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19 janvier 2009 1 19 /01 /janvier /2009 09:46




étude 1: Les séries récentes d'années chaudes sont-elles inhabituelles?

 


http://www.agu.org/pubs/crossref/2008/2008GL036228.shtml



How unusual is the recent series of warm years?


E. Zorita

Institute for Coastal Research, GKSS Research Centre, Geesthacht, Germany

T. F. Stocker

Physics Institute and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland

H. von Storch


Institute for Coastal Research, GKSS Research Centre, Geesthacht, Germany

Previous statistical detection methods based partially on climate model simulations indicate that, globally, the observed warming lies very probably outside the natural variations. We use a more simple approach to assess recent warming at different spatial scales without making explicit use of climate simulations. It considers the likelihood that the observed recent clustering of warm record-breaking mean temperatures at global, regional and local scales may occur by chance in a stationary climate. Under two statistical null-hypotheses, autoregressive and long-memory, this probability turns to be very low: for the global records lower than p = 0.001, and even lower for some regional records. The picture for the individual long station records is not as clear, as the number of recent record years is not as large as for the spatially averaged temperatures.

Received 5 October 2008; accepted 18 November 2008; published 30 December 2008.


Citation: Zorita, E., T. F. Stocker, and H. von Storch (2008), How unusual is the recent series of warm years?, Geophys. Res. Lett., 35, L24706, doi:10.1029/2008GL036228.


résumé :


les séries récentes d'années chaudes aux échelles globales et régionales sont analysées à partir d'une méthode statistique.

La probabilité qu'elles puissent être expliquées par des variations naturelles est extrèmement basse à l'échelle globale p<0.001 et encore plus basse à l'échelle régionale...

 

 


étude 2: forçage orbital et CO2 des inlandsis du Paléozoïque supérieur

 


http://www.agu.org/pubs/crossref/2007/2007GL031188.shtml



Orbital and CO2 forcing of late Paleozoic continental ice sheets


Daniel E. Horton

Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA

Christopher J. Poulsen

Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA

David Pollard

Earth and Environment Science Institute, Pennsylvania State University, University Park, Pennsylvania, USA


Contrasting views of the size, duration, and history of the Gondwanan continental ice sheets have been proposed from late Paleozoic glaciological and sedimentological evidence. To evaluate these differing views, a coupled ice sheet-climate model is used to simulate continental ice sheets under a wide range of late Paleozoic orbital and pCO2 conditions. The model experiments indicate that orbital variations at pCO2 concentrations below 2X pre-industrial atmospheric levels (PAL; 280 ppm) produce large changes in late Paleozoic ice volume (∼1.3 × 1083) and sea level (∼20 to 245 m). Between 2 and 8X PAL Gondwana continental ice is simulated only under the most extreme Southern Hemisphere cold summer orbit, but still produces significant ice volumes (∼8-12 × 1073). Our results highlight the important role of atmospheric CO2 in determining the distribution, volume, and stability of late Paleozoic ice sheets, factors that ultimately impacted sea level, cyclothem deposition, and global climate, and reconcile disparate views of the Late Paleozoic Ice Age. km km

Received 30 June 2007; accepted 30 August 2007; published 11 October 2007.


Citation: Horton, D. E., C. J. Poulsen, and D. Pollard (2007), Orbital and CO2 forcing of late Paleozoic continental ice sheets, Geophys. Res. Lett., 34, L19708, doi:10.1029/2007GL031188.


résumé


des vues contrastées de la taille, de la durée et de l'histoire des inlandsis du Gondwana au cours de l'ère paléozoïque ont été proposées.

Par simulation des conditions orbitales et de teneur en CO2 au cours de cette ère, les auteurs mettent en exergue le rôle important du CO2....

 

 

 

étude 3: qu'est-ce qui provoque la variabilité de la  température des terres?

 



http://www.agu.org/pubs/crossref/2008/2008GL035984.shtml



What is causing the variability in global mean land temperature?


Martin Hoerling

Earth System Research Laboratory, NOAA, Boulder, Colorado, USA

Arun Kumar

Climate Predicton Center, NOAA, Camp Springs, Maryland, USA

Jon Eischeid

Earth System Research Laboratory, NOAA, Boulder, Colorado, USA

Bhaskar Jha

WYLE Information Systems, Gaithersburg, Maryland, USA


Diagnosis of climate models reveals that most of the observed variability of global mean land temperature during 1880-2007 is caused by variations in global sea surface temperatures (SSTs). Further, most of the variability in global SSTs have themselves resulted from external radiative forcing due to greenhouse gas, aerosol, solar and volcanic variations, especially on multidecadal time scales. Our results indicate that natural variations internal to the Earth's climate system have had a relatively small impact on the low frequency variations in global mean land temperature. It is therefore extremely unlikely that the recent trajectory of terrestrial warming can be overwhelmed (and become colder than normal) as a consequence of natural variability.


Received 16 September 2008; accepted 3 November 2008; published 13 December 2008.

Citation: Hoerling, M., A. Kumar, J. Eischeid, and B. Jha (2008), What is causing the variability in global mean land temperature?, Geophys. Res. Lett., 35, L23712, doi:10.1029/2008GL035984.

 

 


résumé:


les modèles montrent qu'une grande partie de la variabilité des températures des terres provient des variations des températures de surface des mers qui elles-mêmes proviennent de forçages radiatifs externes (GES, aérosols, solaire, volcans) en particulier lors des variations multidécennales.

Nos résultats indiquent que les variations internes du climat terrestre ont eu un impact relativement faible sur les variations de basse fréquence de la température terrestre globale.

Il est très improbable que la trajectoire récente du réchauffement terrestre puisse être submergée ( et devienne plus froide que la normale) par la variabilité naturelle.

 

 

 

étude 4: les rétroactions climatiques sous un très large spectre de forçages


http://www.agu.org/pubs/crossref/2009/2008GL036268.shtml



Climate feedbacks under a very broad range of forcing


Robert Colman

Centre for Australian Weather and Climate Research, Melbourne, Victoria, Australia

Bryant McAvaney


An atmospheric general circulation model, coupled to a mixed layer ocean, is subjected to a broad range of forcing away from the current climate between 1/16 to 32 times current CO2 in halving/doubling steps. As climate warms climate sensitivity weakens (although not monotonically), albedo feedback weakens (driving much of the sensitivity weakening), water vapour feedback strengthens (at a rate slightly larger than it would if relative humidity remained unchanged), and lapse rate feedback increases (negatively); this latter change essentially offsetting the water vapour increases. Longwave cloud feedbacks are relatively stable (moderate and positive) across the full range; shortwave cloud feedback remains relatively weak, apart from under the coldest climates. Cloud optical property related components (from total water content, water/ice fraction and cloud thickness) remain remarkably stable. Cloud 'amount' feedbacks show the greatest trends: weakening as temperatures increase. Although cloud feedbacks show an overall consistency of features in different latitudes, precise patterns of changes differ substantially for different baseline climates.


Received 7 October 2008; accepted 2 December 2008; published 3 January 2009.

Citation: Colman, R., and B. McAvaney (2009), Climate feedbacks under a very broad range of forcing, Geophys. Res. Lett., 36, L01702, doi:10.1029/2008GL036268.



résumé


un modèle GCM couplé à une couche océanique bien mélangée est soumis à un spectre de forçages correspondant à 1/16 à 32 fois le forçage provoqué par la teneur en CO2 actuelle.


lorsque le climat se réchauffe, la sensibilité climatique diminue.


la rétroaction albédo diminue fortement (cause principale de la baisse générale)


la rétroaction vapeur d'eau, positive, augmente, mais est compensée par l'augmentation de la rétroaction, négative, due à la diminution du gradient thermique vertical

les rétroactions nuageuses positive (LW) et négative (SW) restent stables

la rétroaction nuageuse, au point de vue quantité de nuages, diminue.

 


 

étude 5: prise en compte des flux de chaleur d'origine anthropique dans les modèles de simulation du climat global



http://www.agu.org/pubs/crossref/2009/2008GL036465.shtml




Integrating anthropogenic heat flux with global climate models


Mark G. Flanner

Advanced Study Program, National Center for Atmospheric Research, Boulder, Colorado, USA


Nearly all energy used for human purposes is dissipated as heat within Earth's land-atmosphere system. Thermal energy released from non-renewable sources is therefore a climate forcing term. Averaged globally, this forcing is only +0.028 W m-2, but over the continental United States and western Europe, it is +0.39 and +0.68 W m-2, respectively. Here, present and future global inventories of anthropogenic heat flux (AHF) are developed, and parameterizations derived for seasonal and diurnal flux cycles. Equilibrium climate experiments show statistically-significant continental-scale surface warming (0.4-0.9°C) produced by one 2100 AHF scenario, but not by current or 2040 estimates. However, significant increases in annual-mean temperature and planetary boundary layer (PBL) height occur over gridcells where present-day AHF exceeds 3.0 W m-2. PBL expansion leads to a slight, but significant increase in atmospheric residence time of aerosols emitted from large-AHF regions. Hence, AHF may influence regional climate projections and contemporary chemistry-climate studies.


Received 24 October 2008; accepted 5 December 2008; published 16 January 2009.

Citation: Flanner, M. G. (2009), Integrating anthropogenic heat flux with global climate models, Geophys. Res. Lett., 36, L02801, doi:10.1029/2008GL036465.


résumé :


Globalement, la chaleur anthropique représente un forçage de seulement 0.028W/m2, mais sur les US et l'ouest de l'Europe, les flux sont de 0.39W/m2 et 0.68 W/m2 respectivement.

Les modélisations climatiques montrent des changements de température locale de 0.4 à 0.9°C pour les flux anthropiques projetés en 2100, mais pas pour les flux actuels et pour ceux de 2040.

Cependant des augmentations significatives dans la température moyenne annuelle et dans l'épaisseur de la couche limite planétaire se produisent dans les cellules où le flux anthropique dépasse 3 W/m2.

L'expansion de la couche limite conduit à une légère mais significative augmentation du temps de résidence des aérosols émis par les régions à haut flux thermique (AHF).

Donc les AHF peuvent influencer les projections climatiques régionales et les études sur les réactions climat/chimie contemporaines.

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