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Tropical SSTs: Natural variations or Global warming?

Classé dans: — group @ 11 septembre 2006

by Michael Mann and Gavin Schmidt

Roughly a year ago, we summarized the state of play in the ongoing scientific debate over the role of anthropogenic climate change in the observed trends in hurricane activity. This debate (as carefully outlined by Curry et al recently) revolves around a number of elements – whether the hurricane (or tropical cyclone) data show any significant variations, what those variations are linked to, and whether our understanding of the physics of tropical storms is sufficient to explain those links.

Several recent studies such as Emanuel (2005 — previously discussed here) and Hoyos et al (2006 — previously discussed here) have emphasized the role of increasing tropical sea surface temperatures (SSTs) on recent increases in hurricane intensities, both globally and for the Atlantic. The publication this week of a comprehensive paper by Santer et al provides an opportunity to assess the key middle question – to what can we attribute the relevant changes in tropical SSTs? And in particular, what can we say about Atlantic SSTs where we have the best data? More »

Fact, Fiction, and Friction in the Hurricane Debate

Classé dans: — group @ 18 août 2006

Michael Mann and Gavin Schmidt

Judith Curry and colleagues have an interesting (and possibly provocative) article, “Mixing Politics and Science in Testing the Hypothesis That Greenhouse Warming Is Causing a Global Increase in Hurricane Intensity” in the latest issue of the Bulletin of the American Meteorological Society (BAMS). The article provides a solid review of the recent developments in the science focusing on potential climate change impacts on tropical cyclones. However, the article is more novel in its approach than the typical scientific review article. For instance, it attempts to deal with the issue of how one should test hypotheses that reflect a complex causal chain of individual hypotheses. This is of course relevant to investigations of climate change influences on tropical cyclone activity, where one is attempting to connect a phenomenon (climate change) that is global in spatial scale and multidecadal in timescale, to a phenomena that is intrinsically “mesoscale” (that is, spans at most hundreds of kilometers) in space and lasts only a few days.

More unusually, the article also takes an introspective look at the role of scientists in communicating societally-relevant science to the public, and provides a critical review of how the science dealing with climate change impacts on tropical cyclones and hurricanes has been reported in the media, and how that reporting has occasionally deepened the polarisation on the issue. In doing so, the article revisits some of the “false objectivity” problems we have talked about before (see here and here). They also assess fairly the quality of the arguments that have been made in response to the Emanuel (2005) and Webster et al (2005) papers in the hope of focussing discussion on the more valid points, rather than some of the more fallacious arguments. The article is unapologetic in advancing their particular point of view, and while we generally share it, we imagine that some readers may disagree. We hope, as we suspect the authors do as well, that it will in any case generate a productive discussion.

NOAA: Hurricane forecasts

Classé dans: — group @ 9 juin 2006

Guest commentary from Thomas Crowley

NOAA has issued its annual forecast for the hurricane season, along with its now-standard explanation that there is a natural cycle of multidecadal (40-60 year) length in the North Atlantic circulation (often referred to as the “Atlantic Multidecadal Oscillation”–see Figure), that is varying the frequency of Atlantic tropical cyclones, and that the present high level of activity is due to a concurrent positive peak in this oscillation. More »

Reactions to tighter hurricane intensity/SST link

Classé dans: — gavin @ 20 mars 2006

There was another twist to the hurricanes/global warming issue in Science Express on Friday where a new paper from the Webster/Curry team just appeared. This study, lead by Carlos Hoyos, crosses a few t’s and dots a couple of i’s on the connection of increasing numbers of intense hurricanes (Cat. 4 and 5) to sea surface temperatures (SST). Basically, they looked at a number of other key variables for hurricane intensity (like wind shear and humidity) and examined whether there was any pattern to those variables across the different ocean basins that they study. Bottom line? None of the other variables have as much explanatory power for the long term trends as SST which is the only consistently trending constituent in the mix. So far, so un-surprising. However, one interesting aspect of this story is that almost all the key players in the ongoing debate were interviewed by different journalists in various media and those comments are probably more useful for gauging the state of play than the details of the new paper itself. More »

Ralentissement de la circulation Atlantique?

Classé dans: — group @ 30 novembre 2005 - (English)

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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