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Can 2°C warming be avoided? Un réchauffement de 2°C peut-il être évité?

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Guest comment by Malte Meinshausen, Reto Knutti and Dave Frame

Yesterday’s BBC article on the “Avoiding Dangerous Climate Change” report of the Exeter meeting last year, carried two messages that have left some a little confused. On the one hand, it said that a stabilization of greenhouse gases at 400-450 ppm CO2-equivalent concentrations is required to keep global mean warming below 2°C, which in turn is assumed to be necessary to avoid ‘dangerous’ climate change. On the other hand, people are cited saying that “We’re going to be at 400 ppm in 10 years’ time”.

So given that we will exceed 400 ppm CO2 in the near future, is a target of 2°C feasible? To make a long story short: the answer is yes. Commentaire par Malte Meinshausen, Reto Knutti and Dave Frame (traduit par Alain Henry)
Hier, un article de la BBC sur le rapport ” Eviter les changements climatiques dangereux” de la conférence d’Exeter de l’an dernier comportait deux messages qui pourraient entretenir une certaine confusion. D’une part, il mentionne qu’une stabilisation des gaz à effet de serre (GES) à 400-450 ppm de CO2 équivalent est nécessaire pour garder le réchauffement global en dessous de 2°C, qui à son tour est supposé nécessaire pour éviter les changements climatiques ‘dangereux’. D’autre part, plusieurs personnes sont citées en disant « Nous atteindrons 400 ppm dans 10 ans ».

Donc, étant donné que nous allons dépasser les 400 ppm de CO2 dans un futur proche, est-ce que la cible de 2°C est-elle faisable? En un mot, la réponse est oui.

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Daily Kos interview

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A brief welcome to anyone coming over from Daily Kos today. Three of us (Mike, Gavin and Stefan) are interviewed by DarkSyde on climate change, this site and walking the line between science and politics. To find something specific, check out the Highlights on the side bar, the index, or use the search bar above.

New look

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Hopefully readers will appreciate the new look we have given the site (you may need to reload for it to work properly). We have added some new features attached to the buttons above – an index which may prove useful in navigating the site, a more prominent Search function (which searches posts and comments), a link to the archives etc. This has allowed us to reduce some of the clutter and hopefully make this site a little more user friendly. If there are any problems, wrinkles that need to be ironed out, or if you have suggestions for further improvement, let us know at contrib -at- realclimate -dot- org.

Polar Amplification

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Guest commentary by Cecilia Bitz, University of Washington

“Polar amplification” usually refers to greater climate change near the pole compared to the rest of the hemisphere or globe in response to a change in global climate forcing, such as the concentration of greenhouse gases (GHGs) or solar output (see e.g. Moritz et al 2002). Polar amplification is thought to result primarily from positive feedbacks from the retreat of ice and snow. There are a host of other lesser reasons that are associated with the atmospheric temperature profile at the poles, temperature dependence of global feedbacks, moisture transport, etc. Observations and models indicate that the equilibrium temperature change poleward of 70N or 70S can be a factor of two or more greater than the global average. [Read more…] about Polar Amplification

One year on…

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RealClimate has been online for just over a year, and so this is probably a good time to review the stories we’ve covered and assess how well the whole project is working out.

Over the last 12 months, we’ve tackled a 100+ scientific topics that range from water vapour feedbacks, the carbon cycle, climate sensitivity, satellite/surface temperature records, glacier retreat, climate modelling to hurricanes. We’ve had guest postings that span questions of Martian climate change to Arctic ozone depletion and solar forcing. We’ve crossed virtual swords with Michael Crichton, the Wall Street Journal’s editorial board, George Will, Nigel Lawson, Fox News and assorted documentary makers (though only one person ever threatened to sue us). Hopefully our contributions have interested, intrigued and occasionally amused (at least a few of you…). [Read more…] about One year on…

2005 temperatures Températures 2005

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Due to a historical quirk (of unknown origin), the World Meterological Organisation releases its summary for each year based on the Dec to Nov ‘meteorlogical year’ means (rather than the more usual calendar year). Anyway, the WMO summary is now available, as is the NASA GISS analysis and the CRU summary. The point upon which all the analyses agree is that 2005 was exceptionally warm and that it continues the long term mean warming trend. All show record warmth in the Northern Hemisphere since 1860, while GISS gives 2005 as the warmest year globally as well (CRU/WMO have it second after 1998). As the summaries indicate, the differences in ranking are on the order of a few hundredths of a degree (smaller than the accuracy of the analysis) and so a definitive ranking is not possible. Differences in how the separate analyses deal with missing data are responsible for most of the apparent variations. Note too that the convention for the base periods for the anomalies differ between the analyses (1961-1990 for CRU/WMO, 1951-1980 for GISS), but this does not affect the rankings.

Update 7pm: The GISS analysis curiously appears to have gone off line….
Update 8am 16 Dec: The GISS summation is still not back up, but the raw data and new figures do seem to be available http://data.giss.nasa.gov/gistemp . Note that as pointed in comment #5, the WMO/CRU/Hadley Centre analysis is for Jan-Nov, and not for the met. year as stated above (though the GISS analysis is). Don’t ask us why!
Final Update 11pm 16 Dec: The GISS analysis is back!

En raison d’une incongruité historique (d’origine inconnue), l’Organisation Météorologique Mondiale publie son compte-rendu annuel basé sur les moyennes d’une “année météorologique” allant de décembre à novembre (plutôt qu’une année civile normale). Ce compte-rendu de l’OMM est désormais disponible, ainsi que l’analyse du GISS-NASA et celle du CRU. Le point sur lequel toutes ces analyses convergent est que l’année 2005 était exceptionnellement chaude et qu’elle poursuit la tendance à long terme d’un réchauffement moyen global. Toutes ces analyses montrent que 2005 correspond à une température record dans l’hémisphère nord depuis 1860, alors que le GISS donne 2005 comme l’année la plus chaude globalement (le CRU l’indique comme la seconde la plus chaude juste après 1998). Comme ces analyses l’indiquent, les différences dans ces classements sont dues à des différences de quelques centièmes de degré (inférieures à la justesse de l’analyse). Les variations apparentes entre ces différentes analyses résultent principalement de la manière par laquelle chaque analyse traite des données manquantes. Un dernier point, la convention qui définit la période de référence pour la détermination des anomalies de température diffère pour chaque analyse (1961-1990 pour le CRU, 1951-1980 pour le GISS), mais ceci n’affecte pas le classement.

(traduit par T. de Garidel)

Decrease in Atlantic circulation? Ralentissement de la circulation Atlantique?

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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?par Gavin Schmidt et Michael Mann (traduit de l’anglais par T. de Garidel)

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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More satellite stuff

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The November 17th issue of Science has an interesting exchange of letters between Christy and Spencer; Mears and Wentz; and Sherwood and Lanzante (ref here; subs required for substance). The context of this discussion is the tropospheric temperature record; see Et tu LT and The tropical lapse rate quandary for two RC posts that discuss the issue, and in particular three papers in the August 11th issue of Science.

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Chaos and Climate

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By James Annan and William Connolley

In this post, we will try to explain a little about chaos theory, and its relevance to our attempts to understand and forecast the climate system. The chaotic nature of atmospheric solutions of the Navier-Stokes equations for fluid flow has great impact on weather forecasting (which we discuss first), but the evidence suggests that it has much less importance for climate prediction.

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Shameless Self Promotion

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Vote for RealClimate!

No, we’re not going to abandon our policy of steering clear of political commentary, but yes, you can nonetheless vote for us!

Some of you may have noticed the new logos up in the upper right hand corner of the RC webpage. As a followup to our selection last month for the “Science & Technology Web Awards 2005” by Scientific American, we are pleased to have now made it to the final round of Deutsche Welle’s 2005 Weblog Awards (for those of you not familiar with them, Deutsche Welle is sort of the German equivalent of Britain’s BBC World Service). [Read more…] about Shameless Self Promotion