A 3 °C global RCP8.5 emission trajectory cancels benefits of European emission reductions on air quality

Despite the international agreement to reduce global warming to below 2 °C, the Intended Nationally Determined Contributions submitted for the COP21 would lead to a global temperature rise of about 3 °C. The relative consequences of such a one-degree additional warming have not yet been investigated for regional air quality. Here we found that a + 3 °C global pollutant emission trajectory with respect to pre-industrial climate (reached along the 2040–2069 period under a RCP8.5 scenario) would significantly increase European ozone levels relative to a 2 °C one (reached along the 2028–2057 period under a RCP4.5 scenario). This increase is particularly high over industrial regions, large urban areas, and over Southern Europe and would annihilate the benefits of emission reduction policies. The regional ozone increase mainly stems from the advection of ozone at Europe’s boundaries, themselves due to high global methane concentrations associated with the RCP8.5 emission scenario. These results make regional emission regulation, combined with emissions-reduction policies for global methane, of crucial importance.

D espite a stabilization over the last decade, tropospheric ozone (O 3 ) remains a serious environmental problem in Europe 1 , as its concentrations are still far from achieving levels of air quality standards of the European Commission (especially the number of exceedances of the daily maximum 8-h average of 120 μg/m 3 ) and of the World Health Organization 2 set for the protection of human health (maximum daily 8-h average of 100 μg/m 3 ). Therefore, further efforts are necessary to improve air quality with respect to ozone: assuming the implementation of current air quality legislation, the European anthropogenic emissions of ozone precursors are expected to decline until 2050 3 . However, climate change can perturb chemical processing, local meteorology, and the long-range transport that influence air pollution [4][5][6][7][8][9][10][11][12] , and could annihilates benefits of European emission reductions on air quality.
As voluntary contributions of parties to reduce greenhouse gas emissions (INDC, Intended Nationally Determined Contributions) submitted before The Paris 2015 climate conference (COP21) would probably result in a global temperature rise near 3°C with respect to pre-industrial levels 13 , we investigated here the impact of a + 3°C climate change (using the RCP8.5 scenario) on future European ozone surface concentrations and compared it to that of a 2°C warming (using the RCP4.5 scenario). To this end, several future air quality scenarios for Europe were performed with a regional chemistry-transport model (CHIMERE 14 ), combined with different global background composition and regional air pollutant emission scenarios in the horizon 2050.

Results
Impact of a 3°C global RCP8.5 emission trajectory. Despite a reduction in anthropogenic emissions in the ECLIPSE-v4a current legislation emissions (CLE) 2050 emissions (decrease by about −67%, −41%, and −49% respectively, for NO x , NMVOC, and CO in ECLIPSE-v4a CLE 2050 compared to the 2005 levels), the mean annual surface ozone concentrations for a 3°C warming (simulation 3C, see Table 1) are about 3.5% higher than the HISTORICAL simulation (annual mean ozone concentrations of 34.41 and 33.28 ppbv, respectively, see Fig. 1). Moreover, with equal regional ozone precursor ECLIPSE-v4a CLE 2050 emissions, mean annual ozone in the 3C simulation is 8% higher than in the 2C simulation. Differences between these scenarios 2C and 3C are statistically significant (means of 31.99 ± 1.01 and 34.41 ± 1.07, respectively).
Over land, and particularly over industrial regions, over megacities and over Turkey (see the increase of about 4 ppbv in Fig. 2c), mean annual ozone concentrations for a + 3°C climate would be indeed higher than for both a historical or + 2°C climate. This means that the efforts made to decrease regional European ozone precursor emissions could be annihilated in the 3C simulation, under the RCP8.5 scenario. As seen in Fig. 1, it should also be noted that, in addition to annual ozone mean, the extremes (both the minimum and the maximum) of the distribution are also increased in the 3C simulation.
The increase of the ozone maximum can also be seen with the maximum daily 8-h average ozone concentrations (MDA8) during the summer (June-September) period. The annual mean number of days per year when summer MDA8 exceeds 100 μg/m 3 (equivalent to 50 ppbv), which is a target air quality guideline value for the World Health Organization, is higher both for 2C and for 3C than for the HISTORICAL simulation in South-eastern Europe (see Fig. 3a, c). It should be noted that ECLIPSE-v4a CLE 2050 emissions are predicted to increase over Turkey with respect to the 2005 levels (see Figs. 4 and 5, for NO x and SO 2 emissions, respectively): the projections show high growth rates in activity and since Turkey lacks stringent laws for stationary combustion, the power plant sector would be responsible for a strong growth in NO x emissions (Klimont. Z., personal communication). The number of ozone exceedance days is much higher for 3C than for 2C in South-eastern Europe and over the Mediterranean sea.   Table 1 Understanding the increase of ozone between 2 and 3°C. This increase of ozone concentrations between 2C and 3C is mostly linked to the boundary conditions under the RCP8.5 scenario.
This can be concluded from a simulation 3C-BOUND2C where the 3C simulation is repeated but with boundary concentrations as for the 2C simulation. This simulation shows strong differences  Table 1. This figure has been generated using the Matplotlib library for the Python programming language (https://matplotlib.org/) in MDA8 exceedance days with the 3C simulation (respectively, Fig. 3c, b), making evident the large impact of boundary conditions (CLE emissions and regional climate input is equal for both simulations). The determining elements of RCP8.5 for the 3C simulation are indeed a doubled global methane concentrations compared to RCP4.5, playing a significant role for ozone concentrations 15 , and a 40-150% greater stratospheric influx of ozone 16 , both leading to an important increase of tropospheric ozone levels. Nevertheless, the regional climate change can also explain a part of this increase in ozone exceedance days in Eastern Europe, with a higher number of hot summer days (i.e., with daily maximum temperature higher than 30°C, see Fig. 6) and a lower thickness of the boundary layer height (see Fig. 7). It is interesting to note that a decrease of ozone exceedance days, only due to regional climate change, is observed over the Mediterranean sea with the 3C-BOUND2C simulation. This could be related, among others, to a larger thickness of the boundary layer over this region (see Fig. 7) and then to enhanced dilution of ozone and of its precursors.  emissions. It should be noted that this increase would not apply to high NO x urban areas where low ozone levels associated to NO titration would persist. Moreover, the number of exceedance days over the WHO threshold (MDA8 > 50 ppb) would be significantly increased if regional ozone precursor emissions would not be reduced. It would be at least 25 days per year at each location in Europe (except over northern countries such as Norway and Finland, see Fig. 8) and could reach about 100 days over the Mediterranean sea in the 3C-EMI2005 simulation. These results demonstrate that the regional emissions decrease projected by the CLE scenario is crucial in order to mitigate the impact of a + 3°C global trajectory under the RCP8.5 scenario. Nevertheless, such a decrease would not be fully sufficient to prevent an increase of European ozone concentrations.

Discussion
The benefits of European anthropogenic emission reductions would probably be annihilated over large regions of Europe with a global + 3°C temperature increase, with ozone background and maximum levels enhanced over Europe compared to the present day. This is due to global changes of climate and background atmospheric composition (with high methane concentrations associated with the RCP8.5 scenario). These results confirm that if European air quality is to be improved, global methane emissions should be regulated providing both positive effects on regional air quality but also on climate change 7,17 . Also, the predicted CLE regional decrease for ozone precursors in the horizon 2050 remains crucial for European air quality, and particularly for the Southern European population. Over the Mediterranean Sea and adjacent areas, if current emissions were not reduced in a + 3°C world under the RCP8.5 scenario, the number of ozone exceedance days could reach 100 per year.
Considering the adverse effects of short-term exposure to daily ozone concentrations 18,19 , this would strongly affect both human health and vegetation.

Methods
The regional chemistry-transport model CHIMERE. The regional chemistrytransport model CHIMERE 14 is driven by weather conditions provided by the IPSL-CM5A-MR global climate model simulations 20 downscaled by the WRF regional climate model as produced for the EURO-CORDEX ensemble 21,22 . The regional simulation domain used here is a grid of 50 × 50 km encompassing Europe and part of the North-Eastern Atlantic Ocean 23 and North Africa. Using this model suite and focusing over Europe, we compare future air quality simulations covering 2°C and 3°C warming periods (see Table 1) with air quality in a reference climate period . They are 30-year periods encompassing the year for which the global average warming reaches the + 2 and + 3°C target, respectively, in the RCP4.5 and RCP8.5 scenarios, a methodology employed in recent studies 21,24 . Set-up of the scenarios. All configurations corresponding to the seven experiments used in this study are described in Table 1