{"id":160,"date":"2005-06-07T09:33:07","date_gmt":"2005-06-07T13:33:07","guid":{"rendered":"\/?p=160"},"modified":"2009-05-26T08:51:00","modified_gmt":"2009-05-26T13:51:00","slug":"how-much-of-the-recent-cosub2sub-increase-is-due-to-human-activities","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2005\/06\/how-much-of-the-recent-cosub2sub-increase-is-due-to-human-activities\/","title":{"rendered":"How much of the recent CO2<\/sub> increase is due to human activities? L’accroissement du CO2 atmosph\u00e9rique: sommes nous enti\u00e8rement responsable?<\/lang_fr>"},"content":{"rendered":"
\n

Contributed by Corinne Le Qu\u00e9r\u00e9<\/a>, University of East Anglia.<\/p>\n

This question keeps coming back, although we know the answer very well: all of the recent CO2<\/sub> increase in the atmosphere is due to human activities, in spite of the fact that both the oceans and the land biosphere respond to global warming. There is a lot of evidence to support this statement which has been explained in a previous posting here <\/a> and in a letter in Physics Today <\/a>. However, the most convincing arguments for scientists (based on isotopes and oxygen decreases in the atmosphere) may be hard to understand for the general public because they require a high level of scientific knowledge. I present simpler evidence of the same statement based on ocean observations, and I explain how we know that not only part of the atmospheric CO2<\/sub> increase is due to human activities, but all of it.
\nCorinne Le Qu\u00e9r\u00e9<\/a>, Universit\u00e9 d’East Anglia.<\/p>\n

C’est une question qui revient sans cesse, bien que nous connaissions d\u00e9j\u00e0 la r\u00e9ponse : nous sommes responsable de la totalit\u00e9 de l’accroissement r\u00e9cent du CO2<\/sub> atmosph\u00e9rique, et ceci, malgr\u00e9 le fait que les oc\u00e9ans et la biosph\u00e8re terrestre r\u00e9pondent tous deux aux changements de r\u00e9chauffement global. Les \u00e9vidences les plus convaincantes pour les scientifiques (bas\u00e9es sur le d\u00e9croissement de l’oxyg\u00e8ne et des isotopes du carbone) ont d\u00e9ja \u00e9t\u00e9 expliqu\u00e9es dans une page pr\u00e9c\u00e9dente disponible ici<\/a> et dans une lettre \u00e0 la revue sp\u00e9cialis\u00e9e Physics Today<\/a>. Cependant, ces \u00e9vidences peuvent \u00eatre difficiles \u00e0 saisir pour les non-sp\u00e9cialistes car elles requi\u00e8rent des connaissances scientifiques importantes. Je pr\u00e9sente ici des \u00e9vidences plus simples qui m\u00e8nent aux m\u00eames conclusions et qui expliquent comment on sait que nous sommes responsables non seulement d’une partie de l’accroissement r\u00e9cent du CO2<\/sub> atmosph\u00e9rique, mais de la totalit\u00e9.
\n(suite…)<\/a><\/p>\n

<\/lang_fr>
\n<\/p>\n

\nOn time-scales of ~100 years, there are only two reservoirs that can naturally exchange large quantities of CO2<\/sub> with the atmosphere: the oceans and the land biosphere (forests and soils). The mass of carbon (carbon is the “C” in CO2<\/sub>) must be conserved. If the atmospheric CO2<\/sub> increase was caused, even in part, by carbon emitted from the oceans or the land, we would measure a carbon decrease in these two reservoirs.\n<\/p>\n

\nNumber of observations of carbon decreasing in the global oceans: zero.<\/p>\n

\nNumber of observations of carbon increasing in the global oceans: more than 20 published studies using 6 independent methods.
\nThe methods are:
\n(1)<\/b> direct observations of the partial pressure of CO2<\/sub> at the ocean surface (Takahashi et al. 2002<\/a>),
\n(2)<\/b> observations of the spatial distribution of atmospheric CO2<\/sub> which show how much carbon goes in and out of the different oceanic regions (Bousquet et al. 2000),
\n(3)<\/b> observations of carbon, oxygen, nutrients and CFCs combined to remove the mean imprint of biological processes (Sabine et al. 2004<\/a>),
\n(4)<\/b> observations of carbon and alkalinity for two time-periods combined with an estimate of water age based on CFCs (McNeil et al. 2002), and the simultaneous observations of atmospheric CO2<\/sub> increase and the decrease in (5)<\/b> oxygen (Keeling et al. 1996), and (6)<\/b> carbon 13 (Ciais et al. 1995) in the atmosphere.<\/p>\n

The principle of the last two methods is that both fossil fuel burning and biospheric respiration consume oxygen and reduce carbon 13 as they produce CO2<\/sub>, but the exchange of CO2<\/sub> with the oceans has only a small impact on atmospheric oxygen and carbon 13. The measure of atmospheric CO2<\/sub> increase together with oxygen or carbon 13 decrease gives the distribution between the different reservoirs.<\/p>\n

All <\/b>the estimates show that the carbon content of the oceans is increasing by ~ 2±1 PgC every year (current burning of fossil fuel is ~7 PgC per year). One method is able to go back in time and shows that the carbon content of the oceans has increased by 118±19 PgC in the last 200 years. There is some uncertainty about the exact amount that the oceans have taken up, but not about the direction of the change. The oceans cannot be a source of carbon to the atmosphere, because we observe them to be a sink of carbon from the atmosphere.<\/p>\n

\nWhat about the land biosphere? We know that deforestation has contributed to the increase in atmospheric CO2<\/sub>. Yet because carbon needs to be conserved, observations of the carbon increase in the atmosphere and the oceans combined with estimates of fossil fuel burning tell us that deforestation has been largely compensated by enhanced growth by the land biosphere. For example, during 1980 to 1999, fossil fuel burning was 117±5 PgC, and the carbon increase in the atmosphere and the oceans were 65±1 and 37±8 PgC, respectively. Thus that leaves 15±9 PgC that has been taken up by the land. This 15±9 PgC includes deforestation (and other land-use changes) which reduced the land biosphere by 24±12 PgC, and an additional land uptake of 39±18 PgC in response to elevated CO2<\/sub> and climate changes (Sabine et al. 2004). Here also there is some uncertainty about the exact amount, but there is no uncertainty that the land biosphere has taken up a quantity of CO2<\/sub> that is roughly equivalent to the deforestation.<\/p>\n

\nWhy are the ocean and land taking up carbon, when we know that warming of the oceans reduces the solubility of CO2<\/sub> and warming of the land accelerates bacterial degradation of the soils? The answer is that warming is not the only process that influences the oceans and land biosphere. The dominant process in the oceans is the response to increasing atmospheric CO2<\/sub> itself. If the oceans had not warmed, they might have taken up even more carbon, although we cannot say for sure because warming may have other impacts, for example on marine biota. On land, bacterial degradation of the soils may have increased in response to warming, but for the moment this effect is smaller than the land response to other processes (for example fertilization by CO2<\/sub> and nitrogen, changes in precipitation, etc).<\/p>\n

\nIs this consistent with what we know of the glaciations? Yes. During glaciations, the balance of processes was very different. Cooling and other climate changes occurred first. The response of the oceans and land biosphere to climate caused the atmospheric CO2<\/sub> to decrease, which caused more cooling (more on the feedbacks between temperature and CO2<\/sub> can be found here<\/a>). During glaciations, there were no external changes in atmospheric CO2<\/sub> and the oceans and land biosphere responded primarily to climate change. In the last 200 years, there have been large changes in atmospheric CO2<\/sub> as a result of human activities, and the oceans and land biosphere respond primarily to rising CO2<\/sub>.<\/p>\n

\nIn summary, we know that the rise in atmospheric CO2<\/sub> is entirely caused by fossil fuel burning and deforestation because many independent observations show that the carbon content has also increased in both the oceans and the land biosphere (after deforestation). If the oceans or land had contributed to the rise in atmospheric CO2<\/sub>, they would hold less carbon. Their response to warming may be real, but it is less than their response to increasing CO2<\/sub> and other climate changes for the moment.<\/p>\n

<\/p>\n

More on the carbon budget can be found in the last IPCC report here<\/a>, which includes budgets and uncertainties for different time periods and additional numbers for the small contribution of volcanoes and other geological reservoirs.<\/p>\n

References:
\nBousquet et al. (2000), Regional changes of CO2<\/sub> fluxes over land and oceans since 1980, Science, Vol 290, 1342-1346.
\nCiais et al. (1995), A Large Northern Hemisphere Terrestrial CO2<\/sub> Sink Indicated by the 13C\/12C Ratio of atmospheric CO2<\/sub>, Science, Vol 269, pp. 1098-1102.
\nKeeling, Piper and Heimann (1996), Global and hemispheric CO2<\/sub> sinks deduced from changes in atmospheric O2<\/sub> concentration, Nature, Vol 381, 218-221.
\nMcNeil et al. (2003), Anthropogenic CO2<\/sub> uptake by the ocean based on the global chlorofluorocarbon data set, Science, Vol 299, 235-239.
\nTakahashi et al. (2002), Global sea-air CO2<\/sub> flux based on climatological surface ocean pCO2<\/sub>, and seasonal biological and temperature effects, Deep Sea Research, Vol 49, 1601-1622.<\/p>\n

<\/small>
\n<\/p>\n

Il n’y a que deux r\u00e9servoirs qui peuvent \u00e9changer de larges quantit\u00e9s de CO2<\/sub> avec l’atmosph\u00e8re sur une p\u00e9riode aussi courte que ~100 ans: l’oc\u00e9an et la biosph\u00e8re terrestre (les for\u00eats et les sols). La quantit\u00e9 totale de carbone (le “C” du CO2<\/sub>) doit \u00eatre conserv\u00e9e. Si le CO2<\/sub> atmosph\u00e9rique \u00e9tait caus\u00e9, m\u00eame en partie, par l’\u00e9mission de carbone provenant de l’oc\u00e9an ou de la biosph\u00e8re terrestre, nous devrions mesurer une diminution du carbone dans ces deux r\u00e9servoirs.<\/p>\n

Nombre d’observations montrant une diminution du carbone dans l’oc\u00e9an global: z\u00e9ro.<\/p>\n

Nombre d’observations montrant une augmentation du carbone dans l’oc\u00e9an global: plus de 20 \u00e9tudes utilisant six m\u00e9thodes ind\u00e9pendantes. Les m\u00e9thodes utilis\u00e9es sont les suivantes:
\n(1)<\/b> mesures directes de la pression partielle du CO2<\/sub> \u00e0 la surface de l’oc\u00e9an (Takahashi et al. 2002<\/a>),
\n(2)<\/b> observations de la distribution spatiale du CO2<\/sub> atmosph\u00e9rique qui indiquent o\u00f9 et combien de carbon entre et sort des diff\u00e9rentes r\u00e9gions de l’oc\u00e9an (Bousquet et al. 2000),
\n(3)<\/b> observations du carbone, de l’oxyg\u00e8ne, des nutriments et des CFCs combin\u00e9s pour soustraire la signature des processus biologiques (Sabine et al. 2004<\/a>),
\n(4)<\/b> observations du carbone et de l’alkalinit\u00e9 s\u00e9par\u00e9es dans le temps combin\u00e9es \u00e0 une estimation de l’\u00e2ge de la masse d’eau bas\u00e9e sur les mesures de CFC (McNeil et al. 2002),
\net les observations simultan\u00e9es de l’augmentation du CO2<\/sub> atmosph\u00e9rique et de la d\u00e9croissance (5)<\/b> de l’oxyg\u00e8ne (Keeling et al. 1996), et (6)<\/b> de l’isotope 13 du carbone (Ciais et al. 1995).<\/p>\n

Les deux derni\u00e8res m\u00e9thodes sont bas\u00e9es sur le fait que la combustion de carbone fossilis\u00e9 et la respiration biosph\u00e9rique consomment l’oxyg\u00e8ne et r\u00e9duisent le carbone 13 tout en produisant du CO2<\/sub>, mais par contre les \u00e9changes oc\u00e9an-atmosph\u00e8re n’ont que peu d’influence sur l’oxyg\u00e8ne et le carbone 13. Donc la mesure de ces trois \u00e9l\u00e9ments permet d’estimer la distribution dans les diff\u00e9rents r\u00e9servoirs.<\/p>\n

Toutes les estimations montrent que l’oc\u00e9an absorbe 2\u00b11 PgC par ann\u00e9e (\u00e0 comparer \u00e0 la combustion de carbone fossilis\u00e9, qui approche les 7 PgC par ann\u00e9e). Une m\u00e9thode permet d’estimer que l’oc\u00e9an a absorb\u00e9 un total de 118\u00b119 PgC durant les 200 derni\u00e8res ann\u00e9es. La quantit\u00e9 exacte de carbone absorb\u00e9e par l’oc\u00e9an est sujette \u00e0 incertitude, mais la direction des changements observ\u00e9s ne fait aucun doute. L’oc\u00e9an ne peut pas avoir contribu\u00e9 \u00e0 l’augmentation du CO2<\/sub> atmosph\u00e9rique puisqu’il est lui-m\u00eame un puit de CO2<\/sub> (et non pas une source).<\/p>\n

Qu’en est-il de la biosph\u00e8re terrestre? Nous savons que la d\u00e9forestation a contribu\u00e9 \u00e0 l’accroissement du CO2<\/sub> atmosph\u00e9rique. Puisque la quantit\u00e9 totale du carbone doit \u00eatre conserv\u00e9e, les observations de l’accroissement du carbone dans l’atmosph\u00e8re et dans l’oc\u00e9an, ainsi que les estimations de la combustion de carbone nous indiquent que la d\u00e9forestation a \u00e9t\u00e9 en grande partie compens\u00e9e par un accroissement de la biosph\u00e8re terrestre m\u00eame. Par exemple entre 1980 et 1999, la combustion de carbone a \u00e9t\u00e9 de 117\u00b15 PgC, et le carbone dans l’atmosph\u00e8re et dans l’oc\u00e9ans augment\u00e9 de 65\u00b11 et 37\u00b18 PgC, respectivement. Il reste 15\u00b19 PgC qui doivent \u00eatre expliqu\u00e9s par des processus terrestre, ce qui inclu la d\u00e9forestation (et autres changements d’exploitation des sols) qui a r\u00e9duit la biosph\u00e8re de 24\u00b112 PgC, et un puit additionel de 39\u00b118 PgC qui est la r\u00e9ponse de la biosph\u00e8re terrestre au CO2<\/sub> atmosph\u00e9rique et aux changements de climat (Sabine et al. 2004). Ici encore , la quantit\u00e9 exacte de carbone absorb\u00e9e est sujette \u00e0 incertitude, mais il ne fait nul doute que la biosph\u00e8re terrestre a absorb\u00e9e une quantite de carbone qui \u00e9quivaut en gros la quantit\u00e9 de carbone \u00e9mises par la d\u00e9forestation.<\/p>\n

Comment cela est-il possible, quand nous savons que le r\u00e9chauffement de l’oc\u00e9an r\u00e9duit la solubilit\u00e9 du CO2<\/sub>, et que le r\u00e9chauffement des sols acc\u00e9l\u00e8re la d\u00e9gradation bact\u00e9rienne? C’est parceque ces processus ne sont pas les seuls \u00e0 influencer l’oc\u00e9an et la biosph\u00e8re terrestre. Dans l’oc\u00e9an, c’est la r\u00e9ponse \u00e0 l’augmentation du CO2<\/sub> dans l’atmosph\u00e8re m\u00eame qui domine. Si l’oc\u00e9an ne s’\u00e9tait pas r\u00e9chauff\u00e9, peut-\u00eatre aurait-t’il absorb\u00e9 plus de carbone, mais nous ne pouvons pas l’affirmer car d’autres processus r\u00e9agissent aussi au r\u00e9chauffement, entre autre les processus biologiques. Dans la biosph\u00e8re terrestre, la d\u00e9gradation des sols a pu augmenter suite au r\u00e9chauffement, mais pour l’instant cet effet est inf\u00e9rieur \u00e0 la r\u00e9ponse de la biosph\u00e8re terrestre aux autres processus (par exemple l’effet de fertilisation du CO2<\/sub> et de l’azote, les changements de pr\u00e9cipitations, etc).<\/p>\n

Cela est-il coh\u00e9rent avec ce que nous connaissons des glaciations? Oui. Lors des glaciations, l’\u00e9quilibre entre les processus \u00e9tait tr\u00e8s diff\u00e9rent. Le refroidissement ainsi que d’autres changements de climats sont d’abord survenus. La r\u00e9ponse de l’oc\u00e9an et de la biosph\u00e8re terrestre a cr\u00e9e une d\u00e9croissance du CO2<\/sub> atmosph\u00e9rique, ce qui a en retour provoqu\u00e9 un refroissement plus important (voir texte sur les feedbacks entre la temp\u00e9rature et le CO2<\/sub> ici)). Lors des glaciations, il n’y a eu aucun apport externe de CO2<\/sub> dans l’atmosph\u00e8re, et l’oc\u00e9an et la biosph\u00e8re terrestre ont r\u00e9pondu principalement aux changements de climat. Lors des deux derniers si\u00e8cles, les activit\u00e9s humaines ont \u00e9mises de grandes quantit\u00e9s de CO2<\/sub> dans l’atmosph\u00e8re, et l’oc\u00e9an et la biosph\u00e8re terrestre ont r\u00e9pondu principalement \u00e0 cet accroissement du CO2<\/sub>.<\/p>\n

En r\u00e9sum\u00e9, nous savons que l’accroissement r\u00e9cent du CO2<\/sub> dans l’atmosph\u00e8re est entirement caus\u00e9 par la combustion de carbone fossilis\u00e9 et par la d\u00e9forestation parceque nombre d’observations ind\u00e9pendantes montrent que le carbone a aussi augment\u00e9 dans l’oc\u00e9an et dans la biosph\u00e8re terrestre (apres avoir tenu compte de la d\u00e9forestation). Si l’oc\u00e9an ou la biosph\u00e8re terrestre avaient contribu\u00e9 \u00e0 l’accroissement du CO2<\/sub> dans l’atmosph\u00e8re, ils contiendraient moins de carbone. Leur r\u00e9ponse au r\u00e9chauffement global peut \u00eatre r\u00e9el, mais il est plus petit que leur r\u00e9ponse \u00e0 l’accroissement du CO2<\/sub> atmosph\u00e9rique et aux autres changements climatiques, pour le moment.<\/p>\n

Voir le dernier rapport du GIEC pour de plus amples informations sur le budget de carbone, qui pr\u00e9sente aussi la faible contribution des \u00e9missions volcaniques et autres r\u00e9servoirs g\u00e9ologiques.<\/p>\n

R\u00e9f\u00e9rences:
\nBousquet et al. (2000), Regional changes of CO2<\/sub> fluxes over land and oceans since 1980, Science, Vol 290, 1342-1346.
\nCiais et al. (1995), A Large Northern Hemisphere Terrestrial CO2<\/sub> Sink Indicated by the 13C\/12C Ratio of atmospheric CO2<\/sub>, Science, Vol 269, pp. 1098-1102.
\nKeeling, Piper and Heimann (1996), Global and hemispheric CO2<\/sub> sinks deduced from changes in atmospheric O2<\/sub> concentration, Nature, Vol 381, 218-221.
\nMcNeil et al. (2003), Anthropogenic CO2<\/sub> uptake by the ocean based on the global chlorofluorocarbon data set, Science, Vol 299, 235-239.
\nTakahashi et al. (2002), Global sea-air CO2 flux based on climatological surface ocean pCO2<\/sub>, and seasonal biological and temperature effects, Deep Sea Research, Vol 49, 1601-1622.<\/p>\n

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Contributed by Corinne Le Qu\u00e9r\u00e9, University of East Anglia. This question keeps coming back, although we know the answer very well: all of the recent CO2 increase in the atmosphere is due to human activities, in spite of the fact that both the oceans and the land biosphere respond to global warming. There is a […]<\/p>\n","protected":false},"author":12,"featured_media":0,"comment_status":"closed","ping_status":"open","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":[1,13,3,19],"tags":[],"class_list":{"0":"post-160","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-science","7":"category-faq","8":"category-greenhouse-gases","9":"category-oceans","10":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/160","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=160"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/160\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=160"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=160"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=160"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}