March 3, 2011

Here we show that human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events found over approximately two-thirds of data-covered parts of Northern Hemisphere land areas.

This statement is, according to NYT opinion blogger Andy Revkin, so unacceptably definitive as to warrant a whole blog post:  “In scientific literature you rarely see statements so streamlined and definitive. For climate science, this is the equivalent of a smoking gun.”

Actually, the statement isn’t a terribly strong one for the scientific literature, particularly given the use of the phrase “have contributed,” and most especially for a study about the trend in increased heavy precipitation, which is a trend many other studies have identified and is a very basic prediction of climate science.

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A key reason Revkin’s piece (which cites a blog post by Roger Pielke, Jr.)  — and one by Time’s Bryan Walsh (which cites Pielke and a blog post by Judith Curry) — are picking the wrong fight is that this particular climate impact is very basic physics.

Dr. Kevin Trenberth, head of NCAR’s Climate Analysis Section, has explained the connection between human-caused global warming and extreme deluges:  “There is a systematic influence on all of these weather events now-a-days because of the fact that there is this extra water vapor lurking around in the atmosphere than there used to be say 30 years ago. It’s about a 4% extra amount, it invigorates the storms, it provides plenty of moisture for these storms and it’s unfortunate that the public is not associating these with the fact that this is one manifestation of climate change. And the prospects are that these kinds of things will only get bigger and worse in the future.”

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Trenberth has further said, “It’s not the right question to ask if this storm or that storm is due to global warming, or is it natural variability. Nowadays, there’s always an element of both.”

I asked Trenberth for this opinion on these critiques.  His reply gets to the heart of the matter:


To me the arguments are all wrong.  Rather than proving that there is a human influence we should be trying to prove there isn’t.  Given that there is a human influence on climate and the oceans have warmed, etc.,  how can there not be an influence on water vapor and precipitation?  The physical relationship is so strong that it has to be,  and one should then have to prove otherwise. To do that one has to use the same flawed models, and so one can not prove the reverse.  That should be the challenge to Pielke Jr and Curry.

So my criticism is first that the scientists even try to do this and pretend (or have a null hypothesis) that there is no human influence when the whole system is crying out that there is. The fact that these studies show a convincing relationship in spite of the inadequate tools shows how robust it is,  and indeed it relates to basic physics.  But the details are certainly questionable in several respects.

The robust features relate to the warming,  the increased water holding capacity, the observed increased atmospheric moisture,  and thus the supply of moisture for all storms.  The changes in storms themselves, where they go,  how long they last, and the overall precipitation amount are more uncertain,  but the increases in intensity of events are surely robust.


Revkin goes on and on about how the Nature piece “Human contribution to more-intense precipitation extremes” (subs. req’d) supposedly failed to put in enough caveats to suit his taste.  But in fact it ends with the sentence, “There are, however, uncertainties related to observational limitations missing or uncertain external forcings and model performance.”

And its key Figure 3 — “the results of four optimal detection analyses using the time evolution of extreme precipitation indices averaged over the Northern Hemisphere, over northern mid-latitudes and northern tropics individually” — has the requisite error bars:


I won’t explain these figures in detail except to point out that they chart  “Best estimates (data points) and 5–95% uncertainty ranges (error bars).”  These are pretty straightforward caveats.

Scientists do not have to caveat every single sentence or paragraph in a scientific paper.

Revkin actually queries coauthor “Gabriele C. Hegerl of the University of Edinburgh, someone whose expertise I’ve drawn on many times over the years” about this.  She replies:

The paper is restricted in length, and that is particularly the case for the abstract (there are tough length guidelines). Therefore, it is not always possible to discuss findings in the detail that would be desirable IN THE ABSTRACT. As Myles [Myles Allen, an author of the other precipitation/climate paper] pointed out, ‘detection and attribution’ is code for a hypothesis test, meaning that we are checking this hypothesis at some confidence level. Every specialist in the area will understand this, and every scientist who looks at abstract AND figures will as well (common practice), and the remaining text is very explicit about methods and the accounted for uncertainties.

Figure 3 shows the results with 5-95% confidence ranges, showing that there is uncertainty in the findings.

The data uncertainties affect all results including the enhancement in observations relative to models.

So, of course there are uncertainties in the findings, as in any attribution and detection result, there is a remaining chance that the observed change is due to internal climate variability (5-ish %) particularly if the models would underestimate that variability. Natural forcings are expected to cause weak changes over that period.

The Nature paper is written for other scientists.

Revkin not only buries the lede, he never bothers to explain precisely what Hegerl’s statement means in lay terms.  Since he doesn’t correct her in the text, I’m going to take her statements at face value.  A couple of commenters, notably dhogaza, who often comments on CP, too, notes, “95% significance is “pretty definitive”.”  We can go a farther than that.

The language that the IPCC uses for statements that are not 100% certain is spelled out here.  For things that are 95% probability, the phrase is “Extremely likely.”  You can see here how the IPCC uses that here in the Fourth Assessment:

The observed warming is highly significant relative to estimates of internal climate variability which, while obtained from models, are consistent with estimates obtained from both instrumental data and palaeoclimate reconstructions. It is extremely unlikely (<5%) that recent global warming is due to internal variability alone such as might arise from El Niño (
Section 9.4.1
). The widespread nature of the warming (Figures 3.9 and 9.6) reduces the possibility that the warming could have resulted from internal variability. No known mode of internal variability leads to such widespread, near universal warming as has been observed in the past few decades.

So the much ado in Revkin’s post is all because the authors say don’t say “it is extremely likely that human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events.”

Misleaders!  Cue the Spanish Inquisition.

Yet for all of the complaining about the lack of sufficient caveats in the attribution, it would be nice if, for once, Revkin would caveat attributions to Pielke’s blog posts by noting that those posts and other writings have been extensively debunked by leading climate scientists (see various links in Foreign Policy’s “Guide to Climate Skeptics” includes Roger Pielke, Jr. and Trenberth’s review of Pielke’s book in Science).  The same goes for Walsh quoting Curry, who has arguably surpassed even Pielke in the breadth and depth of debunking by scientists (see a partial list here, and Skeptical Science here.).

I’m not talking about the occasional critique that everyone who blogs is subject to — I’m talking about detailed scientific or technical take downs of many different posts by many different scientists over an extended period of time.  Surely that warrants at least a brief caveat if one is going to cite either Pielke or Curry in order to critique the work of climate scientists in a major study in Nature, no?

And getting back to Revkin, he bizarrely double downs on the most unscientific statement he has ever written:

As I wrote recently, there seems to be an inverse relationship between the definitiveness of an assertion and its credibility. This doesn’t mean that everything definitive is wrong (only Joe Romm could find a way to interpret it thus). It means that a reporter, or citizen, confronted with a flat statement on a tough issue would do well to dig a bit deeper.

No.  Not even close.

There does NOT seem to be an inverse relationship between the definitiveness of an assertion and its credibility in climate science.  It remains a generally laughable statement for a science journalist — one that is utterly false as a rule in science and especially in climate science.  It is the whole point of science to allow us to make credible assertions as definitively as possible.

Try reading the IPCC reports, especially the summary for policy makers — they are filled with highly caveated statements where definitiveness and credibility are as closely aligned as is possible, as in the quote above.

It is particularly remarkable that Revkin would repeat the statement in this blog post because, as he himself seems to have shown, the attribution finding was “extremely likely” — hardly an inverse relationship between the supposedly definitive nature of the finding, which, again, was merely that “human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events.”  As Trenberth says, that conclusion is obvious and entirely expected.  The burden of proof should be on those trying to disprove that statement.

It is time for Revkin to retract the statement.

I would note that the Nature study does not extend beyond the year 1999, so it misses the hottest decade on record and the wettest year on record.  How much do you want to bet that when the analysis is done in a few years that includes 2000 to 2010 it yields an even more confident conclusion?

This kerfuffle actually misses the most important conclusion of the study:

Changes in extreme precipitation projected by models, and thus the impacts of future changes in extreme precipitation, may be underestimated because models seem to underestimate the observed increase in heavy precipitation with warming.

Après nous le déluge!

Related Posts:

A new study by a Duke University-led team of climate scientists suggests that global warming is the main cause of a significant intensification in the North Atlantic Subtropical High (NASH) that in recent decades has more than doubled the frequency of abnormally wet or dry summer weather in the southeastern United States….

The models – known as  Coupled Model Intercomparison Project Phase 3 (CMIP3) models – predict the NASH will continue to intensify and expand as concentrations of carbon dioxide and other greenhouse gases increase in Earth’s atmosphere in coming decades.“This intensification will further increase the likelihood of extreme summer precipitation variability – periods of drought or deluge – in southeastern states in coming decades,” Li says.