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about Gavin Schmidt

Gavin Schmidt is a climate modeler, working for NASA and with Columbia University.

Michael Crichton’s State of Confusion L’état de confusion de Michael Crichton

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In a departure from normal practice on this site, this post is a commentary on a piece of out-and-out fiction (unlike most of the other posts which deal with a more subtle kind). Michael Crichton’s new novel “State of Fear” is about a self-important NGO hyping the science of the global warming to further the ends of evil eco-terrorists. The inevitable conclusion of the book is that global warming is a non-problem. A lesson for our times maybe? Unfortunately, I think not.

par Gavin Schmidt (traduit par Alain Henry)

Ce message s’écarte des pratiques habituelles de ce site pour commenter une pièce de pure fiction (au contraire des autres messages qui abordent le sujet sous un angle plus subtil). Le nouveau roman de Michael Crichton, « Etat d’urgence » raconte comment une ONG encourage la recherche scientifique sur le réchauffement global pour servir les objectifs de méchants éco-terroristes. Le roman nous amène inévitablement à la conclusion que le réchauffement global est un faux problème. Une leçon pour notre époque? Malheureusement, je ne le pense pas.
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Why does the stratosphere cool when the troposphere warms? Pourquoi la stratosphère refroidit alors que la troposphère se réchauffe ?

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This post is obsolete and wrong in many respects. Please see this more recent post for links to the answer.

14/Jan/05: This post was updated in the light of my further education in radiation physics.
25/Feb/05: Groan…and again.

Recent discussions of climate change (MSU Temperature Record, ACIA) have highlighted the fact that the stratosphere is cooling while the lower atmosphere (troposphere) and surface appear to be warming. The stratosphere lies roughly 12 to 50 km above the surface and is marked by a temperature profile that increases with height. This is due to the absorbtion by ozone of the sun’s UV radiation and is in sharp contrast to the lower atmosphere. There it generally gets colder as you go higher due to the expansion of gases as the pressure decreases. Technically, the stratosphere has a negative ‘lapse rate’ (temperature increases with height), while the lower atmosphere’s lapse rate is positive.
Par Gavin Schmidt (traduit de l’anglais par Vincent Noël)
Des études récentes du changement climatique (MSU température Record, ACIA) ont mis en évidence un refroidissement de la stratosphère, en parallèle a un apparent réchauffement de la surface et la basse atmosphère (troposphère). La stratosphère se situe entre 12 et 50 km d’altitude environ. Elle se caractérise par un profil de température qui augmente avec l’altitude, en raison de l’absorption des radiations solaires ultraviolettes par l’ozone stratosphérique. Les choses sont très différentes dans la troposphère (de 0 a 12 km d’altitude environ), ou, en général, la température baisse lorsque l’altitude augmente, en raison de l’expansion des gaz alors que la pression atmosphérique diminue. En d’autres termes, la stratosphère a un gradient de température négatif, alors que la troposphère a un gradient positif.
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Gavin A. Schmidt

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Gavin Schmidt is a climate modeller at the NASA Goddard Institute for Space Studies and Earth Institute at Columbia University in New York and is interested in modeling past, present and future climate. He works on developing and improving coupled climate models and, in particular, is interested in how their results can be compared to paleoclimatic proxy data. He has worked on assessing the climate response to multiple forcings, including solar irradiance, atmospheric chemistry, aerosols, and greenhouse gases.

He received a BA (Hons) in Mathematics from Oxford University, a PhD in Applied Mathematics from University College London and was a NOAA Postdoctoral Fellow in Climate and Global Change Research. He was cited by Scientific American as one of the 50 Research Leaders of 2004, and has worked on Education and Outreach with the American Museum of Natural History, the College de France and the New York Academy of Sciences. He has over 100 peer-reviewed publications and is the co-author with Josh Wolfe of “Climate Change: Picturing the Science” (W. W. Norton, 2009), a collaboration between climate scientists and photographers. He was awarded the inaugural AGU Climate Communications Prize and was the EarthSky Science communicator of the year in 2011. He tweets at @ClimateOfGavin.

More information about his research and publication record can be found here.

All posts by gavin.

The Arctic Climate Impact Assessment III

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Does the ACIA overstate the problem of ozone depletion? The overview report states that the “stratospheric ozone layer over the Arctic is not expected to improve significantly for at least a few decades”. This is partly because CFC concentrations (that enhance stratospheric ozone destruction) are only expected to decrease slowly as a function of restrictions imposed by the Montreal Protocol and subsequent amendments. Another factor is the fact that stratospheric temperatures are generally cooling as greenhouse gases increase (see MSU Temperature Record, also Why does the stratosphere cool when the troposphere warms?). Due to the temperature dependence on the rates of chemical reactions involving ozone, cooler temperatures also lead to more ozone destruction. Stratospheric temperatures, particularly near the pole are also significantly influenced by dynamical changes, and in particular, the strength of the [Read more…] about The Arctic Climate Impact Assessment III

Antarctic cooling, global warming? Refroidissement de l’Antarctique, réchauffement global ?

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by Eric Steig and Gavin Schmidt
Long term temperature data from the Southern Hemisphere are hard to find, and by the time you get to the Antarctic continent, the data are extremely sparse. Nonetheless, some patterns do emerge from the limited data available. The Antarctic Peninsula, site of the now-defunct Larsen-B ice shelf, has warmed substantially. On the other hand, the few stations on the continent and in the interior appear to have cooled slightly (Doran et al, 2002; GISTEMP). At first glance this seems to contradict the idea of “global” warming, but one needs to be careful before jumping to this conclusion.

par Eric Steig et Gavin Schmidt (traduit par Claire Rollion-Bard)

Les données de température à long terme de l’hémisphère sud sont difficiles à trouver, et au moment où vous accédez au continent Antarctique, les données sont extrêmement éparses. Néanmoins quelques tendances émergent des quelques données disponibles. La Péninsule Antarctique, lieu de la barrière de glace Larsen-B, maintenant disparue, s’est réchauffée substantiellement. D’un autre côté, les quelques stations sur le continent et à l’intérieur semblent s’être légèrement refroidies. (Doran et al., 2002 ; GISTEMP). Au premier coup d’œil, cela semble contradictoire avec l’idée de réchauffement “global”, mais on a besoin d’être prudent avant de sauter sur cette conclusion.
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Michaels misquotes Hansen

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Pat Michaels (under the guise of the Greening Earth society) is particularly fond of misquoting Jim Hansen, director of the NASA GISS laboratory (and in the interests of full disclosure, GS’s boss).

Recently he claimed that Dr. Hansen has now come around to the ‘skeptics’ (i.e. Pat Michaels) way of thinking and suggests that they agree on the (small) amount of warming to be expected in the future. Michaels quotes Hansen from a 2001 PNAS paper:

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Climate model scenarios Les scénarios des modèles climatiques

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A couple of commentators (Pat Michaels, Roy Spencer) recently raised an issue about the standard scenarios used to compare climate models, in this case related to a study on the potential increase in hurricane activity.

The biggest uncertainty in what will happen to climate in the future (say 30 years or more) is the course that the global economy will take and the changes in technology that may accompany that. Since climate scientists certainly don’t have a crystal ball, we generally take a range of scenarios or projections of future emissions of CO2 and other important forcings such as methane and aerosols.
Quelques commentateurs (Pat Michaels, Roy Spencer) ont récemment relancé une question au sujet des scénarios standard utilisés pour comparer les modèles climatiques, dans ce cas relié à une étude sur l’augmentation potentielle de l’activité des ouragans.
La plus grande incertitude dans ce qui va se passer pour le climat du futur (dans 30 ans ou plus) est le cours que va suivre l’économie globale et les changements technologiques qui peuvent l’accompagner. Puisque les climatologues n’ont certainement pas une boule de cristal, nous considérons généralement une gamme de scénarios ou de projections des émissions futures de CO 2 et d’autres forçages importants comme le méthane et les aérosols.

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Climate sensitivity Sensibilité climatique

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Climate sensitivity is a measure of the equilibrium global surface air temperature change for a particular forcing. It is usually given as a °C change per W/m2 forcing. A standard experiment to determine this value in a climate model is to look at the doubled CO2 climate, and so equivalently, the climate sensitivity is sometimes given as the warming for doubled CO2 (i.e. from 280 ppm to 560 ppm). The forcing from doubled CO2 is around 4 W/m2 and so a sensitivity of 3°C for a doubling, is equivalent to a sensitivity of 0.75 °C/W/m2. The principal idea is that if you know the sum of the forcings, you can estimate what the eventual temperature change will be.

We should underscore that the concepts of radiative forcing and climate sensitivity are simply an empirical shorthand that climatologists find useful for estimating how different changes to the planet’s radiative balance will lead to eventual temperature changes. There are however some subtleties which rarely get mentioned. Firstly, there are a number of ways to define the forcings. The easiest is the ‘instantaneous forcing’ – the change is made and the difference in the net radiation at the tropopause is estimated. But it turns out that other definitions such as the ‘adjusted forcing’ actually give a better estimate of the eventual temperature change. These other forcings progressively allow more ‘fast’ feedbacks to operate (stratospheric temperatures are allowed to adjust for instance), but the calculations get progressively more involved.

Secondly, not all forcings are equal. Because of differences in vertical or horizontal distribution of forcings, some changes can have a more than proportional effect on temperatures. This can be described using a relative ‘efficacy’ factor that depends on the individual forcing. For instance, the effect of soot making snow and sea ice darker has a higher efficacy than an equivalent change in CO2 with the same forcing, mainly because there is a more important ice-albedo feedback in the soot case. The ideal metric of course would be a forcing that can be calculated easily and where every perturbation to the radiative balance had an relative efficacy of 1. Unfortunately, that metric has not yet been found!
La sensibilité climatique est une mesure de la variation de température d’équilibre globale de surface atmosphérique pour un forcage donné. Son unité est généralement des °C change par W/m2 de forçage. Pour déterminer sa valeur, une expérience classique est de regarder le climat pour une concentration en CO2 atmosphérique doublée ; ce qui fait que la sensibilité est donnée quelquefois donnée en terme de réchauffement pour un doublement du du CO2(c.a.d. une augmentation de 280 ppm a 560 ppm). Le forçage pour un doublement du CO2 est d’environ 4 W/m2 ce qui implique qu’une sensibilité de 3°C pour un doublement est équivalente a une sensibilité de 0.75 °C/W/m2. L’idée fondamentale est que si on connaît la somme de tous les forçages, il est alors possible d’estimer l’amplitude du changement de température induit.

Il est important de noter que les concepts de forçage radiatif et de sensibilité climatique sont des raccourcis empiriques que les climatologues trouvent utiles pour estimer l’impact de changements dans le bilan radiatif terrestre en termes de changements de températures. Quelques nuances doivent être mentionnées. Premièrement, il existe différentes manières de définir un forçage. La plus simple est le ‘forçage instantané’ – le changement est appliqué et la différence nette de radiation est estimée a la tropopause. Mais, en réalité, d’autres définitions, comme le ‘forçage ajusté’ donnent de meilleurs estimations du changement de température final. Ces autres forçages autorisent progressivement la mise en place de plus de rétroactions ‘rapides’ (les températures stratosphériques peuvent s’ajuster par exemple), mais le niveau de calcul augmente en retour.

Deuxièmement, tous les forçages ne sont pas égaux. En raison de différences dans les distributions verticales ou horizontales des forçages, certains changements peuvent avoir un effet sur les températures supérieur a celui directement proportionnel. Ceci peut être décrit comme un facteur relatif d’efficacité’, spécifique a chaque forçage. Par exemple, l’effet des suies a assombrir la neige et la glace de mer a une efficacité plus élevée qu’un changement équivalent en CO2 avec le même forçage, principalement en raison d’une rétro-action glace-albédo dans le cas des suies. Idéalement, un forçage pourrait être quantifiée par une méthode facile et dans laquelle chaque perturbation du bilan radiatif aurait une efficacité relative de 1. Malheureusement, une telle méthode n’a pas encore été trouvée !

Isotopes

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Isotopes can be thought of as different ‘flavours’ of a particular element (such as oxygen or carbon), that are distinguished by the number of neutrons in their nucleus (and hence their atomic mass). Carbon for instance most commonly has a mass of 12 (written as 12C), but there are also a small fraction of carbon atoms with mass 13 and 14 (13C and 14C), similarly oxygen is normally 16O, but with small amounts of 17O and 18O. All of the isotopes of an element behave in similar way chemically. However, because the mass of each isotope is slightly different there are certain physical processes that will discriminate (or ‘fractionate’) between them. For instance, during evaporation of water, it is slightly easier for the lighter isotopes to escape from the liquid, and so water vapour generally has less 18O than the liquid water from which it came. Because of these physical effects, looking at the ratio of one isotope to another can often be very useful in tracing where these atoms came from.