{"id":406,"date":"2007-02-21T11:37:41","date_gmt":"2007-02-21T16:37:41","guid":{"rendered":"\/?p=406"},"modified":"2007-04-17T16:21:30","modified_gmt":"2007-04-17T21:21:30","slug":"aerosols-the-last-frontier","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2007\/02\/aerosols-the-last-frontier\/","title":{"rendered":"Aerosols: The Last Frontier? Aeross\u00f3is: A \u00daltima Fronteira?<\/lang_po>"},"content":{"rendered":"
\n

Guest commentary from Juliane Fry, UC Berkeley<\/a><\/small><\/p>\n

The recently released IPCC 2007 Fourth Assessment Report Summary for Policymakers<\/a> reminds us that aerosols remain the least understood component of the climate system. Aerosols are solid or liquid particles suspended in the atmosphere, consisting of (in rough order of abundance): sea salt, mineral dust, inorganic salts such as ammonium sulfate (which has natural as well as anthropogenic sources from e.g. coal burning), and carbonaceous aerosol such as soot, plant emissions, and incompletely combusted fossil fuel. As should be apparent from this list, there are many natural sources of aerosol, but changes have been observed in particular, in the atmospheric loading of carbonaceous aerosol and sulphates, which originate in part from fossil fuel burning. While a relatively minor part of the overall aerosol mass, changes in the anthropogenic portion of aerosols since 1750 have resulted in a globally averaged net radiative forcing of roughly -1.2 W\/m2<\/sup>, in comparison to the overall average CO2<\/sub> forcing of +1.66 W\/m2<\/sup>.
\n<\/p>\n

<\/a>Figure SPM-2, shown here, compares the radiative forcing for greenhouse gases and other climate forcing agents, along with an assessment of the level of scientific understanding (\u201cLOSU\u201d) for each component. In this figure, it is clear that while aerosols contribute the largest negative (cooling) radiative forcing, the level of scientific understanding of their climate influence is \u201clow\u201d to \u201cmedium-low\u201d. The aerosol effects are split into two categories: (1) direct effects, meaning the scattering or absorption of radiation by aerosols influencing the net amount of energy reaching the Earth\u2019s surface, and (2) indirect effects, such as the cloud albedo effect, referring to how the presence of aerosol increases cloud reflectivity by providing a larger number of nuclei for cloud droplets, reducing the amount of energy reaching the surface. This is a step up from the last report<\/a>, where the LOSU for aerosols was very low to low, and no most likely value was assigned at all for the ‘indirect’ part.<\/p>\n

This figure also visually hints at why improving our understanding of aerosol\u2019s role in climate is so important: while overall net radiative forcing is positive (warming), aerosols provide the dominant negative (cooling) forcings. Hence, the aerosol currently in our atmosphere is acting to mask some of the greenhouse gas-induced warming. This means that as we get our act together to reduce fossil fuel use to improve air quality and address global warming, we need to be mindful of how changes in emissions will impact aerosol concentrations and composition.<\/p>\n

In addition, our deficient understanding of aerosol forcing also hinders our ability to use the modern temperature record to constrain the \u201cclimate sensitivity\u201d \u2013 the operative parameter in determining exactly how much warming will result from a given increase in CO2<\/sub> concentration. The determination of climate sensitivity has been discussed in this forum previously here<\/a>. The sensitivity parameter can be derived by examining historical records of the correlation of CO2<\/sub> concentration and temperature taking into account other contemporary changes. Aerosols contribute significantly to the uncertainty in climate sensitivity because we cannot model their historical impact on the temperature record with sufficient accuracy, though additional constraints on climate sensitivity such as the last ice age do exist<\/a>. A better understanding of aerosols then may well facilitate more accurate predictions of future climate responses to changing CO2<\/sub>.<\/p>\n

The relative lifetimes of CO2<\/sub> and aerosol in the atmosphere result in the expectation that reducing fossil fuel use will accelerate warming. A CO2<\/sub> molecule has a lifetime of about 100 years in the atmosphere, while an aerosol particle has an average life expectancy of only about 10 days. Therefore, if we instantaneously ceased using combustion engines, the (cooling) fossil fuel-related aerosols would be cleaned out of the atmosphere within weeks, while the (warming) CO2<\/sub> would remain much longer, leaving a net positive forcing from the reduction in emissions for a century or more<\/a>.<\/p>\n

So, what do we need to learn about aerosol to narrow those error bars in Figure SPM-2? To accurately model aerosols\u2019 climate impact, we need to know about the whole lifespan of the aerosols: their diverse sources, aging processes (and how those affect radiative properties), how they mix together and the mechanisms and timescales for its removal from the atmosphere. As the IPCC 2007 4AR will make clear, we\u2019ve come a long way in our understanding of atmospheric aerosol, but there is still plenty of room for improvement.<\/p>\n


\n Coment\u00e1rio convidado de Juliane Fry, UC Berkeley<\/small><\/p>\n

O Quarto Relat\u00f3rio de Avalia\u00e7\u00e3o do IPCC 2007 Sum\u00e1rio para Pol\u00edticos<\/a> recentemente divulgado, nos lembra que os aeross\u00f3is permanecem o menos compreendido componente do sistema clim\u00e1tico. Aeross\u00f3is s\u00e3o part\u00edculas s\u00f3lidas ou l\u00edquidas suspensas na atmosfera, consistindo de (em ordem aproximada de abund\u00e2ncia): sal marinho, poeira mineral, sais inorg\u00e2nicos como o sulfato de am\u00f4nia (que vem de fontes naturais e antropog\u00eanicas, como a queima de carv\u00e3o), e aeross\u00f3is carbonatados como a fuligem, emiss\u00f5es de plantas, e combust\u00edveis f\u00f3sseis incompletamente queimados. Como deve ser aparente desta lista, h\u00e1 muitas fontes de aeross\u00f3is, mas mudan\u00e7as foram observadas, em particular na carga atmosf\u00e9rica de aeross\u00f3is carbonatados e sulfatos, que se originam em parte da queima de combust\u00edveis f\u00f3sseis. Apesar de serem uma parte relativamente pequena da massa total de aeross\u00f3is, mudan\u00e7as na contribui\u00e7\u00e3o antropog\u00eanica dos aeross\u00f3is desde 1750 resultaram numa for\u00e7ante radiativa m\u00e9dia de aproximadamente -1.2 W\/m2, relativa a uma for\u00e7ante global devido ao CO2 de +1.66 W\/m2.<\/p>\n

<\/a>Figura SPM-2, mostrada aqui, compara a for\u00e7ante radiativa para gases de efeito estufa e outros agentes, junto com uma avalia\u00e7\u00e3o do n\u00edvel de entendimento cient\u00edfico (cuja sigla em ingl\u00eas \u00e9 LOSU), para cada componente. Nesta figura, est\u00e1 claro que os aeross\u00f3is contribuem com a maior for\u00e7ante negativa (resfriamento), e que seu n\u00edvel de entendimento varia de \u201cbaixo\u201d a \u201cmeio baixo\u201d. Os efeitos dos aeross\u00f3is se dividem em duas categorias: (1) efeitos diretos, relativos ao espalhamento ou \u00e0 absor\u00e7\u00e3o da radia\u00e7\u00e3o pelos aeross\u00f3is, influenciando a quantidade l\u00edquida de energia que chega \u00e0 superf\u00edcie da Terra, e (2) efeitos indiretos, como o albedo das nuvens, referindo-se a como a presen\u00e7a dos aeross\u00f3is aumenta a refletividade de nuvens ao proporcionar um n\u00famero maior de n\u00facleos para a forma\u00e7\u00e3o de gotas, reduzindo a quantidade de energia que chega a superf\u00edcie. Isto j\u00e1 representa um passo a frente com rela\u00e7\u00e3o ao \u00faltimo relat\u00f3rio<\/a>, , onde o LOSU para os aeross\u00f3is variava de muito baixo a baixo, e nenhum valor mais prov\u00e1vel era assinalado para a parte \u201cindireta\u201d.<\/p>\n

Esta figura virtualmente sugere por que melhorar nossa compreens\u00e3o sobre o papel dos aeross\u00f3is no clima \u00e9 t\u00e3o importante: enquanto a for\u00e7ante radiativa l\u00edquida global \u00e9 positiva (aquecimento), aeross\u00f3is representam as for\u00e7antes negativas (resfriamento) dominante. Consequentemente, os aeross\u00f3is atualmente em nossa atmosfera est\u00e3o atuando de forma a mascarar parte do aquecimento induzido pelos gases de efeito estufa (GEE). Isto significa que ao agirmos para reduzir o uso de combust\u00edveis f\u00f3sseis para melhorar a qualidade do ar e atacar o aquecimento global, devemos ter em mente como estas mudan\u00e7as nas emiss\u00f5es v\u00e3o impactar a concentra\u00e7\u00e3o e a composi\u00e7\u00e3o dos aeross\u00f3is.<\/p>\n

Al\u00e9m disso, nossa defici\u00eancia em compreender os aeross\u00f3is tamb\u00e9m prejudica nossa habilidade de utilizar o moderno registro de temperaturas para restringir a \u201csensibilidade clim\u00e1tica\u201d \u2013 o par\u00e2metro operacional para determinar exatamente quanto de aquecimento resultar\u00e1 de um dado aumento na concentra\u00e7\u00e3o de CO2. A determina\u00e7\u00e3o da sensibilidade clim\u00e1tica foi discutida anteriormente neste f\u00f3rum aqui. O par\u00e2metro de sensibilidade pode ser obtido a partir do exame dos registros hist\u00f3ricos da correla\u00e7\u00e3o entre a concentra\u00e7\u00e3o de CO2 e a temperatura, levando-se em conta outras mudan\u00e7as contempor\u00e2neas. Aeross\u00f3is contribuem significativamente para a incerteza na sensibilidade clim\u00e1tica pois n\u00e3o podemos modelar o seu impacto no registro de temperaturas com precis\u00e3o suficiente, apesar de existirem certas restri\u00e7\u00f5es \u00e0 sensibilidade clim\u00e1tica, como a \u00faltima era glacial. Um melhor entendimento sobre aeross\u00f3is pode permitir previs\u00f5es mais acuradas sobre a resposta do clima futura a varia\u00e7\u00f5es no CO2.<\/p>\n

Os tempos de vida relativos do CO2 e dos aeross\u00f3is na atmosfera resultam na expectativa de que uma redu\u00e7\u00e3o no uso de combust\u00edveis f\u00f3sseis acelerar\u00e1 o aquecimento. A mol\u00e9cula do CO2 tem um tempo de vida de aproximadamente 100 anos na atmosfera, enquanto uma part\u00edcula de aerossol possui uma expectativa de vida m\u00e9dia de apenas 10 dias em m\u00e9dia. Assim, se n\u00f3s instantaneamente pararmos de usar motores a combust\u00e3o, os aeross\u00f3is (resfriadores) associados aos combust\u00edveis f\u00f3sseis seriam removidos da atmosfera em algumas semanas, enquanto as (aquecedoras) mol\u00e9culas de CO2 permaneceriam por muito mais tempo, deixando uma for\u00e7ante l\u00edquida positiva devido a redu\u00e7\u00e3o das emiss\u00f5es por um s\u00e9culo ou mais.<\/p>\n

Assim, o que precisamos saber mais sobre os aeross\u00f3is para estreitar as barras de erro da Figura SPM-2? Para modelar precisamente o seu impacto no clima, necessitamos saber mais sobre todos os aspectos da vida dos aeross\u00f3is: suas diversas fontes, seu processo de envelhecimento (e como este afeta suas propriedades radiativas), como se misturam e os mecanismos e as escalas de tempo de sua remo\u00e7\u00e3o da atmosfera. Como o IPCC 2004 4AR deixar\u00e1 claro, percorremos um longo caminho na compreens\u00e3o dos aeross\u00f3is atmosf\u00e9ricos, mas h\u00e1 ainda muito espa\u00e7o para melhorarmos.<\/p>\n

traduzido por Fernando M. Ramos e Ivan B. T. Lima <\/small>
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Guest commentary from Juliane Fry, UC Berkeley The recently released IPCC 2007 Fourth Assessment Report Summary for Policymakers reminds us that aerosols remain the least understood component of the climate system. Aerosols are solid or liquid particles suspended in the atmosphere, consisting of (in rough order of abundance): sea salt, mineral dust, inorganic salts such […]<\/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":[17,1,13,23],"tags":[],"class_list":{"0":"post-406","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-aerosols","7":"category-climate-science","8":"category-faq","9":"category-ipcc","10":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/406","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=406"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/406\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=406"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=406"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=406"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}