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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?

Why greenhouse gases heat the ocean

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Guest commentary by Peter Minnett (RSMAS)

Observations of ocean temperatures have revealed that the ocean heat content has been increasing significantly over recent decades (Willis et al, 2004; Levitus et al, 2005; Lyman et al, 2006). This is something that has been predicted by climate models (and confirmed notably by Hansen et al, 2005), and has therefore been described as a ‘smoking gun’ for human-caused greenhouse gases.

However, some have insisted that there is a paradox here – how can a forcing driven by longwave absorption and emission impact the ocean below since the infrared radiation does not penetrate more than a few micrometers into the ocean? Resolution of this conundrum is to be found in the recognition that the skin layer temperature gradient not only exists as a result of the ocean-atmosphere temperature difference, but also helps to control the ocean-atmosphere heat flux. (The ‘skin layer‘ is the very thin – up to 1 mm – layer at the top of ocean that is in direct contact with the atmosphere). Reducing the size of the temperature gradient through the skin layer reduces the flux. Thus, if the absorption of the infrared emission from atmospheric greenhouse gases reduces the gradient through the skin layer, the flow of heat from the ocean beneath will be reduced, leaving more of the heat introduced into the bulk of the upper oceanic layer by the absorption of sunlight to remain there to increase water temperature. Experimental evidence for this mechanism can be seen in at-sea measurements of the ocean skin and bulk temperatures. [Read more…] about Why greenhouse gases heat the ocean

Chinese whispers in Australia

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We decided months ago that we would not comment on leaks of the draft of the upcoming IPCC report (due Feb 2007) but we are prepared to correct obvious errors. The ongoing revisions of the text and the numerous drafts make any such commentary, let alone conclusions drawn from it, pretty pointless. This is even more true when the leaks are obviously confused about a central point. The principle error in the latest ‘exclusive’ is that the writer confuses a tightening of the estimate of climate sensitivity to 2xCO2 (as discussed here) with projections of climate change in 2100. These projections obviously depend on the uncertainties in the scenarios of future technology, economic progress and population (etc.) plus uncertainties in feedbacks related to the carbon or methane cycles. Unfortunately these have not been reduced since the last assessment report (and in some cases have actually increased).

That occasional stories will come out that get basic things wrong is unfortunate but not surprising. What is more troubling is that they subsequently get picked up by Reuters and UPI, and republished in places (such as Scientific American, though in their defence, it is simply a posting of the wire report) where the editors should know better. Worse still, the wire service stories are too brief to make the source of the error obvious, and thus the error gets propagated in an ever more confused state. As usual the blogsphere is playing a key role in amplifying and further muddying the story. The advantage of blogs is that errors can be corrected quickly, and the comments on Prometheus for instance, quickly revealed the confusion and the potential agenda of the original story.

There will be plenty of time to discuss the new IPCC report when it comes out and where everyone can read for themselves what has and what hasn’t changed since 2001. Until then, we would counsel against journalists and editors jumping at supposed ‘exclusives’ and – more dangerously – going ahead with them without even a basic sanity check of the details.

Nature’s press advisories

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Articles in Nature (or Science) are embargoed for the media until the day of publication. A week before publication, Nature sends out a press advisory describing the week’s upcoming papers that is separate from any press releases that the authors or their institutions might put out. Journalists read these, and if interested, spend the time before the actual publication talking to sources and doing background so that they have (hopefully) well-thought out stories ready to go when the embargo lifts. The resulting media splash for the most interesting papers is usually good for the magazine and the authors. But not always.

Last week, the press advisory for one paper gave a slightly misleading account of one apsect of the work described in the article. Normally this would probably not get much attention, but the paper in question dealt with the highly emotional and politicized topic of stem cell research. I don’t want to get into the specifics of this paper (comments on that should be directed elsewhere) but the New York Times reported on the clarification that Nature subsequently put out and noted that the principal author Robert Lanza, had neither seen nor approved the text of the press advisory.

This might seem strange, but this is actually the normal state of affairs. Nature‘s editors write the advisories, which only go out to journalists and are not in general ever seen by the public. Often though, these short blurbs set the tone for the subsequent media attention but, if there is a problem, it can lead to a very widespread mis-communication. Colleagues of mine have been in the odd position of having to ask the journalists to read out the release concerning their own work!

We have previously discussed the problems of getting the press release just right when dealing with articles on potentially controversial science topics, and we strongly urged scientists to be more aware of how press releases are crafted and the message they send. This is obviously very difficult to do if the scientists are not involved in the process.

Coincidentally, I recently had a casual conversation with one of the Nature editors concerning this exact issue (prior to last week’s kerfuffle) and learned that the magazine was thinking about making the advisories public at the time of publication. This would definitely be a step forward for openness. I would go further and also suggest that the principal authors be given a chance to comment on the advisories before they go out. Getting things wrong – even subtly – in contentious fields doesn’t help anyone and the slight extra effort to try and prevent mis-communication is well worth it.

In a similar vein, I have found that journalists who take the time to check back with scientists on their quotes or explanations of the science often catch ambiguities or errors at an early stage and this should be encouraged as much as possible.

At a time when science and science reporting is under pressure from many quarters, journals, press officers, editors and authors need to work more closely together on ensuring that science is reported accurately and effectively.

Followup to the ‘Hockeystick’ Hearings

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The House Energy and Commerce committee held two hearings on the “Hockey Stick” and associated “Wegman Report” in July. We commented on the first of the two hearings previously. The hearings, while ostensibly concerning the studies of Mann and coworkers, were actually most remarkable for the (near) unanimity of the participating scientists on critical key points, such as the importance of confronting the issue of climate change, and the apparent acceptance of those points by the majority of congresspersons present.

The committee subsequently provided followup opportunities to participants to clarify issues that were discussed at the hearings. Mike Mann (Penn State Professor and RealClimate blogger) participated in the second (July 27 2006) of the two hearings, “Questions Surrounding the ‘Hockey Stick’ Temperature Studies: Implications for Climate Change Assessments”. He has posted his responses to five follow-up questions, along with supporting documents. Among the more interesting of these documents are a letter and a series of email requests from emeritus Stanford Physics Professor David Ritson who has identified significant apparent problems with the calculations contained in the Wegman report, but curiously has been unable to obtain any clarification from Dr. Wegman or his co-authors in response to his inquiries. We hope that Dr. Wegman and his co-authors will soon display a willingness to practice the principle of ‘openness’ that they so recommend in their report….

Update: There is an interesting discussion of the Wegman and North reports by Gerald North (talking at TAMU) available through Andrew Dessler’s site….

Is Antarctic climate changing?

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Is the Antarctic ice sheet getting bigger or smaller? Is it warming or cooling?

As we’ve reported in earlier posts (here and here), getting accurate answers to these questions is non-trivial, because the available instrumental data remain sparse and generally date back only a few decades, at best. While modern satellite-based techniques such as laser altimetery and gravity anomaly measurements provide important information on very recent changes, to get at the longer term we must rely on less direct methods. In the last 5 years or so, an effort has been under way, much of it under the banner “International Trans Antarctic Scientific Expedition” (ITASE), to do this by collecting many dozens of ice cores from across the Antarctic continent. Two papers out this month represent the first major compilations of results from these efforts. The first, in Science on August 11th, provides a new estimates of Antarctic snowfall changes over the last 50 years. The second, in Geophysical Research Letters (August 30th) provides the first statistical reconstruction of Antarctic temperature change, extending about 200 years into the past.
[Read more…] about Is Antarctic climate changing?

New public opinion poll on global warming

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There is a new Zogby poll on public attitudes in the US towards global warming and the potential connection between severe weather events and climate change.

Unsurprisingly to us (but maybe not to others), most of the US public feel that global warming is happening (around 70%), and roughly the same amount of people report being more or much more convinced of this over the last two years.

More interestingly, the pollsters asked whether people believed that global warming was having an effect on intense hurricanes, droughts, heat waves and the like. Again, roughly 70% of people thought that global warming was having either some effect or a major effect on these weather extremes (note that the question was not phrased to ask whether any specific event was thought likely to have been caused by global warming (which was probably a good choice)).

This begs the question whether people’s experience of severe weather has convinced them that climate change is occuring. Televangelist Pat Robertson, for instance, said very recently that it was the latest heat wave that finally convinced him. I think this is likely to be true for most of the public who are not following the issue very carefully (which is most of them of course!). The most significant single event in this context was probably Katrina, regardless of how much climate change can or can’t be associated with Katrina the Hurricane (let alone Katrina the Disaster!).

I would guess that this is likely to be a very common way for public opinion to be formed across a whole number of issues. That is, when a dominant theme is very prevalent across a wide spectra of media, everyday occurrences or new information are often processed with that in mind, and given our extraordinary ability to see patterns in noisy data, we often end up associating the theme with our own experiences. Other examples surely abound in medical or political contexts.

Given that pattern, it is probably overly optimistic to expect scientists, who continually stress that single weather events can’t in general be attributed to climate change but that changes in statistics might be, to have much success in conveying these finer points to the public directly. Instead, their skills are probably best used in clarifying these points to those (e.g. journalists, policy-makers) that set the dominant themes in the first place.

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.

Ocean heat content: latest numbers

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Net ocean heat content changes are very closely tied to the net radiative imbalance of the planet since the ocean component of the climate system has by far the biggest heat capacity. Thus we have often made the point that diagnosing this imbalance through measurements of temperature in the ocean is a key metric in evaluating the response of the system to changes in CO2 and the other radiative forcings (see here).
In a paper I co-authored last year (Hansen et al, 2005), we compared model results with the trends over the 1993 to 2003 period and showed that they matched quite well (here). Given their importance in evaluating climate models, new reports on the ocean heat content numbers are anticipated quite closely.

Recently, a new preprint with the latest observations (2003 to 2005) has appeared (Lyman et al, hat tip to Climate Science) which shows a decrease in the ocean heat content over those two years, decreasing the magnitude of the long-term trend that had been shown from 1993 to 2003 in previous work (Willis et al, 2004) – from 0.6 W/m2 to about 0.33 W/m2. This has generated a lot of commentary in some circles, but in many cases the full context has not been appreciated.
[Read more…] about Ocean heat content: latest numbers

Short and simple arguments for why climate can be predicted

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Sometimes, I encounter arguments suggesting that since we cannot predict the weather beyond a couple of weeks, then it must be impossible to predict the climate in 100 years. Such statements tend to present themselves as a kind of revelation, often in social settings and parties after I have revealed for some of the guests that I’m a climatologist (if I say I work for the Meteorological Institute, I almost always get the question “so, what’s the weather going to be like tomorrow?”). Such occasions also tend to be times when I’m not too inclined to indulge in deep scientific or technical explanations. Or when talking to a journalist who wants an easy answer. In those cases I try to provide a short and simple, but convincing, explanation that is easy for most people to understand why climate can be predicted despite the chaotic nature of the weather (a more theoretical discussion is provided in the earlier post Chaos and Climate). One approach is to try to relate the topic to something with which they are familiar, such as to point to empirical observations which most accept (I suppose with hindsight it could be similar to the researchers in the early 20th century trying to convince that nuclear reactions were possible – just look at the Sun, and there is the proof! Or before that, the debate about whether atoms were real or not – just look at the blue sky, and you look at the proof…). I like to emphasised the words ‘weather‘ and ‘climate‘ above, because they mean different things.

[Read more…] about Short and simple arguments for why climate can be predicted