{"id":181,"date":"2005-09-02T09:53:32","date_gmt":"2005-09-02T13:53:32","guid":{"rendered":"\/?p=181"},"modified":"2025-09-24T19:51:16","modified_gmt":"2025-09-25T00:51:16","slug":"hurricanes-and-global-warming","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2005\/09\/hurricanes-and-global-warming\/","title":{"rendered":"Hurricanes and Global Warming – Is There a Connection? Huracanes y calentamiento global \u00bfHay conexi\u00f3n?<\/lang_sp>Ouragans et r\u00e9chauffement global – existe t’il un lien ?<\/lang_fr>"},"content":{"rendered":"
\n\n

by Stefan Rahmstorf, Michael Mann, Rasmus Benestad, Gavin Schmidt, and William Connolley<\/small><\/p>\n\n\n\n

On Monday August 29, Hurricane Katrina ravaged New Orleans, Louisiana and Missisippi, leaving a trail of destruction in her wake. It will be some time until the full toll of this hurricane can be assessed, but the devastating human and environmental impacts are already obvious.<\/p>\n\n\n\n

Katrina was the most feared of all meteorological events, a major hurricane making landfall in a highly-populated low-lying region. In the wake of this devastation, many have questioned whether global warming may have contributed to this disaster. Could New Orleans be the first major U.S. city ravaged by human-caused climate change?<\/p>\n\n\n\n

[lang_fr]
by Stefan Rahmstorf, Michael Mann, Rasmus Benestad, Gavin Schmidt, and William Connolley (traduit par Claire Rollion Bard)<\/small>
Le lundi 29 ao\u00fbt, l’ouragan Katrina a ravag\u00e9 la Nouvelle-Orl\u00e9ans, la Louisiane et le Mississipi, laissant une tra\u00een\u00e9e de destruction dans son sillage. Il va se passer du temps avant que le bilan total de cet ouragan soit estim\u00e9, mais les impacts environnementaux et humains sont d\u00e9j\u00e0 apparents.
Katrina \u00e9tait le plus craint des \u00e9v\u00e8nements m\u00e9t\u00e9orologiques, un ouragan majeur laissant un terrain vide dans une r\u00e9gion tr\u00e8s peupl\u00e9e de faible \u00e9l\u00e9vation. Dans le sillage de sa d\u00e9vastation, beaucoup se sont demand\u00e9s si le r\u00e9chauffement global pouvait avoir contribu\u00e9 \u00e0 ce d\u00e9sastre. La Nouvelle-Orl\u00e9ans pourrait-elle \u00eatre la premi\u00e8re ville majeure des Etats-Unis \u00e0 \u00eatre ravag\u00e9e par le changement climatique caus\u00e9 par les humains ?
(suite…)<\/a><\/p>\n\n\n\n

[\\lang_fr]
<\/p>\n\n\n\n\n\n\n\n

[lang_es]Traducci\u00f3n disponible aqui<\/a> (gracias a Mario Cuellar).[\/lang_es]<\/p>\n\n\n\n

The correct answer–the one we have indeed provided in previous posts (Storms & Global Warming II<\/a>, Some recent updates<\/a> and Storms and Climate Change<\/a>) –is that there is no way to prove that Katrina either was, or was not, affected by global warming. For a single event, regardless of how extreme, such attribution is fundamentally impossible. We only have one Earth, and it will follow only one of an infinite number of possible weather sequences. It is impossible to know whether or not this event would have taken place if we had not increased the concentration of greenhouse gases in the atmosphere as much as we have. Weather events will always result from a combination of deterministic factors (including greenhouse gas forcing or slow natural climate cycles) and stochastic factors (pure chance).<\/p>\n\n\n\n

Due to this semi-random nature of weather, it is wrong to blame any one event such as Katrina specifically on global warming – and of course it is just as indefensible to blame Katrina on a long-term natural cycle in the climate.<\/p>\n\n\n\n

Yet this is not the right way to frame the question. As we have also pointed out in previous posts, we can<\/em> indeed draw some important conclusions about the links between hurricane activity and global warming in a statistical<\/em> sense. The situation is analogous to rolling loaded dice: one could, if one was so inclined, construct a set of dice where sixes occur twice as often as normal. But if you were to roll a six using these dice, you could not blame it specifically on the fact that the dice had been loaded. Half of the sixes would have occurred anyway, even with normal dice. Loading the dice simply doubled the odds. In the same manner, while we cannot draw firm conclusions about one single hurricane, we can<\/em> draw some conclusions about hurricanes more generally. In particular, the available scientific evidence indicates that it is likely that global warming will make – and possibly already is making – those hurricanes that form more destructive than they otherwise would have been.<\/p>\n\n\n\n

The key connection is that between sea surface temperatures (we abbreviate this as SST) and the power of hurricanes. Without going into technical details about the dynamics and thermodynamics involved in tropical storms and hurricanes (an excellent discussion of this can be found here<\/a>), the basic connection between the two is actually fairly simple: warm water, and the instability in the lower atmosphere that is created by it, is the energy source of hurricanes. This is why they only arise in the tropics and during the season when SSTs are highest (June to November in the tropical North Atlantic).<\/p>\n\n\n\n

SST is not the only influence on hurricane formation. Strong shear in atmospheric winds (that is, changes in wind strength and direction with height in the atmosphere above the surface), for example, inhibits development of the highly organized structure that is required for a hurricane to form. In the case of Atlantic hurricanes, the El Nino\/Southern Oscillation<\/a> tends to influence the vertical wind shear<\/a>, and thus, in turn, the number of hurricanes that tend to form in a given year. Many other features of the process of hurricane development and strengthening, however, are closely linked to SST.<\/p>\n\n\n\n

Hurricane forecast models (the same ones that were used to predict Katrina’s path) indicate a tendency for more intense (but not<\/em> overall more frequent) hurricanes when they are run for climate change scenarios<\/a> (Fig. 1).<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Figure 1. <\/b>Model Simulation of Trend in Hurricanes (from Knutson et al, 2004<\/a>)<\/p>\n\n\n\n

In the particular simulation shown above, the frequency of the strongest (category 5) hurricanes roughly triples in the anthropogenic climate change scenario relative to the control. This suggests that hurricanes may indeed become more destructive (1) as tropical SSTs warm due to<\/a> anthropogenic impacts.<\/p>\n\n\n\n

But what about the past? What do the observations of the last century actually show? Some past studies (e.g. Goldenberg et al, 2001) assert that there is no evidence of any long-term increase in statistical measures of tropical Atlantic hurricane activity, despite the ongoing global warming. These studies, however, have focused on the frequency<\/em> of all<\/em> tropical storms and hurricanes (lumping the weak ones in with the strong ones) rather than a measure of changes in the intensity<\/em> of the storms. As we have discussed elsewhere on this site<\/a>, statistical measures that focus on trends in the strongest category storms, maximum hurricane winds, and changes in minimum central pressures, suggest a systematic increase in the intensities of those storms that form. This finding is consistent with the model simulations.<\/p>\n\n\n\n

A recent study in Nature<\/em> by Emanuel (2005) examined, for the first time, a statistical measure of the power dissipation<\/em> associated with past hurricane activity (i.e., the “Power Dissipation Index” or “PDI”–Fig. 2). Emanuel found a close correlation between increases in this measure of hurricane activity (which is likely a better measure of the destructive potential of the storms than previously used measures) and rising tropical North Atlantic SST, consistent with basic theoretical expectations. As tropical SSTs have increased in past decades, so has the intrinsic destructive potential of hurricanes.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Figure 2. <\/b>Measure of total power dissipated annually by tropical cyclones in the North Atlantic (the power dissipation index “PDI”) compared to September tropical North Atlantic SST (from Emanuel, 2005<\/a>)<\/p>\n\n\n\n

The key question then becomes this: Why has SST increased in the tropics? Is this increase due to global warming (which is almost certainly in large part due to human impacts on climate)? Or is this increase part of a natural cycle?<\/p>\n\n\n\n

It has been asserted (for example, by the NOAA National Hurricane Center<\/a>) that the recent upturn in hurricane activity is due to a natural cycle, e.g. the so-called Atlantic Multidecadal Oscillation (“AMO”)<\/a>. The new results by Emanuel (Fig. 2) argue against this hypothesis being the sole explanation: the recent increase in SST (at least for September as shown in the Figure) is well outside the range of any past oscillations. Emanuel therefore concludes in his paper that “the large upswing in the last decade is unprecedented, and probably reflects the effect of global warming.” However, caution is always warranted with very new scientific results until they have been thoroughly discussed by the community and either supported or challenged by further analyses. Previous analysis of the AMO and natural oscillation modes in the Atlantic (Delworth and Mann, 2000; Kerr, 2000) suggest that the amplitude of natural SST variations averaged over the tropics is about 0.1-0.2 \u00baC, so a swing from the coldest to warmest phase could explain up to ~0.4 \u00baC warming.<\/p>\n\n\n\n

What about the alternative hypothesis: the contribution of anthropogenic greenhouse gases to tropical SST warming? How strong do we expect this to be? One way to estimate this is to use climate models. Driven by anthropogenic forcings, these show a warming of tropical SST in the Atlantic of about 0.2 – 0.5 \u00baC. Globally, SST has increased by ~0.6 \u00baC<\/a> in the past hundred years. This mostly reflects the response to global radiative forcings, which are dominated by anthropogenic forcing over the 20th Century. Regional modes of variability, such as the AMO, largely cancel out and make a very small contribution in the global mean SST changes.<\/p>\n\n\n\n

Thus, we can conclude that both a natural cycle (the AMO) and anthropogenic forcing could have made roughly equally large contributions to the warming of the tropical Atlantic over the past decades, with an exact attribution impossible so far. The observed warming is likely the result of a combined effect: data strongly suggest that the AMO has been in a warming phase for the past two or three decades, and we also know that at the same time anthropogenic global warming is ongoing.<\/p>\n\n\n\n

Finally, then, we come back to Katrina. This storm was a weak (category 1) hurricane when crossing Florida, and only gained force later over the warm waters of the Gulf of Mexico. So the question to ask here is: why is the Gulf of Mexico so hot at present – how much of this could be attributed to global warming, and how much to natural variability? More detailed analysis of the SST changes in the relevant regions, and comparisons with model predictions, will probably shed more light on this question in the future. At present, however, the available scientific evidence suggests that it would be premature to assert<\/a> that the recent anomalous behavior can be attributed entirely to a natural cycle.<\/p>\n\n\n\n

But ultimately the answer to what caused Katrina is of little practical value. Katrina is in the past. Far more important is learning something for the future, as this could help reduce the risk of further tragedies. Better protection against hurricanes will be an obvious discussion point over the coming months, to which as climatologists we are not particularly qualified to contribute. But climate science can help us understand how human actions influence climate. The current evidence strongly suggests that:
(a) hurricanes tend to become more destructive as ocean temperatures rise, and
(b) an unchecked rise in greenhouse gas concentrations will very likely increase ocean temperatures further, ultimately overwhelming any natural oscillations.
Scenarios for future global warming show tropical SST rising by a few degrees, not just tenths of a degree (see e.g. results from the Hadley Centre model<\/a> and the implications for hurricanes shown in Fig. 1 above). That is the important message from science. What we need to discuss is not what caused Katrina, but the likelyhood that global warming will make hurricanes even worse in future.<\/p>\n\n\n\n

_____________________<\/small>
1. By ‘destructive’ we refer only to the intrinsic ability of the storm to do damage to its environment due to its strength. The potential increases that we discuss apply only to this intrinsic meteorological measure. We are not taking into account the potential for increased destruction (and cost) due to increasing population or human infrastructure.<\/small><\/p>\n\n\n\n

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

Delworth, T.L., Mann, M.E., Observed and Simulated Multidecadal Variability in the Northern Hemisphere, Climate Dynamics, 16, 661-676, 2000.<\/p>\n\n\n\n

Emanuel, K. (2005), Increasing destructiveness of tropical cyclones over the past 30 years, Nature, online publication; published online 31 July 2005 | doi: 10.1038\/nature03906<\/p>\n\n\n\n

Goldenberg, S.B., C.W. Landsea, A.M. Mestas-Nu\u00f1ez, and W.M. Gray. The recent increase in Atlantic hurricane activity. Causes and implications. Science, 293:474-479 (2001).<\/p>\n\n\n\n

Kerr, R.A., 2000, A North Atlantic climate pacemaker for the centuries: Science, v. 288, p. 1984-1986.<\/p>\n\n\n\n

Knutson, T. K., and R. E. Tuleya, 2004: Impact of CO2-induced warming on simulated hurricane intensity and precipitation: Sensitivity to the choice of climate model and convective parameterization. Journal of Climate, 17(18), 3477-3495.<\/p>\n\n\n\n

[lang_fr]
La r\u00e9ponse correcte — celle que nous avons fournie dans les articles pr\u00e9c\u00e9dents (Storms & Global Warming II<\/a>, Some recent updates<\/a> and Storms and climate change<\/a>) — est qu’il n’y a aucune mani\u00e8re de prouver que Katrina \u00e9tait ou n’\u00e9tait pas affect\u00e9e par le r\u00e9chauffement global. Pour un seul \u00e9v\u00e9nement, sans regarder \u00e0 quel point il est extr\u00eame, une telle attribution est fondamentalement impossible. Nous avons seulement une Terre et elle suivra seulement une des infinies possibilit\u00e9s de s\u00e9quence du temps. Il est impossible de savoir si cet \u00e9v\u00e9nement aurait eu lieu ou pas si nous n’avions pas eu d’augmentation de la concentration de gaz \u00e0 effet de serre dans l’atmosph\u00e8re autant que nous avons. Les \u00e9v\u00e8nements du temps r\u00e9sultent toujours d’une combinaison de facteurs d\u00e9terminants (incluant le for\u00e7age des gaz \u00e0 effet de serre ou les cycles lents naturels du climat) et de facteurs stochastiques (pure chance).<\/p>\n\n\n\n

D\u00fb au semi-hasard de la nature du temps, nous avons tort d’imputer un seul \u00e9v\u00e9nement tel que Katrina au r\u00e9chauffement global – et, bien s\u00fbr, il est juste ind\u00e9fendable de bl\u00e2mer Katrina d’un cycle naturel \u00e0 long terme dans le climat.<\/p>\n\n\n\n

N\u00e9anmoins, ce n’est pas la bonne mani\u00e8re d’encadrer la question. Comme nous l’avons indiqu\u00e9 dans des articles pr\u00e9c\u00e9dents, nous pouvons en effet dessiner quelques conclusions importantes sur les liens entre l’activit\u00e9 des ouragans et le r\u00e9chauffement global dans un sens statistique. La situation est analogue \u00e0 faire rouler un d\u00e9 pip\u00e9 : on peut, si on a l’envie, construire un ensemble de d\u00e9s o\u00f9 les 6 sortent deux fois plus souvent que la normale. Mais si on voulait tirer un six avec ces d\u00e9s, on ne pourrait pas les bl\u00e2mer sp\u00e9cifiquement sur le fait que les d\u00e9s ont \u00e9t\u00e9 pip\u00e9s. La moiti\u00e9 des 6 seront tir\u00e9s de toute fa\u00e7on, m\u00eame avec un d\u00e9 normal. Charger les d\u00e9s a doubl\u00e9 simplement les impairs. De la m\u00eame mani\u00e8re, bien que nous ne puissions pas tirer des conclusions fermes sur un seul ouragan, on peut tirer certaines conclusions sur des ouragans plus g\u00e9n\u00e9ralement. En particulier, la documentation scientifique \u00e0 disposition indique qu’il est possible que le r\u00e9chauffement global fera – et l’a peut-\u00eatre d\u00e9j\u00e0 fait – que ces ouragans ont une forme plus destructive qu’ils auraient eu sinon.<\/p>\n\n\n\n

Le lien cl\u00e9 est celui entre les temp\u00e9ratures des eaux de la mer de surface (abr\u00e9viation TES – SST en anglais) et la puissance des ouragans. Sans aller trop loin dans les d\u00e9tails techniques sur la dynamique et la thermodynamique impliqu\u00e9es dans les temp\u00eates tropicales et les ouragans ( une excellent discussion de cela peut \u00eatre trouv\u00e9e ici<\/a>), le lien fondamental entre les deux est assez simple : l’eau chaude, et l’instabilit\u00e9 dans la basse atmosph\u00e8re que cela cr\u00e9e, est la source d’\u00e9nergie des ouragans. C’est pourquoi ils surviennent seulement dans les tropiques et durant la saison o\u00f9 les TESs sont les plus \u00e9lev\u00e9es (Juin \u00e0 Novembre dans l’Atlantique Nord tropical).<\/p>\n\n\n\n

La TES n’est pas la seule influence sur la formation des ouragans. Un fort cisaillement dans les vents atmosph\u00e9riques (c’est-\u00e0-dire, des changements dans la force du vent et la direction avec la hauteur d’atmosph\u00e8re au-dessus de la surface), par exemple limite le d\u00e9veloppement de la structure hautement organis\u00e9e qui est requise pour la formation d’un ouragan. Dans le cas des ouragans de l’Atlantique, l’ El Ni\u00f1o\/Oscillation Australe<\/a> a tendance \u00e0 influencer le cisaillement vertical du vent<\/a> et ainsi, en retour, le nombre d’ouragans qui se forment en une ann\u00e9e donn\u00e9e. Beaucoup d’autres caract\u00e9ristiques du processus du d\u00e9veloppement des ouragans et leur force sont, cependant, \u00e9troitement li\u00e9es \u00e0 la TES.<\/p>\n\n\n\n

Les mod\u00e8les de pr\u00e9vision m\u00e9t\u00e9orologique des ouragans (les m\u00eames qui ont \u00e9t\u00e9 utilis\u00e9s pour pr\u00e9dire le passage de Katrina) indiquent une tendance pour des ouragans plus intenses (mais, dans l’ensemble, pas plus fr\u00e9quents) quand ils simulent pour des sc\u00e9narii de changement climatique<\/a> (Fig. 1).<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Figure 1. <\/b>Simulation d’un mod\u00e8le de tendance pour les ouragans (d’apr\u00e8s Knutson et al., 2004<\/a>)<\/p>\n\n\n\n

Dans la simulation particuli\u00e8re montr\u00e9e ci-dessus, la fr\u00e9quence des ouragans les plus forts (cat\u00e9gorie 5) triplent grosso modo dans un sc\u00e9nario de changement anthropique du climat. Cela sugg\u00e8re que les ouragans pourraient, en effet, devenir plus destructifs (1)<\/a> puisque les TESs tropicales se r\u00e9chauffent \u00e0 cause des impacts anthropiques.<\/p>\n\n\n\n

Mais qu’en est-il du pass\u00e9 ? Que montrent vraiment les observations du dernier si\u00e8cle ? Certaines \u00e9tudes du pass\u00e9 (par exemple, Goldenberg et al., 2001) soutiennent qu’il n’y a aucune \u00e9vidence d’une augmentation \u00e0 long terme dans les mesures statistiques de l’activit\u00e9 des ouragans de l’Atlantique tropical, malgr\u00e9 le r\u00e9chauffement global en cours. Ces \u00e9tudes cependant se sont focalis\u00e9es sur la fr\u00e9quence de tous les ouragans et temp\u00eates tropicaux (r\u00e9unissant les faibles et les forts) plut\u00f4t que de mesurer des changements dans l’intensit\u00e9 des temp\u00eates. Comme nous l’avons discut\u00e9 auparavant<\/a> sur ce site, les mesures statistiques qui se focalisent sur des tendances dans les temp\u00eates de cat\u00e9gorie la pus forte, le maximum des vents des ouragans, et des changements dans le minimum des pressions centrales, sugg\u00e8rent une augmentation syst\u00e9matique dans les intensit\u00e9s des temp\u00eates qui se forment. Cette conclusion est coh\u00e9rente avec les simulations des mod\u00e8les.
Une \u00e9tude r\u00e9cente dans Nature<\/em> par Emanuel (2005) a examin\u00e9, pour la premi\u00e8re fois, une mesure statistique de la dissipation de la puissance associ\u00e9e \u00e0 l’activit\u00e9 des ouragans pass\u00e9s (i.e. “Indice de Dissipation de la Puissance” ou “IDP” — Fig. 2). Emanuel a trouv\u00e9 une \u00e9troite relation entre l’augmentation dans la mesure de l’activit\u00e9 de l’ouragan (qui est susceptible d’\u00eatre une meilleure mesure du potentiel destructif des temp\u00eates que les mesures pr\u00e9c\u00e9demment utilis\u00e9es) et la hausse de la TES de l’Atlantique Nord tropical, ce qui est coh\u00e9rent avec les attentes th\u00e9oriques. Comme les TESs tropicales ont augment\u00e9 durant les derni\u00e8res d\u00e9cennies, alors le potentiel destructif intrins\u00e8que des ouragans l’a fait aussi.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Figure 2. <\/b>Mesure de la puissance totale dissip\u00e9e annuellement par les cyclones tropicaux (Indice de Dissipation de Puissance “IDP”) compar\u00e9e \u00e0 la TES de Septembre de l’Atlantique Nord tropical (d’apr\u00e8s Emanuel, 2005)<\/p>\n\n\n\n

La question cl\u00e9 devient alors : pourquoi la TES a t-elle augment\u00e9 dans les tropiques ? Est-ce que cette augmentation est due au r\u00e9chauffement global (qui est presque certainement en grande partie d\u00fb \u00e0 l’impact humain sur le climat) ? Ou est-ce que cette augmentation fait partie d’un cycle naturel ?
Il a \u00e9t\u00e9 affirm\u00e9 (par exemple, par le NOAA National Hurricane Center<\/a>) que la r\u00e9cente reprise dans l’activit\u00e9 des ouragans est due \u00e0 une cycle naturel, nomm\u00e9 Atlantic Multidecadal Oscillation<\/a> (“AMO”). Les nouveaux r\u00e9sultats d’Emanuel (Fig. 2) argumentent contre cette hypoth\u00e8se comme seule explication : l’augmentation r\u00e9cente de la TES (au moins pour Septembre comme montr\u00e9 dans la figure) est bien en dehors de la gamme des oscillations pass\u00e9es. Emanuel conclut donc dans cet article que “la large hausse dans la derni\u00e8re d\u00e9cennie est sans pr\u00e9c\u00e9dent, et probablement refl\u00e8te l’effet du r\u00e9chauffement global”. Cependant, la prudence est toujours de mise avec des r\u00e9sultats scientifiques tr\u00e8s nouveaux jusqu’\u00e0 ce qu’ils aient \u00e9t\u00e9 compl\u00e8tement discut\u00e9s par la communaut\u00e9 et soit support\u00e9s, soit affaiblis par des analyses suppl\u00e9mentaires. Des analyses pr\u00e9c\u00e9dentes du AMO et des modes d’oscillations naturelles dans l’Atlantique (Delworth and Mann, 2000 ; Kerr, 2000) sugg\u00e8rent que l’amplitude des variations naturelles de la SST moyenn\u00e9e sur les tropiques est d’environ 0,1 – 0,2 \u00b0C, donc un basculement de la phase la plus froide \u00e0 la phase la plus chaude peut expliquer jusqu’\u00e0 0,4 \u00b0C de r\u00e9chauffement.<\/p>\n\n\n\n

Qu’en est-il de l’hypoth\u00e8se alternative : la contribution des gaz \u00e0 effet de serre anthropiques au r\u00e9chauffement de la TES tropicale ? A quelle force s’attend-on ? Une mani\u00e8re d’estimer cela est d’utiliser des mod\u00e8les climatiques. Diriger par des for\u00e7ages anthropiques, cela montre un r\u00e9chauffement de la TES tropicale dans l’Atlantique d’environ 0,2 – 0,5 \u00b0C. Globalement, la TES a augment\u00e9 d’environ 0,6 \u00b0C <\/a>dans les derniers 100 ans. Cela refl\u00e8te principalement la r\u00e9ponse \u00e0 des for\u00e7ages radiatifs globaux qui sont domin\u00e9s par le for\u00e7age anthropique au cours du XXe si\u00e8cle. Les modes r\u00e9gionaux de variabilit\u00e9, comme l’AMO, se neutralisent largement et font une faible contribution dans les changements globaux de TES moyenne.<\/p>\n\n\n\n

Ainsi, nous pouvons conclure qu’\u00e0 la fois un cycle naturel (l’AMO) et le for\u00e7age anthropique pourrait avoir fait grosso modo \u00e9galement de larges contributions au r\u00e9chauffement de l’Atlantique tropical au cours des derni\u00e8res d\u00e9cennies, avec une attribution exacte impossible jusqu’ici. Le r\u00e9chauffement observ\u00e9 est probablement le r\u00e9sultat d’un effet combin\u00e9 : des donn\u00e9es sugg\u00e8rent fortement que l’AMO a \u00e9t\u00e9 dans une phase de r\u00e9chauffement pour les deux ou trois derni\u00e8res d\u00e9cennies, et nous savons aussi qu’au m\u00eame moment le r\u00e9chauffement anthropique global est en cours.<\/p>\n\n\n\n

Alors, finalement, nous retournons \u00e0 Katrina. Cette temp\u00eate \u00e9tait un faible ouragan (cat\u00e9gorie 1) quand elle a crois\u00e9 la Floride, et a gagn\u00e9 de la force plus tard au-dessus des eaux chaudes du Golfe du Mexique. La question a pos\u00e9 alors ici est : pourquoi le Golfe du Mexique est si chaud actuellement – combien de cela peut \u00eatre attribu\u00e9 au r\u00e9chauffement global et combien \u00e0 la variabilit\u00e9 naturelle ? Des analyses plus d\u00e9taill\u00e9es des changements de la TES dans les r\u00e9gions en question, et des comparaisons avec les pr\u00e9dictions des mod\u00e8les, apporteront probablement de la lumi\u00e8re sur cette question dans l’avenir. \u00c0 pr\u00e9sent, cependant, la documentation scientifique disponible sugg\u00e8re qu’il serait pr\u00e9matur\u00e9 d’affirmer<\/a> que le r\u00e9cent comportement anormal peut \u00eatre attribu\u00e9 enti\u00e8rement \u00e0 un cycle naturel.<\/p>\n\n\n\n

Mais, en fin de compte, la r\u00e9ponse \u00e0 ce qui a caus\u00e9 Katrina est d’une valeur pratique faible. Katrina est dans le pass\u00e9. Beaucoup plus important est d’apprendre quelque chose pour le futur, puisque cela pourrait nous aider \u00e0 r\u00e9duire le risque de trag\u00e9dies suppl\u00e9mentaires. Une meilleure protection contre les ouragans sera un point de discussion \u00e9vident dans les mois \u00e0 venir, auquel, en tant que climatologues, nous ne sommes pas particuli\u00e8rement qualifi\u00e9s pour contribuer. Mais la science du climat peut nous aider \u00e0 comprendre comment les actions humaines influencent le climat. La documentation actuelle sugg\u00e8re fortement que :
(a) les ouragans ont tendance \u00e0 devenir plus destructifs quand les temp\u00e9ratures de l’oc\u00e9an augmentent, et
(b) une hausse incontr\u00f4l\u00e9e dans les concentrations des gaz \u00e0 effet de serre sera fortement susceptible d’augmenter plus les temp\u00e9ratures de l’oc\u00e9an, surpassant en fin de compte toutes oscillations naturelles.
Les sc\u00e9narii pour un r\u00e9chauffement global futur montrent une TES tropicale augmentant de quelques degr\u00e9s, pas juste des dixi\u00e8mes de degr\u00e9 (voir par exemple, r\u00e9sultats du mod\u00e8le du Hadley Centre<\/a> et les implications montr\u00e9es pour les ouragans fig. 1). C’est le message important de la science. Ce dont nous avons besoin de discuter n’est pas ce qui a caus\u00e9 Katrina, mais la probabilit\u00e9 qu’un r\u00e9chauffement climatique fera des ouragans m\u00eame pires dans le futur.

1. Par “destructif” nous nous r\u00e9f\u00e9rons \u00e0 la capacit\u00e9 intrins\u00e8que des temp\u00eates de faire des dommages \u00e0 l’environnement \u00e0 cause de sa force. L’augmentation potentielle dont nous discutons s’applique seulement \u00e0 cette mesure m\u00e9t\u00e9orologique intrins\u00e8que. Nous ne prenons pas en compte le potentiel pour une destruction accrue (et un co\u00fbt) due \u00e0 une population en hausse ou \u00e0 l’infrastructure humaine.
<\/small>
R\u00e9f\u00e9rences :<\/p>\n\n\n\n

Delworth, T.L., Mann, M.E., Observed and Simulated Multidecadal Variability in the Northern Hemisphere, Climate Dynamics, 16, 661-676, 2000.
Emanuel, K. (2005), Increasing destructiveness of tropical cyclones over the past 30 years, Nature, online publication; published online 31 July 2005 | doi: 10.1038\/nature03906
Goldenberg, S.B., C.W. Landsea, A.M. Mestas-Nu\u00f1ez, and W.M. Gray. The recent increase in Atlantic hurricane activity. Causes and implications. Science, 293:474-479 (2001).
Kerr, R.A., 2000, A North Atlantic climate pacemaker for the centuries: Science, v. 288, p. 1984-1986.
Knutson, T.K., and R.E. Tuleya, 2004: Impact of CO2-induced warming on simulated hurricane intensity and precipitation: Sensitivity to the choice of the climate model and convective parametization. Journal of Climate, 17 (18), 3477-3495.<\/p>\n\n\n\n

\u00a0[\\lang_fr]<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

by Stefan Rahmstorf, Michael Mann, Rasmus Benestad, Gavin Schmidt, and William Connolley On Monday August 29, Hurricane Katrina ravaged New Orleans, Louisiana and Missisippi, leaving a trail of destruction in her wake. It will be some time until the full toll of this hurricane can be assessed, but the devastating human and environmental impacts are […]<\/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":[5,1,27,13,21,9,19],"tags":[],"class_list":{"0":"post-181","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-modelling","7":"category-climate-science","8":"category-el-nino","9":"category-faq","10":"category-hurricanes","11":"category-instrumental-record","12":"category-oceans","13":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/181","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=181"}],"version-history":[{"count":2,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/181\/revisions"}],"predecessor-version":[{"id":26220,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/181\/revisions\/26220"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=181"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=181"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=181"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}