Archive for the ‘English’ Category

Logical fallacies in assessing risks from climate change

December 9, 2013

Guest post by Hans Custers. Cross-posted at Planet3.  Nederlandse versie hier.

Fallacies of risk

We, humans, are not very good at estimating and weighing risks. Looking at the definition of risk, this is not so strange:

Risk = Probability * Effect

Most of us will have some understanding of both elements ‘Probability’ and ‘Effect’, but the combination of the two is rather abstract. In judging risks, we tend to focus on one of the two elements, and more or less neglect the other one. The figure below shows the difference between our perception of certain risks and their actual magnitude.

Risk perception and actual hazards

The definition of risk might suggest that it is always possible to calculate it, or give a quantitative estimate. But often, it’s not that easy. Sometimes, it can be difficult to define the exact “Effect”, and the parameter that can be used to quantify it. Effects can go from economic or financial costs to damage to nature, from a small decrease in well-being, to large numbers of casualties. Our judgment of risks depends a lot on the type of effect.

To make things even more complicated, the debate on risk often takes place at the border of science and politics, of logical reasoning and subjective judgment. Whatever we do to try and find objective parameters and criteria to assess and weigh risks, decisions what we do and what we do not find acceptable depend to some extent on value judgments. There are no 100% objective criteria to make these types of decisions.

It’s obvious that many fallacies can come up in this minefield for logic. Last week, Judith Curry blogged on the article “Fallacies of risk” by Sven Ove Hansson, trying to identify the fallacies in the debate on climate. In his article, Hansson seems to mainly focus on risks of (new) technologies, especially the ones with a low probability and large effects. Applying the same fallacies to the debate on climate is not as straightforward as it might seem. Curry seems to be making some mishaps. She ends up making quite a few comments on Hansson that totally miss the point. Here’s my attempt to improve on hers.


Cowtan and Way global average temperature observations compared to CMIP5 models

November 15, 2013

It is well known that the Arctic is warming up much faster than the rest of the globe. As a consequence, datasets which omit this region (HadCRUT and NOAA) underestimate the global warming trend. A new paper by Cowtan and Way addresses this cool bias by using satellite data to fill in these data gaps. They make a good case that this method also improves upon the NASA GISS dataset, which uses extrapolated data from surface stations to partly fill in the data sparse regions. Combining their new method of infilling with the most up-to-date sea surface temperatures gives a substantially larger trend over the last 15 years than the abovementioned datasets do. The temporary slowdown in global surface warming (also dubbed “the pause”) nearly disappears. As Michael Tobis notes:

This demonstrates is how very un-robust the “slowdown” is.

The corrections don’t amount to a huge change in absolute temperature change, and the new data actually fall inside the uncertainty envelope provided by HadCRUT4. As the paper correctly states:

While short term trends are generally treated with a suitable level of caution by specialists in the field, they feature significantly in the public discourse on climate change.

In the figure below (made by Jos Hagelaars) the global average temperature as calculated by Cowtan and Way (“C&W hybrid”) is compared to both the HadCRUT4 dataset and the CMIP5 multi-model mean as well as its 5% and 95% percentile values (RCP8.5): [Update: The figure below has
been replaced, since the original was found to be in error during discussions on CA). The confidence interval of this corrected graph is substantially narrower than the erroneous original one. Note that the current graph shows the 5 to 95 percentile range of model runs (i.e. the 90% confidence interval), whereas the previous ones showed the 95% confidence interval. At the bottom of the post a similar figure with both confidence intervals as well as the two sigma range is shown.


Also with these data improvements, recent observations are at the low side of the CMIP5 model range. The comparison of observations to models has to be interpreted with caution however. Some people like to jump to preferred conclusions, but it’s good to keep in mind that the expected warming at a specific point in time depends on a combination of factors. Any of these factors -as well as shortcomings in the observational data, such as those discussed by Cowtan and Way- could contribute to a mismatch between observations and models:

- radiative forcing

- equilibrium climate sensitivity

- climate response time

- natural unforced variability

The last factor means that one shouldn’t expect the multi-model mean (in which most variability is cancelled out) to be identical to the observations (which are the result of a particular realisation of natural variability).

Cowtan and Way made a very clear video in which the main results of their paper are explained in just a few minutes. Highly recommended watching:

More commentary on the paper on e.g. RC (Rahmstorf), SkS (Cowtan and Way), Guardian (Nuccitelli), P3 (Tobis), Victor Venema, Neven. See also this very useful background information provided by the authors.

[some typos corrected and clarifications added, 16-11. Erroneous figure replaced 21-11.]

Update: Below a similar figure as above, with different confidence intervals for the model runs shown. 


Update 2 (Feb 2014):

Jos Hagelaars added Cowtan and Way’s data for 2013 to a figure comparing observations to model projections:

Jos Hagelaars - comparison_cmip5_hadcrut4_cowtanway_2013

BBC interview: global warming pause, climate sceptics, long timescales

September 27, 2013

I was interviewed by Matt McGrath from the BBC last week, as were several other Dutch climate spokespeople (including PBL’s senior scientist Arthur Petersen and skeptical science writer/journalist Marcel Crok). Short parts of these interviews have appeared on the web  and on Radio 4 (“The World Tonight”, 26-09). Below I try to provide a bit of context to my quotes.

Both pieces are centred, as is fashionable these days, on the apparent smaller rate of surface warming in the past 15 years. The web piece is entitled “Climate sceptics claim warming pause backs their view”. Of course they claim it does. What sceptics did achieve –credit where credit is due- is to put this so-called “pause” on the agenda of mainstream media, until it got so fashionable that they all feel forced to use it as an anchor for any reporting on climate. But, as Gavin Schmidt is quoted as saying:

focus on a global warming pause over the past 15 years is a “misplaced” distraction that misses the big picture. He said, “The IPCC and the issue of climate change is not about the weather next year or the next five years; it’s about the long-term climate change that we are engendering.”

See also this useful figure from Stefan Rahmstorf, underscoring the silliness of drawing all too strong conclusions from 15-year trends.

giss2012c - Rahmstorf - Global temp with two silly trendlines

Figure showing NASA GISS global average temperatures with trendlines from 1992-2006 (light blue) and 1998-2012 (green) as well as the most recent 30-year trend in red. Naturally, starting in a very cold volcano-influenced or very warm El Nino influenced year will inflate or deflate the trend. (source: Stefan Rahmstorf)

I am quoted in the BBC piece as follows:

Bart Verheggen is an atmospheric scientist and blogger who supports the mainstream view of global warming. He said that sceptics have discouraged an open scientific debate.

“When scientists start to notice that their science is being distorted in public by these people who say they are the champions of the scientific method, that could make mainstream researchers more defensive.

“Scientists probably think twice now about writing things down. They probably think twice about how this could be twisted by contrarians.”

The discussion was about to what extent climate science isn’t open/transparent enough, as contrarians routinely claim. Matt also asked to what extent skeptics actually play a positive role in making science more open/transparent and more self-critical. I said ideally they would. People who are critical usually have a good influence that way. But many climate contrarians don’t just stop at raising partly valid criticism, but go on to distort the science. That has the opposite influence, as scientists noticing this behavior become more careful and more defensive, and(have to) think ahead how their words might get twisted by contrarians. So they may become less open and less frank, and more careful in how they chose their words.

That is the opposite of what contrarians claim they want to achieve, so it’s quite ironic (though entirely logical) that this is the more likely effect of their behavior. It shows quite a lack of self-awareness on their part that they don’t see how their actions and their behavior affect the dynamics of the public debate. For the worse, in most –though not all- cases.

There may also be some lack of self-awareness among the mainstream that they respond in a way that’s not conducive to a long-term open and frank dialogue with society. From an older comment of mine:

If the valid criticisms wouldn’t be packaged in such conspiratorial/accusative/exaggerated (c/a/e) ways, they would be welcomed much more than they currently are. The art that mainstream scientists and their defenders must learn is to take the valid parts of the criticisms and deal with/respond to them, and leave the c/a/e packaging for what it is. That is increasingly difficult because the critics and their supporters will try to keep the c/a/e in (presumably because this packaging is what is most effective at decreasing the scientists’ credibility and sowing doubt). That dynamic needs to be broken. It needs effort from both sides, as difficult and unfair as it sounds.

As I wrote in my earliest (and still rather positive) reflection on the Curry-turn:

There is a tendency of ‘circling the wagons’ within the scientific community, in response to the continuous attacks on the science. Attacks that are mostly based on smear and insinuation rather than solid arguments. (…) I think the ’us-versus-them’ feeling amongst many scientists and their supporters is understandable (as a reaction to the contrarian c/a/e attacks on the science), but counterproductive in the long run.

In the Radio 4 show (at ~33:50 min in; earlier in the downloadable mp3 version), I am saying a few things about the timescale of the problem and of the solution. I brought this up when the discussion was about whether we now have more time to respond to climate change. This is a vastly underappreciated point in the climate discussion:  The climate system will take much longer to cool down than it did to warm up. This is a consequence of how the carbon cycle works. In this context, I said the following:

We’re going somewhere, and if we don’t like where we’re going, we have to turn that wheel in time.  As when you’re on a giant supertanker on the ocean, you can’t say “oh, I’ll wait until I can feel the iceberg with my pinkie and then I’ll turn the wheel”. Then you’re a bit late, so you have to start doing that in time. That’s the other side of the coin. But if you keep banging the drum saying “it’s five to twelve! It’s five to twelve!” doesn’t work either. And that could be counter-effective to engage those who are a bit more skeptical.

Global warming is a problem in slow-motion, hence the “five to twelve” line is not the most useful one to get people on their feet, because if it remains five to twelve for too long, they will tune you out. That’s what happened in the aftermath of COP15 in Copenhagen for example (where the 5-to-12 line was used a lot, and not much has changed in the years since). The supertanker analogy is more appropriate I find, since that makes clear that even though the problematic situation that’s on your path isn’t in close proximity yet, it is necessary to change course, if you wish to avoid it.


Tropospheric hot spot

August 19, 2013

The current topic under discussion at ClimateDialogue is the tropospheric hot spot: Is it there, and if not, so what? Invited discussant are Steven Sherwood of the University of New South Wales in Sydney, Carl Mears of Remote Sensing Systems (working on the RSS satellite product) and John Christy of the University of Alabama in Huntsville (working on the UAH satellite product).

I’ll provide a short overview here (loosely based on the intro over at CD), interspersed with my own and other people’s commentary.

Based on theoretical considerations and simulations with General Circulation Models (GCMs), it is expected that any warming at the surface will be amplified in the upper troposphere. The reason for this is as follows: More warming at the surface means more evaporation and more convection. Higher in the troposphere the (extra) water vapour condenses and heat is released. Calculations with GCMs show that the lower troposphere warms about 1.2 times faster than the surface. For the tropics, where most of the moist is, the amplification is larger, about 1.4.

This means that, contrary to what some people claim, the hot spot is not specific to the enhanced greenhouse effect: Any surface warming (or cooling) would be expected to be magnified higher aloft, at least in the tropics. Lindzen says it as follows:

We know that the models are correct in this respect since the hot spot is simply a consequence of the fact that tropical temperatures approximately follow what is known as the moist adiabat. This is simply a consequence of the dominant role of moist convection in the tropics.


The fallacy of the middle ground

August 5, 2013

There’s been quite some climate discussion in the Political Science section of the Guardian lately. Warren Pearce had an invited post in which he asked the rhetorical question “Are climate sceptics the real champions of the scientific method?

He makes some good observations about the dynamics of the public debate and the nature of skepticism (e.g. most contrarians don’t deny the basic physics underlying the greenhouse effect, but rather dispute the magnitude of warming that would result from an increased greenhouse effect). On the other hand, he misses the mark in other areas (e.g. he correctly describes how contrarians see themselves but doesn’t investigate how their argumentation really stacks up; often they are guilty of what they accuse mainstream science of).

My main beef with his piece though is his flawed argument of why a well-known contrarian blogger like Anthony Watts, according to Pearce, should be seen as someone who “seeks to uphold standards, through transparent and auditable scientific practice” and “a ‘mainstream’ sceptic who can challenge key areas of climate science without entering into pseudoscience”. Why this praise? Because Watts publicly disagreed with the fringe group Principia Scientific who deny the basic physics underlying the greenhouse effect (which was first established in the 19th century).

That is not a logical argument to make though: Regardless of what one may think of Watts, contrasting an extremist with someone who is even more extreme doesn’t make him mainstream. Regardless of what one thinks of Watts, contrasting someone who frequently flirts with pseudoscience with an all-out pseudo-science lover doesn’t free the former from any link with pseudo-science.

That is what I would call the fallacy of the middle ground.


The Dutch view on the future of the IPCC – what it does and what it does not say

July 15, 2013

Guest post by Hans Custers. Dutch version is here.

The IPCC invited the governments of all participating countries to give their view on the future of the climate panel. The IPCC is a complex organization, dealing with a very complex subject, so perfection will be impossible to achieve. Or, from an optimistic point of view: there’s always room for improvement. And of course, a transparent process of self-reflection is a very good start for improvements. But well, this is the IPCC, so there is a catch. Whatever happens in this process, it will be spun by the anti IPCC and anti climate science campaign as ‘evidence’ for their claims. Every bit of criticism on the IPCC’s procedures and methods will be spun as substantial criticism on the scientific content of the assessment reports. And if governments would be reluctant to criticize, because they know it will be taken out of context, it would be seen as ‘evidence’ for the huge climate conspiracy.

The Netherlands has finished their submission and it was published (pdf) on KNMI’s website early last month. In my opinion they did what they should have done: they presented a straightforward view, not worrying about the inevitable spin by the skeptic campaign. And, oh yeah, inevitable it was. The next quote appeared in internet discussions again and again, after skeptics found the submission:

The IPCC needs to adjust its principles. We believe that limiting the scope of the IPCC to human-induced climate change is undesirable, especially because natural climate change is a crucial part of the total understanding of the climate system, including human-induced climate change. The Netherlands is also of the opinion that the word ‘comprehensive’ may have to be deleted, because producing comprehensive assessments becomes virtually impossible with the ever expanding body of knowledge and IPCC may be more relevant by producing more special reports on topics that are new and controversial.

Skeptics don’t seem to understand, or don’t want to understand, that this is about the IPCC organization only, and not about the content of assessment reports, or even climate science in general. They suggest the Dutch government thinks that natural influences and controversial topics may have been underestimated in previous assessments. There are some things they overlook.


EGU General Assembly: The Arctic, Models, and Data

June 7, 2013

Guest post by Heleen van Soest

In April, the annual European Geosciences Union conference was held in Vienna, Austria. Heleen van Soest, MSc student Climate Studies at Wageningen University, attended the conference, and shares some thoughts and tweets (@Hel1vs).

The opening reception, April 7, reveals that geoscientists are fond of beer. I get to talk to some nice people and hand out my first business cards. Yay! I talk with Walter Schmidt,  President of the Division on Geosciences Instrumentation and Data Systems, about observations and data. Lesson learned: data are important, but never take them for granted. Especially from satellites: they basically measure counts and voltages. To interpret the numbers and get something useful, we already need models, i.e. algorithms. Usually, model skill is tested against data. Disagreement between them is often blamed on model errors, assumptions, etc. Keep in mind that data might be wrong, too. Fortunately, raw data is increasingly archived as such, together with the algorithms used to interpret them. In that way, data can still be used if the algorithms are updated. I dedicate my first #egu2013 tweet to this conversation and go home. I am happy to find a Va Piano (Italian restaurant) in ‘my’ street. Together with Sherlock Holmes (the book, that is), I eat my pasta.

Tweet At #egu2013 opening reception, interesting conversation about models and data: “important, but never take them for granted” (Walter Schmidt)

Monday, 8 April

Permafrost day. An important issue, as permafrost contains about half of the world’s soil carbon. If permafrost thaws, the organic carbon becomes available for microbes to degrade. Greenhouse gas (methane) emissions are a result, further increasing temperatures. This positive feedback is sometimes compared to a time bomb. Modelling studies of permafrost do show it will degrade under further warming. For example, Greenland permafrost south of 76°N will disintegrate this century. However, see RealClimate before you start to worry that this bomb is about to explode.

But today is not only permafrost; I’ve also got something on ice observations.


Consensus: Behind the numbers

May 17, 2013

The much reported paper by John Cook et al finds a very strong consensus about human caused climate change in the scientific literature: Of those abstracts expressing a position on the cause of global warming, 97% (implicitly or explicitly) endorsed human causation of this warming.

Over at Lucia’s, Brandon Shollenberger  found a way to search the results of 12,280 out of 12,465 papers. Based on this search method and the SkS paper rating guidelines, Marcel Crok reports the following breakdown of results:

Category 1 (explicit endorsement with quantification): 65
Category 2 (Explicit Endorsement without quantification): 934
Category 3 (Implicit Endorsement): 2933
Category 4 (Neutral): 7930 [the reported number]
Category 5 (Implicit Rejection): 53
Category 6 (Explicit Rejection without quantification): 15
Category 7 (Explicit Rejection with quantification): 10

The 97% is arrived at by adding up categories 1 to 3 and taking that as a percentage of all categories except 4. This percentage is actually 98% using the numbers above, but these are obtained via a shortcut.

Of course, various other fractions could be calculated from this list, each with a slightly different meaning.  E.g. of those abstracts making a statement about the quantitative contribution of human activity to the warming, 87% (65/75) endorsed dominant human causation. And of those abstracts expressing an explicit position on the cause of global warming, 97.6% (999/1024) endorsed human causation.

Any way you slice it, a strong consensus it is.


Long term persistence and internal climate variability

April 30, 2013

After a long hiatus, Climate Dialogue has just opened a second discussion. This time it’s about the presence of long term persistence in timeseries of global average temperature, and its implications (if any) for internal variability of the climate system and for trend significance. This discussion is strongly related to the question of whether global warming could just be a random walk, a question vigorously debated on this blog (incl my classic  april fool’s day post three years ago).

Invited expert participants in the discussion include Rasmus Benestad (of RealClimate fame), Demetris Koutsoyiannis and Armin Bunde. The introduction text here slightly differs from that posted on


Documentairy “Thin Ice” now available on the web

April 22, 2013

The documentairy “Thin Ice“, with spectacular images and interviews with a few dozen of well-known and lesser well-known climate scientists, is available for viewing tonight on their website (which features lots of other interesting content btw). At the same time, various public screenings are being organized all over the world (unfortunately not in the Netherlands, mea culpa). The free viewing via their website is probably temporary, though I don’t know for how long (my guess is a few days). The premiere has of course been timed to coincide with Earth Day.


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