{"id":87,"date":"2004-12-22T16:09:45","date_gmt":"2004-12-22T20:09:45","guid":{"rendered":"\/?p=87"},"modified":"2017-06-05T14:47:05","modified_gmt":"2017-06-05T19:47:05","slug":"how-do-we-know-that-recent-cosub2sub-increases-are-due-to-human-activities-updated","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2004\/12\/how-do-we-know-that-recent-cosub2sub-increases-are-due-to-human-activities-updated\/","title":{"rendered":"How do we know that recent CO2<\/sub> increases are due to human activities? Comment savons-nous que l’augmentation r\u00e9cente du CO2<\/sub> est due aux activit\u00e9s humaines ? (mise-\u00e0-jour<\/em>)<\/lang_fr>"},"content":{"rendered":"
\n

Note:<\/b>This is an update to an earlier post, which many found to be too technical. The original, and a series of comments on it, can be found here<\/a>. See also a more recent post here<\/a> for an even less technical discussion.<\/em><\/small><\/p>\n

Over the last 150 years, carbon dioxide (CO2<\/sub>) concentrations have risen from 280 to nearly 380 parts per million (ppm). The fact that this is due virtually entirely to human activities is so well established that one rarely sees it questioned. Yet it is quite reasonable to ask how we know this.<\/p>\n

Det finns en svensk \u00f6vers\u00e4ttning tillg\u00e4nglig h\u00e4r<\/a><\/lang_sw>
\nUna traducci\u00f3n en espa\u00f1ol est\u00e1 disponible aqu\u00ed<\/a>.<\/lang_sp>
\n
\npar Eric Steig (traduit par Gilles Delaygue)<\/small><\/p>\n

Note :<\/b>Ceci est une mise-\u00e0-jour d’un article pr\u00e9c\u00e9dent, que beaucoup ont trouv\u00e9 trop technique. L’original, ainsi qu’une s\u00e9rie de commentaires, se trouvent ici<\/a>.<\/em><\/small><\/p>\n

Pendant les 150 derni\u00e8res ann\u00e9es, la concentration en dioxyde de carbone (CO2<\/sub>) a augment\u00e9 de 280 \u00e0 380 parties par million (ppm). Le fait que cette augmentation soit due pratiquement enti\u00e8rement aux activit\u00e9s humaines est si bien \u00e9tabli qu’on le voit rarement remis en question. Pourtant, il est tout \u00e0 fait raisonnable de se demander comment nous le savons.<\/p>\n

(suite…<\/a>)
\n<\/lang_fr><\/p>\n

<\/p>\n

One way that we know that human activities are responsible for the increased CO2<\/sub> is simply by looking at historical records of human activities. Since the industrial revolution, we have been burning fossil fuels and clearing and burning forested land at an unprecedented rate, and these processes convert organic carbon into CO2<\/sub>. Careful accounting of the amount of fossil fuel that has been extracted and combusted, and how much land clearing has occurred, shows that we have produced far more CO2<\/sub> than now remains in the atmosphere. The roughly 500 billion metric tons of carbon we have produced is enough to have raised the atmospheric concentration of CO2<\/sub> to nearly 500 ppm. The concentrations have not reached that level because the ocean and the terrestrial biosphere have the capacity to absorb some of the CO2<\/sub> we produce.* However, it is the fact that we produce CO2<\/sub> faster<\/i> than the ocean and biosphere can absorb it that explains the observed increase.<\/p>\n

Another, quite independent way that we know that fossil fuel burning and land clearing specifically are responsible for the increase in CO2<\/sub> in the last 150 years is through the measurement of carbon isotopes. Isotopes<\/a> are simply different atoms with the same chemical behavior (isotope means \u201csame type\u201d) but with different masses. Carbon is composed of three different isotopes, 14<\/sup>C, 13<\/sup>C and 12<\/sup>C. 12<\/sup>C is the most common. 13<\/sup>C is about 1% of the total. 14<\/sup>C accounts for only about 1 in 1 trillion carbon atoms.<\/p>\n

CO2<\/sub> produced from burning fossil fuels or burning forests has quite a different isotopic composition from CO2<\/sub> in the atmosphere. This is because plants have a preference for the lighter isotopes (12<\/sup>C vs. 13<\/sup>C); thus they have lower 13<\/sup>C\/12<\/sup>C ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same 13<\/sup>C\/12<\/sup>C ratio \u2013 about 2% lower than that of the atmosphere. As CO2<\/sub> from these materials is released into, and mixes with, the atmosphere, the average 13<\/sup>C\/12<\/sup>C ratio of the atmosphere decreases.<\/p>\n

Isotope geochemists have developed time series of variations in the 14<\/sup>C and 13<\/sup>C concentrations of atmospheric CO2<\/sub>. One of the methods used is to measure the 13<\/sup>C\/12<\/sup>C in tree rings, and use this to infer those same ratios in atmospheric CO2<\/sub>. This works because during photosynthesis, trees take up carbon from the atmosphere and lay this carbon down as plant organic material in the form of rings, providing a snapshot of the atmospheric composition of that time. If the ratio of 13<\/sup>C\/12<\/sup>C in atmospheric CO2<\/sub> goes up or down, so does the 13<\/sup>C\/12<\/sup>C of the tree rings. This isn\u2019t to say that the tree rings have the same<\/i> isotopic composition as the atmosphere \u2013 as noted above, plants have a preference for the lighter isotopes, but as long as that preference doesn\u2019t change much, the tree-ring changes wiil track the atmospheric changes.<\/p>\n

Sequences of annual tree rings going back thousands of years have now been analyzed for their 13<\/sup>C\/12<\/sup>C ratios. Because the age of each ring is precisely known** we can make a graph of the atmospheric 13<\/sup>C\/12<\/sup>C ratio vs. time. What is found is at no time in the last 10,000 years are the 13<\/sup>C\/12<\/sup>C ratios in the atmosphere as low as they are today. Furthermore, the 13<\/sup>C\/12<\/sup>C ratios begin to decline dramatically just as the CO2<\/sub> starts to increase — around 1850 AD. This is exactly what we expect if the increased CO2<\/sub> is in fact due to fossil fuel burning. Furthermore, we can trace the absorption of CO2<\/sub> into the ocean by measuring the 13<\/sup>C\/12<\/sup>C ratio of surface ocean waters. While the data are not as complete as the tree ring data (we have only been making these measurements for a few decades) we observe what is expected: the surface ocean 13<\/sup>C\/12<\/sup>C is decreasing. Measurements of 13<\/sup>C\/12<\/sup>C on corals and sponges — whose carbonate shells reflect the ocean chemistry just as tree rings record the atmospheric chemistry — show that this decline began about the same time as in the atmosphere; that is, when human CO2<\/sub> production began to accelerate in earnest.***<\/p>\n

In addition to the data from tree rings, there are also of measurements of the 13<\/sup>C\/12<\/sup>C ratio in the CO2<\/sub> trapped in ice cores. The tree ring and ice core data both show that the total change in the 13<\/sup>C\/12<\/sup>C ratio of the atmosphere since 1850 is about 0.15%. This sounds very small but is actually very large relative to natural variability. The results show that the full glacial-to-interglacial change in 13<\/sup>C\/12<\/sup>C of the atmosphere — which took many thousand years — was about 0.03%, or about 5 times less than that observed in the last 150 years.<\/p>\n

For those who are interested in the details, some relevant references are:
\nStuiver, M., Burk, R. L. and Quay, P. D. 1984. 13C\/12C ratios and the transfer of biospheric carbon to the atmosphere. J. Geophys. Res. 89, 11,731-11,748.
\nFrancey, R.J., Allison, C.E., Etheridge, D.M., Trudinger, C.M., Enting, I.G., Leuenberger, M., Langenfelds, R.L., Michel, E., Steele, L.P., 1999. A 1000-year high precision record of d13Cin atmospheric CO2. Tellus 51B, 170\u2013193.
\nQuay, P.D., B. Tilbrook, C.S. Wong. Oceanic uptake of fossil fuel CO2: carbon-13 evidence. Science 256 (1992), 74-79
\n—————————
\nNotes<\/b>
\n*How much they can be expected to absorb in the long run is an interesting and important scientific question, discussed in some detail in Chapter 3 of the IPCC report<\/a>. Clearly, though, it is our ability to produce CO2 faster than the ocean and biosphere can absorb that it is the fundamental cause of the observed increase since pre-industrial times.
\n**The development of continuous series of tree rings going back thousands of years by using trees of overlapping age, is known as dendrochronology (see the Arizona Tree Ring lab web pages for more information on this).<\/a>
\n***There is a graph illustrating such sponge data here<\/a>. Thanks to F. Boehm for providing.<\/font><\/p>\n


\n<\/a>
\nL’une des fa\u00e7ons de savoir que les activit\u00e9s humaines sont responsables de l’augmentation en CO2<\/sub> est tout simplement de regarder les enregistrements historiques de ces activit\u00e9s. Depuis la R\u00e9volution Industrielle, nous avons brul\u00e9 des combustibles fossiles, coup\u00e9 et brul\u00e9 des for\u00eats \u00e0 un rythme jamais atteint auparavant, et ces processus convertissent du carbone organique en CO2<\/sub>. Une comptabilisation minutieuse de la quantit\u00e9 extraite et brul\u00e9e de combustible fossile, et de la surface d\u00e9forest\u00e9e, montre que nous avons \u00e9mis beaucoup plus de CO2<\/sub> qu’il n’en reste actuellement dans l’atmosph\u00e8re. Les 500 milliards de tonnes de carbone, approximativement, que nous avons produit, sont suffisants pour augmenter la concentration atmosph\u00e9rique du CO2<\/sub> jusqu’\u00e0 presque 500 ppm. La concentration n’a pourtant pas atteint ce niveau, parce que les oc\u00e9ans et la biosph\u00e8re ont la capacit\u00e9 d’absorber une partie de ce que nous produisons.* N\u00e9anmoins, c’est le fait que nous produisons du CO2<\/sub> plus vite<\/i> que les oc\u00e9ans et la biosph\u00e8re ne peuvent l’absorber qui explique l’augmentation observ\u00e9e.<\/p>\n

Une autre m\u00e9thode, compl\u00e8tement ind\u00e9pendante de la pr\u00e9c\u00e9dente, de savoir que ce sont bien les combustibles fossiles et la d\u00e9forestation qui sont responsables de l’augmentation du CO2<\/sub> pendant les derniers 150 ans est obtenue par la mesure des isotopes du carbone. Des isotopes<\/a> sont simplement des atomes diff\u00e9rents ayant le m\u00eame comportement chimique (isotope signifie “m\u00eame type”), mais de masses diff\u00e9rentes. Le carbone est form\u00e9 de trois isotopes diff\u00e9rents, 14<\/sup>C, 13<\/sup>C and 12<\/sup>C. 12<\/sup>C est le plus commun. 13<\/sup>C repr\u00e9sente environ 1% du total. 14<\/sup>C n’est pr\u00e9sent qu’une fois sur mille milliards d’atomes de carbone.<\/p>\n

Le CO2<\/sub> produit par la combustion de bois ou de combustibles fossiles (hydrocarbures, charbon) a une composition isotopique tr\u00e8s diff\u00e9rente du CO2<\/sub> atmosph\u00e9rique. C’est \u00e0 cause de la pr\u00e9f\u00e9rence des plantes pour les isotopes l\u00e9gers (12<\/sup>C par rapport \u00e0 13<\/sup>C), ainsi elles ont des rapports 13<\/sup>C\/12<\/sup>C plus faibles. Comme les combustibles fossiles sont issus de plantes disparues, les plantes et les combustibles fossiles ont tous \u00e0 peu pr\u00e8s le m\u00eame rapport 13<\/sup>C\/12<\/sup>C \u2013environ 2% plus bas que celui de l’atmosph\u00e8re. Comme le CO2<\/sub> de ces mati\u00e8res est \u00e9mis et se m\u00e9lange dans l’atmosph\u00e8re, le rapport moyen 13<\/sup>C\/12<\/sup>C de l’atmosph\u00e8re d\u00e9cro\u00eet.<\/p>\n

Les g\u00e9ochimistes des isotopes ont d\u00e9velopp\u00e9 des s\u00e9ries temporelles de variations des concentrations atmosph\u00e9riques en 14<\/sup>C et 13<\/sup>C. L’une des m\u00e9thodes utilis\u00e9es est de mesurer le rapport 13<\/sup>C\/12<\/sup>C des cernes d’arbres, et d’utiliser ceci pour estimer ce m\u00eame rapport pour le CO2<\/sub> atmosph\u00e9rique. Ca marche parce que durant la photosynth\u00e8se, les arbres absorbent leur carbone \u00e0 partir de l’atmosph\u00e8re et fixent ce carbone en mati\u00e8re organique sous forme de cernes, fournissant un instantan\u00e9 de la composition atmosph\u00e9rique de cette \u00e9poque. Si le rapport 13<\/sup>C\/12<\/sup>C du CO2<\/sub> atmosph\u00e9rique monte ou descend, il en est de m\u00eame pour le rapport 13<\/sup>C\/12<\/sup>C des cernes d’arbres. Ce qui ne veut pas dire que les cernes d’arbre ont la m\u00eame<\/i> composition isotopique que l’atmosph\u00e8re \u2013comme mentionn\u00e9 pr\u00e9c\u00e9demment les plantes ont une pr\u00e9f\u00e9rence pour les isotopes l\u00e9gers, mais tant que cette pr\u00e9f\u00e9rence ne change pas beaucoup, les variations des cernes d’arbre vont suivre celles de l’atmosph\u00e8re.<\/p>\n

Des s\u00e9quences de cernes annuels d’arbres longues de plusieurs milliers d’ann\u00e9es ont maintenant \u00e9t\u00e9 analys\u00e9es pour leur rapport 13<\/sup>C\/12<\/sup>C. Puisque l’\u00e2ge de chaque cerne est connu pr\u00e9cis\u00e9ment** nous pouvons faire un graphe du rapport 13<\/sup>C\/12<\/sup>C dans l’atmosph\u00e8re au cours du temps. On trouve que jamais sur les derniers 10000 ans le rapport 13<\/sup>C\/12<\/sup>C dans l’atmosph\u00e8re n’a \u00e9t\u00e9 plus bas qu’aujourd’hui. De plus, le rapport 13<\/sup>C\/12<\/sup>C commence \u00e0 diminuer de fa\u00e7on importante juste quand le CO2<\/sub> commence \u00e0 augmenter \u2013vers 1850. C’est exactement ce \u00e0 quoi on s’attend si l’augmentation du CO2<\/sub> est bien due \u00e0 l’utilisation de combustibles fossiles. De plus, nous pouvons suivre l’absorption du CO2<\/sub> par les oc\u00e9ans en mesurant le rapport 13<\/sup>C\/12<\/sup>C des eaux de surface. M\u00eame si les donn\u00e9es ne sont pas aussi compl\u00e8tes que pour les cernes d’arbres (nous avons seulement commenc\u00e9 ces mesures depuis quelques d\u00e9cennies), nous observons ce qui est attendu : le rapport 13<\/sup>C\/12<\/sup>C des eaux de surface diminue. Des mesures de 13<\/sup>C\/12<\/sup>C sur des coraux et des \u00e9ponges \u2013dont les squelettes carbonat\u00e9s refl\u00e8tent la composition chimique de l’oc\u00e9an comme les cernes d’arbres enregistrent celle de l’atmosph\u00e8re\u2013 indiquent que cette diminution a commenc\u00e9 \u00e0 peu pr\u00e8s au m\u00eame moment que dans l’atmosph\u00e8re, c’est-\u00e0-dire lorsque la production anthropique de CO2<\/sub> commen\u00e7a r\u00e9ellement \u00e0 s’acc\u00e9l\u00e9rer.*** <\/p>\n

En plus des donn\u00e9es provenant des cernes d’arbres, il y a aussi des mesures du rapport 13<\/sup>C\/12<\/sup>C sur le CO2<\/sub> emprisonn\u00e9 dans les carottes de glace. Les donn\u00e9es issues \u00e0 la fois des cernes d’arbre et des carottes de glace montrent que la variation totale du rapport 13<\/sup>C\/12<\/sup>C de l’atmosph\u00e8re depuis 1850 est d’environ 0,15%. Ce qui parait tr\u00e8s faible mais est en fait tr\u00e8s important par rapport \u00e0 la variabilit\u00e9 naturelle. Les r\u00e9sultats montrent que le changement total glaciaire-interglaciaire du rapport 13<\/sup>C\/12<\/sup>C dans l’atmosph\u00e8re \u2013changement qui a pris plusieurs milliers d’ann\u00e9es\u2013 \u00e9tait \u00e0 peu pr\u00e8s 0,03%, soit environ 5 fois moins que celui observ\u00e9 sur les derniers 150 ans.<\/p>\n

Pour ceux qui d\u00e9sireraient plus de d\u00e9tails, voici quelques publications pertinentes :
\nStuiver, M., Burk, R. L. and Quay, P. D. 1984. 13<\/sup>C\/12<\/sup>C ratios and the transfer of biospheric carbon to the atmosphere. J. Geophys. Res. 89, 1731-1748.
\nFrancey, R.J., Allison, C.E., Etheridge, D.M., Trudinger, C.M., Enting, I.G., Leuenberger, M., Langenfelds, R.L., Michel, E., Steele, L.P., 1999. A 1000-year high precision record of d13<\/sup>Cin<\/font> atmospheric CO2<\/sub>. Tellus 51B, 170-193.
\nQuay, P.D., B. Tilbrook, C.S. Wong. Oceanic uptake of fossil fuel CO2<\/sub>: carbon-13 evidence. Science 256 (1992), 74-79
\n—————————
\nNotes<\/b>
\n* Combien ils pourraient absorber sur le long terme est une question scientifique int\u00e9ressante et importante, discut\u00e9e en d\u00e9tails dans le Chapitre 3 du rapport du GIEC<\/a>. Clairement, la cause fondamentale de l’augmentation observ\u00e9e depuis la p\u00e9riode pr\u00e9-industrielle provient de notre capacit\u00e9 \u00e0 produire du CO2<\/sub> plus rapidement que les oc\u00e9ans et la biosph\u00e8re ne peuvent l’absorber.
\n** Le d\u00e9veloppement de s\u00e9ries continues de cernes d’arbres remontant sur plusieurs milliers d’ann\u00e9es, en utilisant des arbres qui se recouvrent dans le temps, est connu comme la dendrochronologie (voir les pages internet du laboratoire de l’Arizona sur les cernes d’abres<\/a> pour plus d’informations).
\n***Un graphe illustrant les donn\u00e9es des \u00e9ponges se trouve ici<\/a>. Merci \u00e0 F. Boehm d’avoir indiqu\u00e9 ce lien.<\/font><\/p>\n

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

Note:This is an update to an earlier post, which many found to be too technical. The original, and a series of comments on it, can be found here. See also a more recent post here for an even less technical discussion. Over the last 150 years, carbon dioxide (CO2) concentrations have risen from 280 to […]<\/p>\n","protected":false},"author":8,"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":[1,13,3,2],"tags":[],"class_list":{"0":"post-87","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-science","7":"category-faq","8":"category-greenhouse-gases","9":"category-paleoclimate","10":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/87","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\/8"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=87"}],"version-history":[{"count":9,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/87\/revisions"}],"predecessor-version":[{"id":20440,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/87\/revisions\/20440"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=87"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=87"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=87"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}