{"id":147,"date":"2005-04-26T10:16:50","date_gmt":"2005-04-26T14:16:50","guid":{"rendered":"\/?p=147"},"modified":"2009-06-03T09:21:19","modified_gmt":"2009-06-03T14:21:19","slug":"pollution-climate-connections","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2005\/04\/pollution-climate-connections\/","title":{"rendered":"Pollution-Climate Connections Liens entre Pollution et Climat<\/lang_fr>"},"content":{"rendered":"
\n

Guest commentary by Loretta Mickley<\/a>, Harvard University<\/small><\/p>\n

Every summer over much of the United States, we brace ourselves for heat waves. During these periods, the air turns muggy and usually smoggy. After a few days, a cold front moves in, sweeping away the pollution and ending the heat. Given that we are on a path towards global warming, atmospheric chemists are asking how climate change could affect air quality. Will warmer temperatures mean more pollution during these episodes? Will episodes last longer? Most importantly, what effect will changes in air quality have on human health?
\nArticle invit\u00e9 par Loretta Mickley<\/a>, Harvard University (Traduit par Pierre Allemand)<\/small><\/p>\n

Tous les \u00e9t\u00e9s, dans la plupart des r\u00e9gions des USA, nous nous pr\u00e9parons aux vagues de chaleur. Pendant ces p\u00e9riodes, l\u2019air devient tr\u00e8s humide et g\u00e9n\u00e9ralement brumeux. Apr\u00e8s quelques jours, un front froid survient, balayant la pollution et mettant fin \u00e0 la chaleur. Etant donn\u00e9 que nous nous acheminons vers un r\u00e9chauffement global, les sp\u00e9cialistes de la chimie de l\u2019atmosph\u00e8re se demandent de quelle fa\u00e7on le changement climatique pourrait affecter la qualit\u00e9 de l\u2019air. Des temp\u00e9ratures plus \u00e9lev\u00e9es signifieront-elles plus de pollution durant ces \u00e9pisodes ? Ces \u00e9pisodes dureront-ils plus longtemps ? Plus important : quels sont les effets des changements dans la qualit\u00e9 de l\u2019air sur la sant\u00e9 humaine ?
\n(suite…)<\/a>
\n<\/lang_fr><\/p>\n


\nRecently the National Resource Defense Council<\/a> (NRDC) released Heat Advisory<\/a>, a report warning that surface air quality could suffer greatly as a result of climate change. In response, a group called the Pacific Research Institute<\/a> (PRI), together with another group called United for Jobs<\/a>, published Air Quality False Alarm<\/a>, a detailed criticism of the NRDC forecast. PRI argues, among other things, that anthropogenic emissions in the U.S. will drop sharply in coming decades. In their view, air pollution will become a thing of the past, no matter what happens to climate.<\/p>\n

What\u2019s the story here? First, a little background on ozone and particulate matter (PM), two major components of smog. Surface ozone is formed from a mix of natural and anthropogenic precursors like nitrogen oxides and volatile organic carbon. We have measurements of surface ozone dating back to the late 1800s which imply that ozone in some regions has increased 2-5 times due to emissions of ozone precursors from cars, industry, and power plants. As for PM, there are many different kinds \u2013 e.g., organic carbon, soot, and sulfate-ammonium-nitrate. Some kinds of PM, like soot, are directly emitted into the air, but other kinds condense from gas-phase molecules. Like ozone, PM has both natural and anthropogenic ingredients. <\/p>\n

Many factors govern the severity and timing of pollution episodes. An obvious factor is the magnitude of precursor emissions. But there are meteorological factors, like how stagnant the surface air is and whether it\u2019s clear or cloudy, warm or cool. The summer of 1998, for example, saw a record number of ozone exceedances averaged over New England. That summer was also the warmest on record for that region. The hot summer that Europe endured in 2003 was also a summer of high pollution levels for that continent. But the cool summer in the U.S. that same year meant that the we saw low levels of pollution. <\/p>\n

So how will pollution evolve over the coming decades as climate changes? The easy answer is: oh, the warmer temperatures mean greater pollution! But it\u2019s more complicated than that. Then there are other meteorological factors to consider. As the surface temperatures rise, will the depth of the boundary layer increase, diluting the pollutants within it? Maybe stronger surface winds will carry all the pollution away. What about changes in cloud cover or rainfall?<\/p>\n

To tackle issues of this complexity, modelers often turn to sensitivity studies. A sensitivity study is one in which you change just one or two variables, and keep everything else constant. By taking the problem apart in this way, you can isolate the effect of one or two factors at a time. <\/p>\n

In one sensitivity study, Aw and Kleeman [2003] imposed a 5\u00baC increase in temperature over the Los Angeles basin, but kept all other meteorological variables (like windspeed) constant in their model. Ozone in the region increased by 10-15%, but concentrations of sulfate-ammonia-nitrate PM decreased by 10-15%. That\u2019s because ammonia condenses less readily at high temperatures. This is an interesting result. But in the real world, stalled high pressure systems, like the one over the Midwest and Northeast last week (April 18-20), can lead to both warm temperatures and high PM. With clear skies and weak winds, PM can accumulate over the source regions. As the climate changes, not only could temperatures change, but also the behavior of these high pressure systems.<\/p>\n

In my research group, we tried a different sensitivity study [Mickley et al., 2004]. We devised our model experiment to test just the effect of changing wind patterns on pollutant concentrations. What we found was that the severity summertime regional pollution episodes in the Midwest and Northeast U.S. increased significantly by 2050, relative to present. Also, the average length of an episode increased from 2 to 3-4 days. Why did this happen? Our model forecast a 20% decline in the frequency of cold fronts sweeping into the U.S., so stagnation events in the model persisted longer. That allowed both gas-phase and PM pollution to build to higher concentrations. <\/p>\n

Another model study [Hogrefe et al., 2004] focused on the effect of climate change on just surface ozone. The authors found that even with emissions of ozone precursors in the model held at 1990s levels, the total number of \u201cexceedance days\u201d increased by about 60% over the eastern U.S. (An exceedance day is a day in which ozone averaged over 8 hours exceeds the EPA threshold of 84 ppb.) Because of the complexity of the study, Hogrefe et al. [2004] could not diagnose precisely all the meteorological changes (temperature? circulation patterns?) contributing to the increased surface ozone in their model. But they did find that one factor accounting for about half the increase was enhanced emissions of natural ozone precursors, which are temperature-sensitive.<\/p>\n

One of the biggest unknowns, of course, is how anthropogenic emissions will evolve in the future. The Clean Air Act has led to tremendous improvements in air quality since the 1970s. But even if our emissions do decline, the consequences for air pollution are uncertain. Fiore et al. [2002] have shown that decreases in U.S. emissions may be offset by increases elsewhere in the world. Specifically, rising methane emissions elsewhere in the world could significantly enhance background levels of ozone over the U.S., leading to as much pollution in 2030 as we saw in the mid-1990s. <\/p>\n

So there\u2019s a lot more to be learned about the links between climate and pollution. Since both surface ozone and PM have adverse effects on human health, understanding the link is important.<\/p>\n

References:<\/b><\/p>\n

Aw, J., and M.J. Kleeman, Evaluating the first-order effect of intraannual air pollution on urban air pollution<\/a>, J. Geophys. Res., 108, 4365, 10.1029\/2002JD002688, 2003.<\/p>\n

Fiore, A.M., D.J. Jacob, B.D. Field, D.G. Streets, S.D. Fernandes, and C. Jang, Linking ozone pollution and climate change: The case for controlling methane<\/a>, Geophys. Res. Lett., 29, 1919, doi:10.1029\/2002GL015601, 2002.<\/p>\n

Hogrefe, C., B. Lynn, K. Civerolo, J.-Y. Ku, J. Rosenthal, C. Rosenzweig, S. Gaffin, K. Knowlton, and P.L. Kinney, Simulating changes in regional air pollution over the eastern United States due to changes in global and regional climate and emissions<\/a>. J. Geophys. Res., 109, D22301, doi:10.1029\/2004JD004690, 2004.<\/p>\n

Mickley, L. J., D. J. Jacob, B. D. Field, and D. Rind, Effects of future climate change on regional air pollution episodes in the United States<\/a>, Geophys. Res. Let., 30, L24103, doi:10.1029\/2004GL021216, 2004.<\/p>\n


\nR\u00e9cemment, le National Resource Defense Council (NRDC)<\/a> a publi\u00e9 un Heat Advisory, un rapport mettant en garde sur le fait que la qualit\u00e9 de l\u2019air ambiant pourrait largement souffrir des changements climatiques. En r\u00e9ponse, un groupe appel\u00e9 le Pacific Research Institute (PRI)<\/a>, en association avec un autre groupe appel\u00e9 United for Jobs<\/a>, a publi\u00e9 le Air Quality False Alarm<\/a>, critique d\u00e9taill\u00e9e des pr\u00e9visions du NRDC. Le PRI soutient, entre autres, que les \u00e9missions anthropog\u00e9niques aux USA diminueront fortement dans les d\u00e9cennies \u00e0 venir. De leur point de vue, la pollution de l\u2019air ne sera plus qu\u2019un souvenir, quelle que soit l\u2019\u00e9volution du climat.
\nO\u00f9 en sommes-nous en fait ?
\nTout d\u2019abord, voyons quelques notions de base concernant l\u2019ozone et les mati\u00e8res particulaires (MP), deux composants essentiels du \u00ab smog \u00bb. La formation de l\u2019ozone de surface est provoqu\u00e9e par le m\u00e9lange de pr\u00e9curseurs naturels et anthropog\u00e9niques comme les oxydes d\u2019azote et les compos\u00e9s organiques volatils. On poss\u00e8de des mesures de l\u2019ozone de surface remontant \u00e0 la fin du 19e si\u00e8cle qui prouvent que la concentration atmosph\u00e9rique de l\u2019ozone a \u00e9t\u00e9 multipli\u00e9e par 2 \u00e0 5 dans certaines r\u00e9gions, en raison des \u00e9missions de pr\u00e9curseurs d\u2019ozone par les voitures, l\u2019industrie et les centrales de production d\u2019\u00e9nergie. Les MP peuvent \u00eatre de plusieurs types : carbone organique, suies, et sulfonitrates d\u2019ammonium. Certains types de MP comme les suies sont directement \u00e9mises dans l\u2019air, mais d\u2019autres types proviennent de la condensation de mol\u00e9cules gazeuses. Comme l\u2019ozone, les MP sont form\u00e9es \u00e0 partir d\u2019ingr\u00e9dients \u00e0 la fois naturels et anthropog\u00e9niques.
\nDe nombreux facteurs gouvernent la s\u00e9v\u00e9rit\u00e9 et la dur\u00e9e des \u00e9pisodes de pollution. Un facteur \u00e9vident est l\u2019importance des \u00e9missions de pr\u00e9curseurs. Mais, il y a des facteurs m\u00e9t\u00e9orologiques comme les propri\u00e9t\u00e9s de l\u2019air de surface : le fait qu\u2019il soit stagnant, clair ou nuageux, chaud ou froid. Par exemple, l\u2019\u00e9t\u00e9 1998 a vu un record du nombre de pics d\u2019 ozone sur la Nouvelle Angleterre. Cet \u00e9t\u00e9-l\u00e0 battit aussi le record de chaleur de la r\u00e9gion. L\u2019\u00e9t\u00e9 chaud que l\u2019Europe a subi en 2003 a aussi \u00e9t\u00e9 un \u00e9t\u00e9 pr\u00e9sentant un haut niveau de pollution pour le continent. Ce m\u00eame \u00e9t\u00e9 fut frais aux USA , avec un niveau de pollution bas.
\nAinsi, quelle sera l\u2019\u00e9volution de la pollution r\u00e9sultant des changement climatiques des prochaines d\u00e9cennies ? Une r\u00e9ponse simple serait : des temp\u00e9ratures plus \u00e9lev\u00e9es signifieront un degr\u00e9 de pollution plus \u00e9lev\u00e9 ! C\u2019est en r\u00e9alit\u00e9 un peu plus compliqu\u00e9. Il y a en effet d\u2019autres facteurs m\u00e9t\u00e9orologiques \u00e0 consid\u00e9rer. Avec l\u2019accroissement de la temp\u00e9rature de surface, est-ce que l\u2019\u00e9paisseur de la couche limite va augmenter, diluant de ce fait les polluants ? Peut-\u00eatre que les vents de surface plus forts chasseront toute la pollution Quels changements de la couverture nuageuse et des pr\u00e9cipitations va-t-on observer ?
\nPour venir \u00e0 bout des probl\u00e8mes pos\u00e9s par cette complexit\u00e9, les exp\u00e9rimentateurs de mod\u00e8les se tournent vers les \u00e9tudes de sensibilit\u00e9. Une \u00e9tude de sensibilit\u00e9 consiste \u00e0 changer seulement une ou deux variables et \u00e0 garder tout le reste constant. Cette m\u00e9thode de r\u00e9solution permet d\u2019isoler les effets d\u2019un seul ou de deux facteurs \u00e0 la fois.
\nDans une \u00e9tude de sensibilit\u00e9, Aw et Kleemann (2003) ont impos\u00e9 un accroissement de 5\u00b0C sur le bassin de Los Angeles, mais ont gard\u00e9 constantes toutes les autres variables m\u00e9t\u00e9orologiques (comme la vitesse du vent) dans leur mod\u00e8le. L\u2019ozone dans la r\u00e9gion s\u2019est accru de 10 \u2013 15 %, mais la concentration des MP de sulfonitrate d\u2019ammonium a d\u00e9cru de 10-15 %. Cela parce que l\u2019ammoniac se condense moins rapidement \u00e0 plus haute temp\u00e9rature. C\u2019est un r\u00e9sultat int\u00e9ressant. Mais, dans le monde r\u00e9el, les syst\u00e8mes de hautes temp\u00e9ratures stables comme celui qui s\u2019\u00e9tait \u00e9tabli sur le Midwest et le Northeast la semaine derni\u00e8re (18-20 avril) peuvent conduire \u00e0 la fois \u00e0 des temp\u00e9ratures \u00e9lev\u00e9es et des hautes teneurs en MP. Sous l\u2019influence de ciels clairs et de vents faibles, les MP peuvent s\u2019accumuler au-dessus des r\u00e9gions sources. Sous l\u2019effet des changements climatiques, non seulement les temp\u00e9ratures peuvent changer, mais aussi le comportement de ces syst\u00e8mes de hautes pressions.
\nDans mon groupe de recherche, nous avons essay\u00e9 une \u00e9tude de sensibilit\u00e9 diff\u00e9rente [Mickley et al., 2004]. Nous avons con\u00e7u notre mod\u00e8le afin de tester uniquement l\u2019effet du changement des vents sur la concentration des polluants. Nous avons trouv\u00e9 que la s\u00e9v\u00e9rit\u00e9 des \u00e9pisodes de pollution r\u00e9gionale dans le Midwest et le Northeast augmentait significativement autour de 2050, par rapport aux valeurs actuelles. Nous avons trouv\u00e9 \u00e9galement que la dur\u00e9e moyenne d\u2019un \u00e9pisode passait de 2 \u00e0 3-4 jours. Que s\u2019est-il pass\u00e9 ? Notre mod\u00e8le pr\u00e9voit une baisse de 20 % de la fr\u00e9quence des fronts froids balayant les USA, ce qui fait que les p\u00e9riodes de stagnation dans le mod\u00e8le persistent plus longtemps. Ce qui permet \u00e0 la pollution issue de phases gazeuses et de MP d\u2019atteindre des concentrations plus \u00e9lev\u00e9es.
\nUne autre \u00e9tude de mod\u00e8le [Hogrefe et al., 2004] s\u2019est concentr\u00e9e sur les effets des changement climatique sur l\u2019ozone de surface. Les auteurs ont trouv\u00e9 que m\u00eame avec des \u00e9missions de pr\u00e9curseurs d\u2019ozone maintenues \u00e0 leur niveau des ann\u00e9es 1990, le nombre total des \u00ab jours de d\u00e9passement \u00bb augmentait d\u2019environ 60 % sur l\u2019est des USA. (un \u00ab jour de d\u00e9passement \u00bb est un jour o\u00f9 la moyenne du taux d\u2019ozone d\u00e9passe le seuil de l\u2019EPA (84 ppb) pendant 8 heures). En raison de la complexit\u00e9 de l\u2019\u00e9tude, Hogrefe et al. [2004] n\u2019ont pas pu d\u00e9terminer pr\u00e9cis\u00e9ment tous les changements m\u00e9t\u00e9orologiques (temp\u00e9rature ? mod\u00e8le de circulation ?) ayant une influence sur l\u2019ozone de surface de leur mod\u00e8le. Mais, ils ont trouv\u00e9 qu\u2019un facteur contribuant pour environ la moiti\u00e9 de l\u2019accroissement \u00e9tait l\u2019augmentation des \u00e9missions des pr\u00e9curseurs naturels de l\u2019ozone, qui sont sensibles \u00e0 la temp\u00e9rature.
\nUne des plus grandes inconnues reste, \u00e9videmment, l\u2019\u00e9volution future des \u00e9missions anthropog\u00e9niques. La loi sur la propret\u00e9 de l\u2019air (Clean Air Act) a amen\u00e9 des am\u00e9liorations tr\u00e8s importantes de la qualit\u00e9 de l\u2019air depuis les ann\u00e9es 1970. Mais, m\u00eame si nos \u00e9missions doivent diminuer, les cons\u00e9quences sur la pollution de l\u2019air restent incertaines. Fiore et al. [2002] ont montr\u00e9 que la diminution des \u00e9missions am\u00e9ricaines pourraient \u00eatre annul\u00e9es par des augmentations ailleurs dans le monde.
\nPlus pr\u00e9cis\u00e9ment, l\u2019augmentation des \u00e9missions de m\u00e9thane ailleurs dans le monde pourrait accro\u00eetre les niveaux de base d\u2019ozone sur les USA, conduisant \u00e0 une pollution en 2030 analogue \u00e0 celle que nous avons vue dans les ann\u00e9es 1990.
\nIl y a donc encore beaucoup \u00e0 apprendre sur les liens entre le climat et la pollution.
\nComme l\u2019ozone de surface et les MP ont des effets n\u00e9gatifs sur la sant\u00e9 humaine, il est important de conna\u00eetre ces liens.<\/p>\n

References:<\/b><\/p>\n

Aw, J., and M.J. Kleeman, Evaluating the first-order effect of intraannual air pollution on urban air pollution<\/a>, J. Geophys. Res., 108, 4365, 10.1029\/2002JD002688, 2003.<\/p>\n

Fiore, A.M., D.J. Jacob, B.D. Field, D.G. Streets, S.D. Fernandes, and C. Jang, Linking ozone pollution and climate change: The case for controlling methane<\/a>, Geophys. Res. Lett., 29, 1919, doi:10.1029\/2002GL015601, 2002.<\/p>\n

Hogrefe, C., B. Lynn, K. Civerolo, J.-Y. Ku, J. Rosenthal, C. Rosenzweig, S. Gaffin, K. Knowlton, and P.L. Kinney, Simulating changes in regional air pollution over the eastern United States due to changes in global and regional climate and emissions<\/a>. J. Geophys. Res., 109, D22301, doi:10.1029\/2004JD004690, 2004.<\/p>\n

Mickley, L. J., D. J. Jacob, B. D. Field, and D. Rind, Effects of future climate change on regional air pollution episodes in the United States<\/a>, Geophys. Res. Let., 30, L24103, doi:10.1029\/2004GL021216, 2004.<\/p>\n

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Guest commentary by Loretta Mickley, Harvard University Every summer over much of the United States, we brace ourselves for heat waves. During these periods, the air turns muggy and usually smoggy. After a few days, a cold front moves in, sweeping away the pollution and ending the heat. Given that we are on a path […]<\/p>\n","protected":false},"author":12,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","footnotes":""},"categories":[17,5,1,3],"tags":[],"class_list":{"0":"post-147","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-aerosols","7":"category-climate-modelling","8":"category-climate-science","9":"category-greenhouse-gases","10":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/147","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/users\/12"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=147"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/147\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=147"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=147"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=147"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}