3</sub></span> and <span class="inline-formula">PM<sub>2.5</sub></span> from individual CMIP6 models highlights important regional differences due to the simulated interaction of aerosols, chemistry, climate and natural emission sources within models. The projection of regional air pollutant concentrations from the latest climate and Earth system models used within CMIP6 shows that the particular future trajectory of climate and air quality mitigation measures could have important consequences for regional air quality, human health and near-term climate. Differences between individual models emphasise the importance of understanding how future Earth system feedbacks influence natural emission sources, e.g. response of biogenic emissions under climate change.</p>
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