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Hurricanes

El Nino, Global Warming, and Anomalous U.S. Winter Warmth

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Translations: (Slovenčina) (Svenska)

It has now become all too common. Peculiar weather precipitates immediate blame on global warming by some, and equally immediate pronouncements by others (curiously, quite often the National Oceanic and Atmospheric Administration in recent years) that global warming can’t possibly be to blame. The reality, as we’ve often remarked here before, is that absolute statements of neither sort are scientifically defensible. Meteorological anomalies cannot be purely attributed to deterministic factors, let alone any one specific such factor (e.g. either global warming or a hypothetical long-term climate oscillation).

Lets consider the latest such example. In an odd repeat of last year (the ‘groundhog day’ analogy growing ever more appropriate), we find ourselves well into the meteorological Northern Hemisphere winter (Dec-Feb) with little evidence over large parts of the country (most noteably the eastern and central U.S.) that it ever really began. Unsurprisingly, numerous news stories have popped up asking whether global warming might be to blame. Almost as if on cue, representatives from NOAA’s National Weather Service have been dispatched to tell us that the event e.g. “has absolutely nothing to do with global warming”, but instead is entirely due to the impact of the current El Nino event.

[Update 1/9/07: NOAA coincidentally has announced today that 2006 was officially the warmest year on record for the U.S.]
[Update 2/11/08: It got bumped to second place. ]
[Read more…] about El Nino, Global Warming, and Anomalous U.S. Winter Warmth

Tropical SSTs: Natural variations or Global warming?

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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? [Read more…] about Tropical SSTs: Natural variations or Global warming?

Fact, Fiction, and Friction in the Hurricane Debate

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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

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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. [Read more…] about NOAA: Hurricane forecasts

Reactions to tighter hurricane intensity/SST link

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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. [Read more…] about Reactions to tighter hurricane intensity/SST link

Decrease in Atlantic circulation?

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Translations: (Français)

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?
[Read more…] about Decrease in Atlantic circulation?

Hurricanes and Global Warming – Is There a Connection?

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Translations: (Français) (Español)

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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Storms & Global Warming II

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Earlier this year, we posted two discussions on the association between climate change and storms: Storms and Climate Change and Some recent updates. I will use the abbreviation TC here in the loose meaning of a tropical cyclone. These posts discussed the high number of TCs during the previous hurricane season and an essay on the relation between TCs and climate change. The uncertainty surrounding trends in storminess was underlined, and a point was being made about this subject being controversial. [Read more…] about Storms & Global Warming II

On record-breaking events

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NASA’s Earth Observatory reports that there was a record low Arctic sea ice concentration in June 2005. There was a record-number of typhoons over Japan in 2004. In June, there were reports of a number of record-breaking events in the US. And on July 28, the British News paper The Independent reported on record-breaking rainfall (~1 m) in India, claiming hundreds of lives. These are just a few examples of recent observations. So, what is happening?

[Read more…] about On record-breaking events

Some recent updates

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A couple of recent papers in Science this week relate to discussions we have had on RealClimate recently. The first by Curry and Mauritzen relates to the Gulf Stream Slowdown? post and describes the amount of fresh water that has been added to the North Atlantic over the last few decades (calculated from a database of salinity measurements) and what impact that has had on density and overturning circulations. There is a press release available at WHOI which is quite informative (hat tip to Joseph O’Sullivan for the link).

The second is related to the Storms and Climate Change post and is a perspective by Kevin Trenberth on the potential for a hurricanes and global warming link. The NCAR press release is available here.

These will undoubtedly not be the last word on the subject, and so we will probably be revisiting these topics at some point soon…