{"id":422,"date":"2007-03-25T09:59:47","date_gmt":"2007-03-25T14:59:47","guid":{"rendered":"\/?p=422"},"modified":"2007-04-25T21:53:28","modified_gmt":"2007-04-26T02:53:28","slug":"does-a-global-temperature-exist","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2007\/03\/does-a-global-temperature-exist\/","title":{"rendered":"Does a Global Temperature Exist?Existe uma temperature global?<\/lang_po>"},"content":{"rendered":"
\n

Does a global temperature exist? This is the question asked in a recently published article<\/a> in Journal of Non-Equilibrium Thermodynamics<\/em> by Christopher Essex, Ross McKitrick, and Bjarne Andresen. The paper argues that the global mean temperature is not physical, and that there may be many other ways of computing a mean which will give different trends. <\/p>\n

The common arithmetic mean<\/a> is just an estimate that provides a measure of the centre value of a batch of measurements (centre of a cloud of data points, and can be written more formally as the integral of x f(x) dx<\/em>. The whole paper is irrelevant in the context of a climate change because it missed a very central point. CO2<\/sub> affects all surface temperatures on Earth, and in order to improve the signal-to-noise ratio, an ordinary arithmetic mean<\/em> will enhance the common signal in all the measurements and suppress the internal variations which are spatially incoherent (e.g. not caused by CO2<\/sub> or other external forcings). Thus the choice may not need a physical justification, but is part of a scientific test which enables us to get a clearer ‘yes’ or ‘no’. One could choose to look at the global mean sea level instead, which does have a physical meaning because it represents an estimate for the volume of the water in the oceans, but the choice is not crucial as long as the indicator used really responds to the conditions under investigation. And the global mean temperature is indeed a function of the temperature over the whole planetary surface. <\/p>\n

<\/p>\n

Is this paper a joke then? It is old and traditional knowledge that the temperature measurements made in meteorological and climatological studies are supposed to be representative of a certain volume of air, i.e. the arithmetic mean. Essex et al.<\/em> argue that it is not really physical, but surely the temperature measurements do have clear practical implications? Temperature itself can be inferred directly from several physical laws, such as the ideal gas law<\/a>, first law of thermodynamics<\/a> and the Stefan-Boltzmann law<\/a>, so it’s not the temperature itself which is ‘unphysical’. Even though the final temperature of two bodies in contact may not be the arithmetic mean, it will still be a weighted<\/em> arithmetic mean of the temperatures of the two initial temperatures if no heat is lost to the surroundings. Besides, grid-box sizes for numerical weather models often have a minimum spatial scale of 10-20km, and the temperature may be regarded as a mean for this scale. Numerical weather models usually provide useful forecasts. <\/p>\n

And what distinguishes the mean temperature representing a small volume to a larger one? Or do Essex et al.<\/em> think the limit is at greater scales. For instance at the synoptic<\/a> spatial scale (~1000 km)? The funny thing then is that the concept of regional mean temperature<\/em> would also not be meaningful according to Essex et al.<\/em> And one may also wonder if the problem of computing a mean temperature is meaningful in time, such as the summer-mean temperature or winter-mean temperature? <\/p>\n

Essex et al.<\/em> suggest that there are many different ways of computing the mean, and it is difficult to know which make more sense. But when they compute the geometric mean, they should not forget that the temperature should be in degrees Kelvin (the absolute temperature) as opposed to Celsius. One argument used by Essex et al.<\/em> is that the temperatures are not in equilibrium. Strictly speaking, this applies to most cases. But in general, these laws still give a reasonable results because the temperatures are close to being in equilibrium in meteorology and climatology. The paper doesn’t bring any new revelations – I thought that these aspects were already well-known.<\/p>\n

Update:<\/strong> Rabett Run<\/a> has a very detailed set of posts pulling apart this paper more thoroughly.<\/p>\n

<\/p>\n

Existe uma temperature global? Esta \u00e9 a quest\u00e3o formulada num recentemente publicado artigo<\/a>
\nno Journal of Non-Equilibrium Thermodynamics<\/em> por Christopher Essex, Ross McKitrick, e Bjarne Andresen. O trabalho argumenta que uma temperatura m\u00e9dia global n\u00e3o \u00e9 fisicamente aceit\u00e1vel, e que deve haver outras maneiras de computar uma m\u00e9dia, as quais dariam diferentes tend\u00eancias. <\/p>\n

A usual media aritm\u00e9tica<\/a> \u00e9 somente uma estimativa que fornece uma medida do valor central de um conjunto de medidas (centro de uma nuvem de pontos, e pode ser definida formalmente como a integral de x f(x) dx<\/em>). <\/p>\n

Todo o artigo \u00e9 irrelevante no contexto de mudan\u00e7as clim\u00e1ticas pois ele ignorou um ponto muito importante. CO2<\/sub> afeta todas as temperaturas superficiais da terra, e para melhorar a rela\u00e7\u00e3o sinal-ru\u00eddo, uma media aritm\u00e9tica<\/em> ordin\u00e1ria melhorar\u00e1 o sinal comum a todas as medidas e suprimir\u00e1 as varia\u00e7\u00f5es internas que s\u00e3o espacialmente incoerentes (por exemplo, n\u00e3o causadas pelo CO2<\/sub> ou outros for\u00e7antes). Assim esta escolha n\u00e3o requer uma justificativa f\u00edsica, mas \u00e9 parte de um teste cient\u00edfico que nos permite obter um \u2018sim\u2019 ou \u2018n\u00e3o\u2019 mais claro. Pode-se escolher olhar para o n\u00edvel m\u00e9dio dos oceanos, que n\u00e3o tem um sentido f\u00edsico pois representa uma medida do volume de \u00e1gua nos oceanos, mas a escolha n\u00e3o \u00e9 crucial na medida que o indicador utilizado responde \u00e0s condi\u00e7\u00f5es que s\u00e3o investigadas. E a temperatura m\u00e9dia global \u00e9 de fato uma fun\u00e7\u00e3o da temperatura sobre toda a superf\u00edcie do planeta.<\/p>\n

Este trabalho \u00e9 ent\u00e3o uma brincadeira? \u00c9 um conhecimento antigo e tradicional que as temperaturas medidas em meteorologia e em estudos climatol\u00f3gicos s\u00e3o por hip\u00f3tese representativas de um certo volume de ar, isto \u00e9, uma m\u00e9dia aritm\u00e9tica. Essex et al.<\/em> afirmam que esta n\u00e3o \u00e9 realmente f\u00edsica, mas certamente medidas de temperatura t\u00eam implica\u00e7\u00f5es pr\u00e1ticas claras. Temperatura propriamente dita pode ser inferida diretamente de diferentes leis f\u00edsicas, como a lei dos gases ideais<\/a>, a primeira lei da termodin\u00e2mica <\/a> ea lei de Stefan-Boltzmann<\/a>, assim n\u00e3o \u00e9 a temperatura per se que \u00e9 \u2018n\u00e3o-f\u00edsica\u2019. Apesar da temperatura de dois corpos em contato n\u00e3o ser necessariamente a m\u00e9dia aritm\u00e9tica, ainda sim ela ser\u00e1 uma m\u00e9dia ponderada<\/em> das temperaturas iniciais se nenhum calor \u00e9 perdido para o ambiente. Al\u00e9m disso, os tamanhos de grade dos modelos de previs\u00e3o num\u00e9rica do tempo usualmente possuem uma escala espacial m\u00ednima de 10-20km, e neste caso a temperatura pode ser interpretada como uma m\u00e9dia nesta escala. Modelos num\u00e9ricos de tempo geralmente fornecem previs\u00f5es \u00fateis. <\/p>\n

E o que distingue a temperatura m\u00e9dia de um volume pequeno de um grande? Ou ser\u00e1 que Essex et al.<\/em> acreditam que o limite est\u00e1 nas grandes escalas? Por exemplo, na escala sin\u00f3tica<\/a>(~1000 km)? \u00c9 engra\u00e7ado pensar que ent\u00e3o o conceito de temperatura m\u00e9dia regional<\/em> tamb\u00e9m n\u00e3o teria sentido segundo Essex et al.<\/em> E pode-se tamb\u00e9m questionar se o problema de calcular a temperatura m\u00e9dia faz sentido no tempo, por exemplo, a temperatura m\u00e9dia do ver\u00e3o ou inverno?<\/p>\n

Essex et al.<\/em> sugerem que h\u00e1 diferentes maneiras de calcular a m\u00e9dia, e que \u00e9 dif\u00edcil saber qual faz mais sentido. Mas quando eles calculam a m\u00e9dia geom\u00e9trica, eles n\u00e3o deveriam esquecer que a temperatura deveria ser medida em graus Kelvin (a temperatura absoluta) e n\u00e3o em Celsius. Um argumento utilizado por Essex et al.<\/em> \u00e9 que as temperaturas n\u00e3o est\u00e3o em equil\u00edbrio. Rigorosamente falando, isto \u00e9 v\u00e1lido para a maior parte dos casos. Mas em geral, estas leis ainda sim fornecem resultados razo\u00e1veis porque as temperaturas est\u00e3o pr\u00f3ximas de estar em equil\u00edbrio em meteorologia e climatologia. O trabalho n\u00e3o traz nenhum fato novo \u2013 eu pensava que estes aspectos j\u00e1 estavam bem conhecidos. <\/p>\n

Atualiza\u00e7\u00e3o:<\/strong> Rabett Run<\/a> tem um bem detalhado conjunto de posts destrinchando cuidadosamente este trabalho.<\/p>\n

traduzido por Fernando M. Ramos e Ivan B. T. Lima<\/a> <\/small><\/p>\n

<\/lang_po><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Does a global temperature exist? This is the question asked in a recently published article in Journal of Non-Equilibrium Thermodynamics by Christopher Essex, Ross McKitrick, and Bjarne Andresen. The paper argues that the global mean temperature is not physical, and that there may be many other ways of computing a mean which will give different […]<\/p>\n","protected":false},"author":11,"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":[1,13,26],"tags":[],"class_list":{"0":"post-422","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-science","7":"category-faq","8":"category-rc-forum","9":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/422","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\/11"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=422"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/422\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=422"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=422"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=422"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}