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The Mediterranean region under climate change

Jean-Paul Moatti
Stéphane Thiébault

Part 1. Mechanisms, observed trends, projections

Sub-chapter 1.4.6. The (uncertain) future of air quality

Texte intégral

1Assessing the future evolution of air quality requires taking climate projections into account and designing environmental policies. In the context of adaptation to climate change, the geophysical changes expected in coming decades will have an impact on chronic and extreme air pollution events (Jacob and Winner, 2009). But air quality is also sensitive to climate mitigation strategies: the social and technological changes required to reduce greenhouse gas emissions will be accompanied by changes in the emission of air pollutants and of their precursors. There are potentially large co-benefits between air quality and climate change mitigation that could help efforts to identify win-win strategies. Nevertheless, mitigating climate change can also cause collateral damage to air quality. It is thus very important to identify the co-benefits and possible collateral damage to maximize the former while minimizing the latter. Here we briefly review recent results on the impacts of climate change on air quality in the Mediterranean region in terms of ozone and particles, and describe the positive and negative feedback of climate change on air quality.

Adaptation: the impact of climate change on air quality

Surface ozone

2Ozone concentrations in the troposphere are driven by many chemical and dynamic processes including emissions of ozone precursors and meteorological variables. Climate change has an impact on the tropospheric ozone through its effects on biogenic emissions of ozone precursors (mainly volatile organic compounds - VOCs), meteorological parameters (temperature, precipitation, humidity) and atmospheric chemistry (chemical budget, photochemical regimes). Climate change will be accompanied by a reduction in rainfall over southern Europe, creating wintertime deficits that reduce soil water content, thereby further increasing average temperatures and the frequency and severity of heat waves (Fiore et al. 2012; Vautard et al. 2013) with major consequences for summertime ozone pollution in Europe and the Mediterranean, which have already been pointed out (Langner et al. 2005; Meleux et al. 2007). A meta-analysis of the 25 projections of ozone pollution in Europe in the context of climate change published between 2007 and 2015 was conducted by Colette et al. (2015a) to explore the robustness of the projected impact of climate change on surface ozone (Fig. 11). The corresponding climate ozone penalty is defined as the incremental change in ozone that can be attributed to climate change alone, in the absence of changes in anthropogenic emissions of ozone or other drivers. The penalty was confirmed over most of continental Europe, especially in European countries located on the Mediterranean rim where such a penalty is robust, i.e. consistent in over two-thirds of the models in the ensemble (diamond symbols in Fig. 11).

Figure 11
Increase in surface summertime ozone concentrations (ppbv) by the middle of the 21st century in the moderate climate change scenario A1B in an ensemble of all the published European model projections (adapted from Colette et al. 2015a).

3The main effect of climate responsible for this increase in surface ozone pollution in Europe is the increase in temperature and solar radiation leading to an increase in biogenic isoprene emissions even if a possible inhibition of these emissions with increasing CO2 concentrations occurs in the long run, thereby yielding major uncertainties (Lathière et al. 2010; Langner et al. 2012). The other impacts of climate on surface ozone are the direct impact of an increase in temperature on the kinetics of atmospheric chemistry, and the direct impact of solar radiation on photochemistry resulting from changes in cloud cover. Both increase photolysis rates, particularly that of nitrogen dioxide, which favors the formation of ozone. In both cases, the increase in temperature and solar radiation can result from gradual changes in the average climate, but they are exacerbated in the case of extreme heat wave events. In addition to meteorological factors, heat waves favor the accumulation of pollution in the absence of atmospheric dispersion.

4In the context of the MISTRALS/ChArMEx project, the global model outputs from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP; Young et al. 2013) are being analyzed to assess future changes in surface ozone over the Euro-Mediterranean region (Jaidan et al. in prep.). Under the pessimistic Representative Concentration Pathway (RCP8.5) scenario, mean temperature will increase by about 5.4K by 2100 compared to 2000 accompanied by a small increase (about 2%) in surface ozone.

5Over European land surfaces, the 95% confidence interval of summertime mean ozone change is estimated to be [0.44; 0.64] and [0.99; 1.50] ppbv for the 2041–2070 and 2071–2100 periods, respectively. This change may seem small, but it is of the same order of magnitude as the ozone trends reported over Europe in the past two decades despite the implementation of ambitious policies (Monks et al. 2015; Colette et al. 2016). This raises serious doubts about our ability to compensate for the climate change penalty by controlling the emissions of ozone precursors.

Particulate matter

6The largest detrimental sanitary impacts of air pollution are currently attributed to atmospheric aerosols from various sources (WHO, 2013). Also called particulate matter (PM), they can originate from anthropogenic or biogenic gaseous precursors (this is the case for example of sulfate, nitrate, ammonium, and secondary organic aerosols), from primary emissions of particulate matter (e.g. elemental carbon (EC), but also heavy metals and persistent organic pollutants (POPs) such as polycyclic aromatic hydrocarbons (PAHs)), or from natural sources (desert dust, sea salt, volcanic ash).

7Future changes in PM pollution in the context of adaptation to climate change is less clear than that of ozone because of the complexity of the often competitive processes involved (Fuzzi et al. 2015). Recent evidence points to a climate change benefit (with a reduction in PM loads, in particular because of an increase in volatility with increasing temperature; Lecœur and Seigneur, 2013; Colette et al. 2013; Lacressonnière et al. 2016; Lemaire et al. 2016) but increases have been reported in the southern parts of Europe (Manders et al. 2012; Hedegaard et al. 2013). Changes in biogenic precursor emission of secondary aerosols (SOA), which are likely to increase substantially in a warmer climate, could lead to an increase in PM concentrations (Megaritis et al. 2013). Changes in scavenging by precipitation, transport patterns and persistence of anticyclonic conditions leading to PM accumulation could also play a role in shaping future aerosol concentrations (Pausata et al. 2013). The frequency of precipitation is more likely to affect PM scavenging than the intensity of precipitation. Simulating accurate precipitation frequencies is very challenging for climate models, and projections are still subject to large uncertainties. Extreme heat events associated with stagnation of air masses are projected to increase, but the relative contribution of changes in their frequency and duration versus changes in the intensity of heat waves is not yet clear (Clark and Brown, 2013). PM pollution is likely to be more sensitive to the extended duration of the events.

8The potential change in PM loads in southern Europe will be largely determined by the mineral dust fraction. Both advection from the Sahara and North African deserts and local mobilization e.g. from agricultural land during dry conditions (Bessagnet et al. 2008) contribute to this fraction. Global and regional climate changes as well as changes in land use may have significant impacts on dust emission and transport. African dust activity has been shown to be correlated with different aspects of climate variability including the El Niño/Southern Oscillation, the North Atlantic Oscillation, the meridional position of the intertropical convergence zone, Sahelian rainfall and surface temperatures over the Sahara Desert, which can affect surface wind activity to varying degrees (Evan et al. 2016). The same authors conclude that the likely tendency for African dust activity is a decrease in a warmer climate. However, changes in PM10 exceedances due to dust over Europe are more likely to be sensitive to changes in the frequency and transport pathways of dust storms rather than to variations in mean emissions or in mean concentrations. Currently, there is no consensus on the sign and magnitude of future regional change in dust concentrations affecting Mediterranean regions and southern Europe.

9As another source of natural aerosol, sea sprays, can account for a major fraction of PM in the coastal regions of Europe. Beside sea salts, a significant proportion of the submicron fraction of sea sprays is organic and comes from biogenic sources. Studies have revealed no significant trend in the activity of sea sprays in the North Atlantic in recent decades (Korhonen et al. 2011) and this is unlikely to change significantly with climate change (Jacobson and Streets, 2009).

10Wildfires are another major source of aerosol and ozone precursors that can severely impact air quality (Hodzic et al., 2007; Miranda et al., 2008) and for which climate and land use change may be determining factors. A dryer climate would tend to increase wildfires but man-driven changes in land use also have a very strong impact, especially in Europe, where the population density is high. Landscape management and fragmentation and fire suppression tend to reduce wildfires (Knorr et al. 2014). For these reasons, an increase in fire frequency with climate change will not necessarily lead to a net increase in PM emissions as these are not only determined by the number of fires but also by their duration, extent and intensity.

Mitigation: towards win-win solutions to limit global warming and improve air quality

11The evolution towards a low carbon economy will be accompanied by reductions in the emission of air pollutants. A vast array of mitigation measures will have beneficial impacts on both air pollution and climate mitigation, of which several belong to the category of energy efficiency measures, which represent a very substantial pathway towards win-win solutions (Colette et al. 2015b), even if some strategies that favor climate mitigation may be detrimental to air quality (for instance the use of diesel fuel and the domestic burning of wood with outdated appliances).

Figure 12
Win-win strategies: cost benefit analyses at European scale demonstrate that the additional costs related to the climate mitigation (MIT) scenario aiming at limiting warming to 2 °C at the end of the 21st century compared to the business-as-usual scenario (REF) could be largely offset by savings in end of pipe air pollution mitigation costs and avoided damage to health. Adapted from Schucht et al. (2015).

12The sanitary benefits that can be expected from the future evolution of climate and air quality policies were quantified in IIASA (2013) and Likhvar et al. (2015), for example. These authors demonstrate that European countries located along the Mediterranean coasts will benefit the most from a reduction in ozone exposure.

13A quantitative assessment of the costs and benefits associated with climate mitigation in Europe was proposed by Schucht et al. (2015). These authors found that the very substantial costs of shifting to an energy mix that would comply with the 2 °C warming target would be offset by the positive externality represented by reduced air pollution. This is because the low-carbon scenario also yields (i) reduced cost of end of pipe technologies and (ii) direct sanitary benefits (Fig. 12).

Way forward

14Recent evidence demonstrated the link between climate change and air pollution both regarding adaptation and mitigation strategies. It should be emphasized that, at present, most work has been performed at continental scale, through Europe-wide assessments, in addition to a few global studies (Anenberg et al. 2010; West et al. 2013; Lelieveld et al. 2015). There have been few dedicated assessments of such impacts on the Mediterranean region, thereby opening new research perspectives, in which the proposed contribution of the MISTRALS/ChArMEx Program could be instrumental.

15Overall, major uncertainties remain on the likely evolution of aerosols, especially over southern Europe and the Mediterranean basin. Beside process studies, ensembles of high resolution modeling approaches combining climate and aerosols, and including land use change/management scenarios are one possible way to characterize key mechanisms and to quantify and reduce these uncertainties.

16As far as climate adaptation is concerned, the role of climate change in land use and, in turn, in dust resuspension and dispersion remain a key uncertainty. The role of biogenic emissions, as ozone precursors, but also of secondary organic aerosols, is also an important topic.

17There are win-win strategies to be developed in the years to come to improve air quality and to engage in progress towards a low carbon economy. Such benefits have been pointed out in several European studies, but the specific situation of Mediterranean countries deserves a closer look to tailor the most efficient sustainable strategies.

Table des illustrations

Légende Figure 11Increase in surface summertime ozone concentrations (ppbv) by the middle of the 21st century in the moderate climate change scenario A1B in an ensemble of all the published European model projections (adapted from Colette et al. 2015a).
Fichier image/jpeg, 423k
Légende Figure 12Win-win strategies: cost benefit analyses at European scale demonstrate that the additional costs related to the climate mitigation (MIT) scenario aiming at limiting warming to 2 °C at the end of the 21st century compared to the business-as-usual scenario (REF) could be largely offset by savings in end of pipe air pollution mitigation costs and avoided damage to health. Adapted from Schucht et al. (2015).
Fichier image/jpeg, 134k

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