A recent conference presentation at AGU (reported here) while confirming that global sea level is indeed rising (in line with other estimates), showed that Arctic sea levels may actually be falling. On the face of it these preliminary results are a little puzzling (though note that this isn’t yet a properly peer reviewed paper, and so may not reflect what ends up in the journal), but it does reveal some of the complexities in analysing sea level in relatively small enclosed basins and so a brief overview of the different factors involved is probably useful. [Read more…] about Sea level in the Arctic
Oceans
On a Weakening of the Walker Circulation
by Ray Pierrehumbert and Rasmus Benestad
Second article of our 3-part series on atmospheric circulation and global warming
In Part I we outlined some general features of the tropical circulation, and discussed ways in which increases in anthropogenic greenhouse gases might affect El Niño. Now we take up the question of how global warming might affect the quasi-steady east-west overturning circulation known as the Walker Circulation. The Walker circulation affects convection and precipitation patterns, the easterly Trade Winds, oceanic upwelling and ocean biological productivity; hence, changes in this circulation can have far-reaching consequences. It also provides the background state against which El Niño events take place, and so changes in the Walker circulation should form an intrinsic part of thinking about how global warming will affect El Niño. In a paper that recently appeared in Nature, Vecchi, Soden, Wittenberg, Held, Leetmaa and Harrison present intriguing new results which suggest that there has already been a weakening of the Walker circulation in the past century, and that the observed changes are consistent with those expected as a response to increases in anthropogenic greenhouse gases. The discussion in Vecchi et al. also raises some very interesting issues regarding the way the hydrological cycle might change in a warming world.
El Niño and Global Warming
By Rasmus Benestad & Raymond Pierrehumbert
This is the first part of a planned mini-series of 3 posts on tropical climate, circulation, and oceanic response in conjunction with a global warming. Climate change related to a global warming is more than just temperature and precipitation -massive atmospheric circulations change too, and these changes can have consequences.
Sir Nicholas Shackleton
With the recent death of Sir Nicholas Shackleton, paleoclimatology lost one of its brightest pioneers. Over the last ~40 years, Nick made numerous far-reaching contributions to our understanding of how climates varied in the past, and through those studies, he identified factors that are critically important for climate variability in the future. His career neatly encompasses the birth of the new science of paleoceanography to its synthesis into the even newer science of ‘Earth Systems’; he made major contributions to these evolving fields throughout his life, and his insightful papers are required reading for students of paleoclimatology.
[Read more…] about Sir Nicholas Shackleton
Atlantic circulation change summary Changement de la circulation Atlantique – Résumé
Nature this week has an excellent summary of the state of the science with regards to possible changes in the ocean thermohaline (or meridional) circulation in the Atlantic and its impact on climate. Even though it quotes a couple of us, it’s still worth reading if you want to understand how results like the Bryden et al paper – that suggested that the Atlantic overturning had reduced by 30% in recent decades – are assimilated into the scientific picture.
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Was the record Amazon drought caused by warm seas?
On December 11, 2005, The New York Times ran a story on record drought conditions in the Amazonas region of Brasil, linking it to global warming, and specifically the warm ocean temperatures in the North Atlantic that have also been linked to the ferocity of the 2005 Atlantic hurricane season. This prompted a response from Chris Mooney, calling for a comment from RealClimate about whether such an assertion is valid, as we earlier made it very clear that it is impossible to say whether one single extreme event in a very noisy environment – such as Hurricane Katrina – is related to climate change. So we decided to take a look at this phenomena, and address why there might be a connection and what it takes to make an attribution. [Read more…] about Was the record Amazon drought caused by warm seas?
Decrease in Atlantic circulation? Ralentissement de la circulation Atlantique?
by Gavin Schmidt and Michael Mann
In a sure-to-be widely publicized paper in the Dec. 1 Nature, Bryden et al. present results from oceanographic cruises at 25°N across the Atlantic showing a ~30% decline in the ocean overturning circulation. These cruises have been repeated every few years since 1957, and the last two cruises (in 1998 and 2004) show notable changes in the structure of the deep return circulation. In particular, the very deepest part of the return flow (at around 3000 to 5000 m) has reduced and moved up in the water column compared to previous decades. How solid is this result and what might it imply for climate?
Dans un article largement commenté dans la presse (voir par exemple ici et la) dans le numéro du 1er déc. de Nature, Bryden et al. présentent des résultats de croisières océanographiques à 25°N à travers l’Océan Atlantique qui montrent un déclin d’environ 30% de la circulation océanique “générale”–dite circulation thermohaline-. Ces croisières ont été répétées régulièrement depuis 1957, et les deux dernières croisières (en 1998 et 2004) montrent des changements notables de la structure de la circulation de retour profonde. En particulier, le flux dans la partie la plus profonde du courant de retour (entre environ 3000 et 5000 m) a diminué et est remonté dans la colonne de l’eau par rapport aux décennies précédentes. Quelle est la robustesse de ces résultats et quelles en sont les implications potentielles pour le climat ?
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[Read more…] about Decrease in Atlantic circulation?
Saltier or not? Plus salé ou pas ?
In a recent (of Sept. 16, 2005) publication in Science, Hatun et al. find that record-high salinities have been observed over the past decade in the region where water from the Atlantic flows into the northern oceans. They combine an analysis of observations with simulations using an ocean model, concluding that the salinity of the inflow to the northern oceans is controlled by ocean dynamics and the circulation in the sub-polar gyre. The observations by Hatun et al. may suggest that at the moment the warm and salty waters from the south are especially warm and salty.
In another publication paper in Science from June 17th 2005, on the other hand, Curry & Mauritzen conclude that as a whole the northern North Atlantic has become significantly fresher (less salty) in recent decades. The latter study was based entirely on observations (hydrographic data between Labrador and Europe in the past 50 years). The recent evidence for salinification provided by Hatun et al. has been interpreted by some as being inconsistent with the evidence for high-latitude North Atlantic freshening found in previous reports. So what is really happening? Is the salinity increasing or decreasing? And can the two recent Science studies be consistent with each other?
traduit de l’anglais par Claire Rollion-Bard
Dans une publication récente (du 16 Sept. 2005) dans Science, Hatun et al. ont trouvé que de fortes salinités ont été observées durant la dernière décennie dans la région où l’eau de l’Atlantique s’écoule dans les océans du Nord. Ils ont combiné une analyse des observations avec des simulations utilisant un modèle océanique, concluant que la salinité du courant vers les océans du Nord est contrôlée par la dynamique océanique et la circulation de l’enroulement sub-polaire. Les observations de Hatun et al. peuvent suggérer que les eaux chaudes et salées venant du sud sont spécialement chaudes et salées.
Dans une autre publication dans Science du 17 juin 2005, Curry & Mauritzen concluent que, dans l’ensemble, le nord de l’Atlantique Nord est devenu significativement plus doux (moins salé) au cours des récentes décennies. Cette étude était basée entièrement sur des observations (données hydrographiques entre le Labrador et l’Europe dans les derniers 50 ans). L’évidence récente pour une augmentation de la salinité fournie par Hatun et al. a été interprétée par certains comme étant inconsistante avec l’évidence de l’Atlantique Nord des hautes latitudes moins salé trouvé dans les précédentes publications. Que se passe-t’il donc réellement ? Est-ce que la salinité augmente ou baisse ? Et est-ce que les deux récentes études dans Science peuvent être compatibles l’une avec l’autre ?
[Read more…] about Saltier or not?
Hurricanes and Global Warming – Is There a Connection? Huracanes y calentamiento global ¿Hay conexión? Ouragans et réchauffement global – existe t’il un lien ?
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 already obvious.
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?
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by Stefan Rahmstorf, Michael Mann, Rasmus Benestad, Gavin Schmidt, and William Connolley (traduit par Claire Rollion Bard)
Le lundi 29 août, l’ouragan Katrina a ravagé la Nouvelle-Orléans, la Louisiane et le Mississipi, laissant une traînée de destruction dans son sillage. Il va se passer du temps avant que le bilan total de cet ouragan soit estimé, mais les impacts environnementaux et humains sont déjà apparents.
Katrina était le plus craint des évènements météorologiques, un ouragan majeur laissant un terrain vide dans une région très peuplée de faible élévation. Dans le sillage de sa dévastation, beaucoup se sont demandés si le réchauffement global pouvait avoir contribué à ce désastre. La Nouvelle-Orléans pourrait-elle être la première ville majeure des Etats-Unis à être ravagée par le changement climatique causé par les humains ?
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The Acid Ocean – the Other Problem with CO2 Emission
The Royal Society has just issued a summary report on the effects of CO2 on the pH chemistry of seawater and aquatic organisms and ecosystems. In addition to its pivotal role in the atmosphere in the regulation of global climate, CO2 and its sister chemical species, HCO3– and CO32- comprise the carbonate buffer system which regulates the pH of seawater. The new report can be found here. Acidifying the ocean is particularly detrimental to organisms that secrete shell material made of CaCO3, such as coral reefs and a type of phytoplankton called coccolithophorids [Kleypas et al., 1999]. The ocean pH change will persist for thousands of years. Because the fossil fuel CO2 rise is faster than natural CO2 increases in the past, the ocean will be acidified to a much greater extent than has occurred naturally in at least the past 800,000 years [Caldeira and Wicket, 2003].
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