Archive for the ‘Aerosol’ Category

FAQ for the article “Scientists’ Views about Attribution of Global Warming”

August 11, 2014

published in Environmental Science and Technology (open access), DOI: 10.1021/es501998e, Supporting Information here.

A formal version of the FAQ is also available at the website of the PBL Netherlands Environmental Assessment Agency. A blog post with a brief description of the main conclusions is here.



1. What are the objectives of this survey?

The PBL aimed to characterize the spectrum of scientific opinion about physical climate science issues. The research was focused on issues that are a frequent topic of public debate, and explored questions such as:

  • On which issues is there widespread agreement amongst scientists?
  • On which issues do scientists hold varied opinions?
  • How does the spectrum of scientific opinion compare to IPCC assessments?
  • How do scientists view skeptical arguments and viewpoints?

2. What is the relevance of an opinion survey or of measurement of consensus in trying to assess the science?

Science is based on the critical evaluation of available evidence in the context of existing knowledge. It is not “just an opinion.” With this survey, we tried to identify how scientists assess the different viewpoints that exist in public discussions of climate science. If the evidence for a certain viewpoint has become sufficiently strong and stable over time, the scientists’ aggregated opinion could be expected to reflect that.

3. Are the survey results publicly available?

The full survey results are not publicly available, because the PBL intends to use the data for further analyses.
Graphs with the ‘straight counts’ for every question (i.e. the number of responses for each answer option) are available in a background report. These results are not segregated in different sub-groups.



4. How does this study compare to the often-quoted 97% consensus?

Our results are consistent with similar studies, which all find high levels of consensus among scientists, especially among scientists who publish more often in the peer-reviewed climate literature.

Cook et al. (2013) found that 97% of papers that characterized the cause of recent warming indicated that it is due to human activities. (John Cook, the lead author of that analysis, is co-author on this current article.) Similarly, a randomized literature review found zero papers that called human-induced climate change into question (Oreskes, 2004).

Other studies surveyed scientists themselves. For instance, Doran and Kendall-Zimmermann (2009) found lower levels of consensus for a wider group of earth scientists (82% consensus) as compared to actively publishing climatologists (97% consensus) on the question of whether or not human activity is a “significant contributor” to climate change. Our results are also in line with those of e.g. Bray and von Storch (2008) and Lichter (2007).

In our study, among respondents with more than 10 peer-reviewed publications (half of total respondents), 90% agree that greenhouse gases are the largest – or tied for largest – contributor to recent warming. The level of agreement is ~85% for all respondents.

While these findings are consistent with other surveys, several factors could explain the slight differences we found:

  • Surveys like ours focus on opinions of individual scientists, whereas in a literature analyses the statements in individual abstracts are tallied. Literature analyses have generally found higher levels of consensus than opinion surveys, since the consensus is stronger amongst more heavily published scientists.
  • This study sets a more specific and arguably higher standard for what constitutes the consensus position than other studies. For instance, Doran and Kendall-Zimmermann (2009) asked about human activity being a “significant contributor” to global warming, and Anderegg et al. (2010) investigated signatories of public statements, while we asked specifically about the degree to which greenhouse gases are contributing to climate change in comparison with other potential factors.
  • Contrarian viewpoints are somewhat overrepresented in our survey and they may have overestimated their self-declared level of expertise (see question 9).

5. How is the consensus or agreement position defined?

The consensus position was defined in two ways:

  • Greenhouse gases contributed more than 50% to global warming since the mid-20th (Question 1). This is analogous to what was written in IPCC AR4.
  • Greenhouse gases have caused strong or moderate warming since pre-industrial times (Question 3). “Moderate” warming was only interpreted as the consensus position if no other factor was deemed to have caused “strong” warming. This response means that greenhouse gases were considered the strongest –or tied for strongest- contributor to global warming.

The former definition exactly mirrors the main attribution statement in IPCC AR4 and served as a ‘calibration’ for the latter.

6. What does “relative response” mean on the y-axis of many Figures?

This gives the percentage of the respondents (often within a certain sub-group) for the specific answer option. We opted to show the relative response rather than the absolute response to enable comparing the responses of different sub-groups (with differing group sizes as denoted by N=…) within one graph.

7. What are “undetermined” answers?

Those are the sum of responses “I don’t know”, “unknown” and “other”.

8. Why do IPCC AR4 authors show a higher consensus than the other respondents?

AR4 authors are generally domain experts, whereas the survey respondents at large comprise a very broad group of scholars, including for example scientists studying climate impacts or mitigation. Hence we consider this to be an extension of the observation -in this study and in e.g. Anderegg et al. (2010) and Doran and Kendall-Zimmermann (2009) – that the more expert scientists report stronger agreement with the IPCC position. Moreover, on the question of how likely the greenhouse contribution exceeded 50%, many respondents provided a stronger statement than was made in AR4. Using a smaller sample of scientists, Bray (2010) found no difference in level of consensus between IPCC authors and non-authors.

9. How reliable are the responses regarding the respondent’s area of expertise and number of peer-reviewed publications?

Respondents were tagged with expertise fields, though these were in many cases limited and not meant to be exhaustive. These tags were mainly used to ensure that the group of respondents was representative of the group that the survey was sent to. A subset of respondents was also tagged with a Google Scholar metric. Those who were tagged as “unconvinced” reported more expertise fields than the total group of respondents and also a higher number of publications compared to their Google Scholar metrics, if available (see Supplemental Information).

10. Since most scientists agree with the mainstream and therefore most media coverage is mainstream, what is the problem with “false balance”?

Scientists with dissenting opinions report receiving more media attention than those with mainstream opinions. This results in a skewed picture of the spectrum of scientific opinion. Whether that is problematic is in the eye of the beholder, but it may partly explain why public understanding lags behind scientific discourse (e.g. the “consensus gap”).


Survey Respondents

11. How many responses did you get to the survey?

Out of 6550 people contacted, 1868 filled out the survey (either in part or in full).

12. How did you compile the list of people to be surveyed?

Respondents were selected based on

  • keyword search in peer-reviewed publications (“global climate change” and “global warming”)
  • recent climate literature (various sources)
  • highly cited climate scientists (as listed by Jim Prall)
  • public criticisms of mainstream climate science (as listed by Jim Prall)

13. Are all of the survey invitees climate scientists?

The vast majority of invitees are scientists who published peer-reviewed articles about some aspect of climate change (this could be climate science, climate impacts, mitigation, etc.). Not all of them necessarily see themselves as climate scientists.

14. Why did you invite non-scientist skeptics to take part in the survey?

They were included in the survey to ensure that the main criticisms of climate science would be included. They constitute less then 5% of the survey respondents. Viewpoints that run counter to the prevailing consensus are therefore somewhat magnified in our results.

15. How representative are the survey responses of the “scientific opinion”?

It’s difficult to ascertain the extent to which our sample is representative, especially because the target group is heterogeneous and hard to define. We have chosen to survey the wider scientific field that works on climate change issues. Due to the criteria we used and the number of people invited we are confident that our results are indeed representative of this wider scientific field studying various aspects of global warming. We checked that those who responded to the survey were representative of the larger group of invitees by using various pieces of meta-information.

16. Did you take into account varying levels of expertise of respondents?

Respondent were asked to list their area(s) of expertise and their number of peer-reviewed publications. These and other attributes were used to interpret differences in responses.

17. How did you prevent respondents from manipulating the survey results, e.g. by answering multiple times?

An automatically generated, user specific token ensured that respondents could only respond once.

18. How did you ensure respondent anonymity?

Survey responses were analyzed by reference to a random identification number.


Survey Questions

19. Are the survey questions public?

Yes, survey questions and answer options are available on the PBL website and as Supporting Information (part 2) to the article.

20. How did you decide on the questions to ask?

The survey questions are related to physical science issues which are a frequent topic of public debate about climate change.

21. Was the survey reviewed before it was sent to respondents?

Yes, before executing the survey it has been extensively tested and commented on by various climate scientists, social scientists and science communicators with varying opinions, to ensure that questions were both clear and unbiased. Respondents were not steered to certain answers.


Reference: Bart Verheggen, Bart Strengers, John Cook, Rob van Dorland, Kees Vringer, Jeroen Peters, Hans Visser, and Leo Meyer, Scientists’ Views about Attribution of Global Warming, Environmental Science and Technology, 2014. DOI: 10.1021/es501998e. Supporting Information available here.

Please keep discussions on this thread limited to what is mentioned in this FAQ and to other questions you may have about the survey or the article. Discussion of the survey results should be directed at the more generic blog post.


Confusing the net cloud effect with a cloud feedback: Very different beasts

September 20, 2011

I stumbled upon a new post at WUWT today:

New peer reviewed paper: clouds have large negative-feedback cooling effect on Earth’s radiation budget

Note that Anthony has since changed the title to leave out the word “feedback”, which was the source of his confusion. It starts out as follows:

Oh dear, now we have three peer reviewed papers (Lindzen and Choi, Spencer and Braswell, and now Richard P. Allan) based on observations that show a net negative feedback for clouds, and a strong one at that. (…) The key paragraph from the new paper:

…the cloud radiative cooling effect through reflection of short wave radiation is found to dominate over the long wave heating effect, resulting in a net cooling of the climate system of −21 Wm−2.

The attentive reader will immediately spot the problem here. Watts is confusing two issues:

– the net radiative effect of clouds on climate (i.e. in comparison with having no clouds at all)

– the net feedback of clouds in response to a change in climate

The paper addresses the first, whereas Anthony interpreted it as if it addresses the second.

These are two distinctly different issues. The latter (clouds as feedback) is about how cloud cover and properties might change in response to a warming or cooling of the climate: Will the net cloud radiative effect (i.e. the former) become more or less negative.

The net radiative effect of clouds on Earth’ climate has long been known to be negative (i.e. cooling). See e.g this quote from the paper:

The overall global net cloud radiative effect is one of cooling as documented previously (Ramanathan et al., 1989).

That can be verified in any textbook on the subject and most introductions of papers on this topic. Or in my introductory post on aerosols, clouds and climate.

I pointed this error out in the thread, as did more than a few others after me (including Roy Spencer). Only after the author of the paper, Richard Allan, came in to say that this post mis-interpreted the paper, did Anthony change the title and added an update. The mistaken interpretations are still in the body of the text though.

Richard Allan wrote to me in email (reproduced with permission):

I was surprised that this paper was linked to cloud feedback since, as you mention, it attempts to quantify the well known influence of cloud on Earth’s radiation budget (at the top of the atmosphere, at the surface and within the atmosphere and also during day and night) and does not attempt to diagnose cloud feedback.

Watts goes on to say (bold in original):

The cooling effect is found to be -21 Watts per meter squared, more than 17 times the posited warming effect from a doubling of CO2 concentrations which is calculated to be ~ 1.2 Watts per meter squared.

He’s comparing apples and oranges. The 21 W/m2 is the top of the atmosphere (TOA) cloud forcing in reference to having no clouds at all (see table 1 in the paper); the 1.2 W/m2 is the surface forcing due to a doubling in CO2 concentrations. The TOA forcing of a doubling in CO2 is closer to 4 W/m2. But that’s not the “zero” point. The total greenhouse effect (due to water vapor, clouds, CO2 and other GHG) is about 150 W/m2.                                                                                                                   

In other words, this paper falls squarely within the mainstream; it further quantifies a (previously known) net cooling effect of clouds on the Earth’ climate; it does not quantify how clouds may change in response to warmer climate (cloud feedback), though it does provide a carrot stick in saying that these types of analyses are important “in assessing cloud climate feedbacks which contribute substantially to uncertainty in climate prediction.”. That may very well be, but it hasn’t been done in this paper (as confirmed by its author). 

Judging by the comments, many at WUWT took this, in combination with the whopping -21 W/m2, to mean that they discovered a gigantic negative feedback. Nope.

Tallbloke (from Lisbon fame) still insists that

if [cloud forcing] becomes slightly less negative, it’s still very negative, and overwhelms the effect of changes in co2.

… being very confused. Comments vary over a very wide range though. Many are confused (e.g. stating that as specific humidity goes up in a warmer world, so should cloud cover, whereas cloud formation depends on relative humidity rather than on specific humidity), there’s lots of laughing-at-the-scientists going on, but there are also sensible comments that either offer insight or good questions.

Mosher makes the following observation:

it is also fascinating because of what we dont see. usually you will see a whole crew of commeters pounce on the word “model”. This time they didnt.

They didnt because they thought the paper supported spencer. But it was on an entirely different topic. That misunderstanding kinda silenced the usual “models are bad” crew.

Radiative forcing by aerosol used as a wild card: NIPCC vs Lindzen

February 22, 2011

(This is also featured as a guest post on Skeptical Science)

The greatest source of uncertainty in understanding climate change is arguably due to the role of aerosols and clouds. This uncertainty offers fertile ground for contrarians to imply that future global warming will be much less than commonly thought. However, some (e.g. Lindzen) do so by claiming that aerosol forcing is overestimated, while others (e.g. the NIPCC) by claiming that aerosol forcing is underestimated. Even so, they still arrive at the same conclusion…

Let’s have a look at their respective arguments. Below is a figure showing the radiative forcing from greenhouse gases and from aerosols, as compared to pre-industrial times. The solid red curve gives the net forcing from these two factors. The wide range of possible values is primarily due to the uncertainty in aerosol forcing (blue dotted line).

The greenhouse gas forcing (dashed red curve) is relatively well known, but the aerosol forcing (dashed blue curve) is not. The resulting net anthropogenic forcing (red solid curve) is not well constrained. The height of the curve gives the relative probability of the associated value, i.e. the net climate forcing is probably between 1 and 2 W/m2, but could be anywhere between 0 and 3 W/m2. (From IPCC, 2007, Fig 2.20)

NIPCC’s argument

The NIPCC report (a skeptical document, edited by Craig Idso and Fred Singer, made to resemble the IPCC report) says:

“The IPCC dramatically underestimates the total cooling effect of aerosols.”

They hypothesize that natural emissions of aerosol precursors will increase in a warming climate, causing a negative feedback so as to dampen the warming. Examples of such gaseous aerosol precursors are dimethyl sulfide (DMS) emitted by plankton or iodocompounds created by marine algae. They use these putative negative feedbacks to claim that

“model-derived sensitivity is too large and feedbacks in the climate system reduce it to values that are an order of magnitude smaller.”


These are intriguing processes (the “CLAW hypothesis” first got me interested in aerosols, when I assisted with DMS measurements on some remote Scottish islands), but their significance on a global scale is ambiguous and highly uncertain. As a review article about the DMS-climate link says:

“Determining the strength and even the direction, positive or negative, of the feedbacks in the CLAW hypothesis has proved one of the most challenging aspects of research into the role of the sulfur cycle on climate modification.”

The NIPCC report exaggerates the uncertainty in climate science, but seems to put a lot of faith in elusive and hardly quantified processes such as natural aerosol feedbacks coming to our rescue.

Lindzen’s argument

On to Lindzen:

“The greenhouse forcing from man made greenhouse gases is already about 86% of what one expects from a doubling of CO2 (…) which implies that we should already have seen much more warming than we have seen thus far (…).”

Lindzen again (E&E 2007):

“How then, can it be claimed that models are replicating the observed warming? Two matters are invoked.”

The “two matters” he refers to are aerosol cooling and thermal inertia from the oceans (a.k.a. “warming in the pipeline”). He then proceeds to argue that both of these factors are much smaller than generally thought, perhaps even zero. E.g. on aerosols, Lindzen writes:

“a recent paper by Ramanathan et al (2007) suggests that the warming effect of aerosols may dominate – implying that the sign of the aerosol effect is in question.”

By downplaying the importance of these two factors, Lindzen argues that the observed warming implies a small climate sensitivity. The same line of argument is also used by Dutch journalist Marcel Crok, who writes in his recent book (in Dutch, my translation):

aerosols probably cool much less than commonly thought


While it is true that aerosols can warm and cool the climate (by absorption and reflection of solar radiation, respectively, besides influencing cloud properties), most evidence suggests that globally, cooling is dominant. Whereas Ramanthan et al (2007) don’t quantify the net aerosol effect (in contrast to Lindzen’s implicit claim), Ramanathan and Carmichael (2008) (quoted by Crok) do. They estimate both the warming ánd cooling effects to be stronger than most other estimates, but the net forcing (-1.4 W/m2) is right in line with (even a little stronger than) the IPCC estimate (see the above figure). Taking into account realistic estimates of aerosol forcing and ocean thermal inertia, the earth has warmed as much as expected, within the admittedly rather large uncertainties. Ironically, it is exactly because aerosol forcing is so uncertain and because the climate hasn’t equilibrated yet that the observed warming since pre-industrial times is only a very weak constraint on climate sensitivity. Lindzen seems very certain of something that most scientists would readily admit is very uncertain.


So we have the peculiar situation that both of these approaches try to claim that climate sensitivity is small, but the NIPCC approach is to claim that aerosol forcing is very large (thus providing a negative feedback to warming), whereas the Lindzen approach is to claim that aerosol forcing is very small (thus necessitating a small sensitivity to explain the observed warming so far). Of course they can’t both be right, and probably neither of them are. Looking back at the figure above, both approaches are based on assuming that aerosol forcing is at the edge of the probability spectrum (as if it were some fudge factor), whereas the most likely value is somewhere in the mid range. Both approaches also ignore the other lines of evidence that point to climate sensitivity likely being in the range of 2 to 4.5 degrees. E.g. a value as small as suggested by the NIPCC (0.3 degrees) is entirely inconsistent with the paleo-climate record of substantial climate changes in the earth’ history. And finally, both approaches implicitly assign high confidence to some of the most uncertain aspects of climate science, even though they routinely mock climate science as if nothing is known at all.

Of course it is not mandatory for all those who dismiss mainstream climate science to agree, but to see two important “spokespeople” for climate contrarians take such mutually inconsistent approaches is peculiar. Even more so when you realize that Lindzen signed the recent “prudent path” letter to US Congress, in which the NIPCC report was approvingly cited… Most people can’t have it both ways, but apparently climate contrarians can.

See also a more detailed critique of the NIPCC and of Lindzen’s argument.

Climate uncertainties

August 23, 2009

The bottom line of my previous post was that the ‘next generation questions’ on climate change should focus on what do we do about it? I also mentioned what major aspects of climate science I thought are most uncertain:
– Regional climate effects
– Equilibrium climate sensitivity
– The role of aerosol and clouds
– Sea level rise (update: added after Heiko’s suggestion)
Tom Fuller asked me to elaborate, based on some specific questions he posed

1. Regional climate effects

I think we can agree that both adaptation and mitigation (emission reduction) are needed, but letting mitigation play second fiddle puts us at risk of ever increasing climate damages, which will cost future generations a lot to adapt to (if at all possible). To limit the future risks to manageable levels, mitigation is of the utmost importance.

Helping countries become richer should likewise be coupled to helping them become more sustainable. Even apart from climate change, this is self-evident: 20% of the population consumes 80% of its resources. For many of these resources, there is not enough to go around for 6 billion people to have the American consumption pattern, plain and simple.

The strong link between wealth and pollution, resource depletion and climate change should be weakened. The only way that developing countries can reach our level of wealth in a sustainable way is if the ecological impact per unit wealth decreases at least as fast as their wealth increases (see eg Michael Tobis). Otherwise the impacts will continue to increase, above and beyond what we can reasonably adapt to.

 2. Climate sensitivity

 To estimate the equilibrium (Charney) sensitivity (change in temperature after a doubling of CO2), the climate forcing needs to be known as well as the climate response after it’s had time to settle into a new equilibrium. For a slowly changing forcing such as we’re experiencing now, the equilibration time is long but not accurately known, and neither is the aerosol forcing, so 20th century data are not particularly useful. Collecting better or more temperature data is not going to help.

Climate sensitivity is primarily constrained by paleo-climate data, while the climate response following a volcanic eruption is also a useful indicator from what I’ve understood. These constraints leave some more wiggle room at the upper end than at the lower end. Combining multiple constraints together leads to a most likely value of 3 deg for a doubling of CO2 (See eg James’ empty blog). Climate models also converge on this value (+/- 1). This has been a remarkable stable estimate over the course of decades, while the uncertainty hasn’t decreased significantly. Perhaps it won’t anytime soon. And perhaps that doesn’t matter too much as far as policy goes, because even with a realistic low estimate we’re still way behind in our policy response.

 3. Aerosols and clouds

 The short lifetime (days to weeks) of aerosols is an important reason for the uncertainty in their role in climate change. It causes their concentration to be highly variable in time and space, and it’s hard to even know what the global concentration is. Add to that their variability in size and chemical composition, and the poorly understood role of clouds, and it’s clear that the uncertainty in aerosol radiative forcing will remain a steady feature of climate science for some time to come.

 Different clouds have different climate effects. They both cool (by reflection of sunlight) and warm (by their trapping of IR radiation, much like GHG) the atmosphere. Which effect dominates depends on the type and altitude of the cloud. Their large variability and myriad of interdependencies involved makes quantifying their global effect very difficult indeed. This won’t change any time soon.

I have some hope that better and more satellite measurements will drive the better quantification in the future, but that’s just a guess. Process based cloud physics studies are equally necessary to elucidate the interdepencies.

With the industrialization, SO2 emissions soared, causing the aerosol burden to increase. More recently we’ve started to clean up our act regarding SO2, so the aerosol burden (at least in Europe and North America) is decreasing again. That’s the reason for their ‘bridging effect’ halfway through the 20th century; it has nothing to do with a political desire. Scientists are a strange bunch; they just want to understand what’s happening.

4. Sea level rise (update)

The dynamics of sea level rise are very uncertain, but very important since they determine to a great extent the speed of sea level rise, which in turn strongly affects the risk posed to society. (Thanks to Heiko for bringing this omission to my attention) The magnitude and speed of sea level rise are amongst the most uncertain,  yet also the potentially most dangerous effects of climate change. I have written more about sea level rise before.

The NIPCC report: don’t be fooled

June 13, 2009

(Nederlandse samenvatting hier)              (For a sneak preview, see the bottom line below)

The new ammunition put forward by “skeptics” seems to be the Heartland InstitutesNIPCC report 2009 (“Climate change reconsidered”). It is made to resemble, at least in format and in name, the IPCC report. According to Dutch “skeptic” (and contributor to the report) Hans Labohm it completely shatters the AGW (anthropogenic global warming) theory (e.g. here, in Dutch). That’s a very bold assertion, which should be backed up by very strong evidence for it to be taken seriously. Let’s take a look at the executive summary…

Second opinion
The preface starts as follows: “Before facing major surgery, wouldn’t you want a second opinion?”
Now that’s funny. I recently described the IPCC process using the same analogy: If you get a second opinion on your health condition, and it confirms what your specialist said in the first place, your trust in the diagnosis probably increases. Now imagine that you collect the interpretations of medical professionals all over the world, and by and large they their conclusions converge to the same broad picture. This happens to be how the IPCC comes to its conclusions.
Their opening statement is actually a strong argument for going with the consensus position on a complex topic. Yet they use it to argue in the opposite direction; very peculiar.

It continues: “When a nation faces an important decision that risks its economic future, or perhaps the fate of the ecology, it should do the same.” (i.e. getting a second opinion)
Huh? Risking our economic future? If they’re talking about the costs of emission reduction, they are seriously exaggerating. Who is being alarmist here? There will be winners and losers, yes, but that’s something entirely different. Everybody has a choice to join the winners or the losers. Different from the horse races, it’s easy this time to predict who (in the long run) will be the winners and who will be the losers. Take your pick.

The usual stuff
The previous NIPCC report has already been commented on by RealClimate, and it doesn’t seem like there’s much news under the sun this time. The same old and tired arguments feature in the current report. The RealClimate article has many links that debunk the various talking points, and I’m not going to repeat all of them here. A presentation from the lead author, Fred Singer, has been briefly discussed at RealClimate as well. It’s a good example of yet another groundhog day. For those who have followed the staged ‘climate debate’, the list of authors is revealing: Many of the usual suspects, with a history so to speak.

There are the usual, to be expected arguments, like that it’s all the sun’s fault. And logical fallacies, like ‘the climate changed before without human activity being involved, so therefore it must be natural now as well’. Try that line of argument in a court of law against a pyromaniac, by saying that forest fires have always happened naturally. It won’t fly, and it reveals that this report is not about science. The good thing is, with such erroneous lines of reasoning, no specialized knowledge is needed to see that.

Degrees of uncertainty
What I didn’t expect, however, was to see otherwise interesting research be put in a context as if it somehow “falsifies the AGW theory”. In many cases, it hardly has any relevance to the attribution of current climate change, or to future projections.

Ironically, their main argument against climate modeling is its associated uncertainty (mistaking it for knowing nothing, and ignoring that uncertainty goes both ways). That doesn’t stop them from putting forward hypothetical feedbacks that have no evidence whatsoever of operating on a globally significant scale. By the way, climate modeling is mocked in the report as merely being “the opinions of scientists transformed by mathematics and obscured by complex writing”. Doesn’t sound like they know what a climate model really is.

The report goes on to describe many hypothetical feedbacks in the climate system. Of course, they are all negative: They counteract the initial warming, independent of the cause for the warming. Their combined effect, is the hope, should be evidence that the climate sensitivity is an order of magnitude (!) smaller than the commonly accepted range (between 1.5 and 4.5 degrees C for a doubling of CO2). Not just 50%, no, a factor of 10, I kid you not. My alarm bells go off. Let’s see what the implications of such low climate sensitivity would be. Any climate forcing (whether natural or human induced) would be so strongly damped as to hardly have any effect on global temperatures. But then how come the globe is warming, and has warmed and cooled in the past? A logical consequence of their theory (negligible climate sensitivity) is that it’s hardly possible for the earth’s climate to change. Indeed, there is no physics-based climate model that can satisfactorily model both the current and past climates with such low climate sensitivity.

Many of the proposed feedbacks involve the cooling effects of aerosols. They suggest that these cooling effects are larger than reported by the IPCC. That is contradicted by climate models providing a very decent match to the observed cooling following a major volcanic eruption (emitting sulfate aerosol in the stratosphere). Moreover, some have argued that a strong aerosol radiative forcing means that the climate sensitivity has to be large in order to still be able to explain the temperature trend of the last 100 years, so they seem to be shooting in their own foot.

They come up with all kinds of hypothetical feedback mechanisms involving more natural aerosol emissions in response to global warming: Dimethylsulfide from marine phytoplankton (although a very intriguing possibility, this has never been confirmed to be a significant feedback mechanism, and there is ample evidence to the contrary, which is omitted from the report), biological aerosols (idem), carbonyl sulfide (idem), nitrous oxide (idem), and iodocompounds (idem), about which they write the following:
“Iodocompounds—created by marine algae— function as cloud condensation nuclei, which help create new clouds that reflect more incoming solar radiation back to space and thereby cool the planet.”
Nou breekt mijn klomp (“Now my clogg breaks”), as I would say in Dutch. This route to atmospheric particle formation may be important at coastal sites with exposed seaweed, but its global importance is questionable to say the very least; at present it could best be considered an interesting thought experiment. Moreover, freshly nucleated particles have to grow by about a factor of 100,000 in mass before they start affecting climate, and a lot can happen to them before they reach the necessary size.

All very interesting research topics, but to claim that they are somehow evidence for negligible climate sensitivity is an extreme example of over-interpretation. In these active areas of research, where no firm conclusions have been reached yet on global significance, they selectively cite only those articles that they can somehow spin to support their desired conclusion. I feel that I’ve read enough of this report to know what it’s worth.

Bottom line
This report exhumes a very strong and unfounded faith in negative feedbacks from nature, which are hypothetical with sometimes sketchy, often contradictory, and sometimes no evidence of actually operating at a globally significant scale. This highlights an inconsistent view of uncertainty, and an unwillingness to weigh the evidence: “If it causes cooling, the uncertainty (or lack of evidence) doesn’t matter; if it causes warming, it’s too uncertain (and no evidence strong enough) to matter”.

How would you know?
Let’s apply some of my own recommendations for non-specialists on judging sources:
– The report clearly misses the forest for the trees.
– It gives a hidden argument for going with the consensus (“second opinion”), but somehow twists that around.
– It’s characterization of the IPCC process has the smell of a conspiracy to it and is full of strawmen arguments.
– To their credit (and my surprise), I couldn’t find any obvious confusion of timescales, such as confusing weather and climate.
– It contains some embarrassing mistakes in basic logic.
– The two way cause-effect relationship between temperature and CO2 is not properly recognized.
– Their strong claim of shaking the foundations of climate science is extremely unlikely; They don’t provide compelling evidence for such an extraordinary claim; They vastly overestimate the likelihood of cooling effects (feedbacks), and underestimate, deny or ignore warming effects.
– They grossly exaggerate the economic risks of emission reduction, and downplay the risk of unmitigated climate change.
– Some of the authors have historical credentials in a relevant discipline, more than a few have not. The list of signatories at the end is very thin on relevant expertise.
– The Heartland Institute is a conservative think-tank and not a reliable source of scientific information.

Guest post at Realclimate on aerosol nucleation and climate

April 24, 2009

Guess it’s a little late notice, but I have a guest post at RealClimate on the potential effects of aerosol nucleation and cosmic rays on climate. For the whole article please see RealClimate. The bottom line is as follows.


Freshly nucleated particles have to grow by about a factor of 100,000 in mass before they can effectively scatter solar radiation or be activated into a cloud droplet (and thus affect climate). They have about 1-2 weeks to do this (the average residence time in the atmosphere), but a large fraction will be scavenged by bigger particles beforehand. What fraction of nucleated particles survives to then interact with the radiative budget depends on many factors, notably the amount of condensable vapor (leading to growth of the new particles) and the amount of pre-existing particles (acting as a sink for the vapor as well as for the small particles). Model-based estimates of the effect of boundary layer nucleation on the concentration of cloud condensation nuclei (CCN) range between 3 and 20%. However, our knowledge of nucleation rates is still severely limited, which hampers an accurate assessment of its potential climate effects. Likewise, the potential effects of galactic cosmic rays (GCR) can only be very crudely estimated. A recent study found that a change in GCR intensity, as is typically observed over an 11 year solar cycle, could, at maximum, cause a change of 0.1% in the number of CCN. This is likely to be far too small to make noticeable changes in cloud properties.

Aerosols, clouds and climate

April 16, 2009


Clouds can cool or warm the underlying surface. They reflect some of the incoming shortwave solar radiation back to space (cooling). They also absorb infrared radiation and re-radiate it to all directions, with a wavelength (and thus energy) that depends on cloud temperature (and thus height). The latter is a quasi-greenhouse effect (warming).


The colder (=higher) the cloud, the less energy will be re-radiated to space, so the more the atmosphere will warm. (The same happens with greenhouse gases: the higher re-radiation into space takes place, the less energy is lost, and the more the atmosphere will warm. In that sense, the sky is the limit in terms of a warming effect of CO2: no matter if the absorption bands at the surface would be saturated, it would just move up the re-radiation to a higher altitude, thereby still adding to the warming effect.)

At night, there’s no solar radiation to reflect, and so clouds mainly warm the surface. However, the net effect of clouds is a cooling.


The basic response of liquid water clouds to an increased CCN population is expected to be a change in the droplet size spectrum: Assuming the liquid water content remains relatively unchanged, more cloud droplets will form, having a smaller average size. Absorption depends on particle volume, whereas reflection depends on its geometric cross-section, and thus the ratio of reflection over absorption of the cloud increases as the droplet size decreases. Dependent on the altitude of the cloud droplets, and the albedo of the underlying surface, this will usually result in cooling. A smaller average droplet size may increase cloud lifetime, via the suppression of rain. Cloud cover could be susceptible to CCN concentration, especially in remote areas. In reality, different feedback mechanisms involving micro-physics and radiative transfer make the picture more complicated. For example, the liquid water content of a cloud is highly variable and not much is known about the response of liquid water content to increasing CCN concentrations.


The effects mentioned here concern only “warm” clouds; much less is known about the response of ice clouds. If through human activities the concentration of ice nuclei has also decreased (e.g. black carbon), there could be a “glaciation indirect effect”: more ice nuclei cause a cloud to rain out quicker, and may thus lead to a decrease in cloud cover (i.e. opposite to the “warm” indirect effect). This is still very hypothetical, however.


From local measurements of ship tracks, relations were found between particle emissions, cloud droplet diameter and cloud albedo. Still, more simultaneous measurements of aerosol particles, cloud droplets and radiative properties are necessary to gain a more accurate understanding of climate change. There are strong indications that besides inorganic salts, some organic compounds may also contribute to the CCN activity of an aerosol. Ultimately though, size may matter more than chemistry for the cloud nucleating ability of aerosols.

The atmospheric aerosol

April 13, 2009


This serves as an introduction to atmospheric aerosols to accompany my guest post on RealClimate. I’ll be discussing more about aerosols in the near future, including their potential role in GeoEngineering.



Atmospheric aerosol refers to the liquid or solid particles suspended in the air. They can be emitted directly into the atmosphere (primary aerosol), or they can be formed in the atmosphere by gas-to-particle conversion (secondary aerosol). The oxidation of precursors, such as SO2, NO2, and suitable organic compounds, can produce compounds with very low vapor pressures. These can condense to form a new particle, often in combination with other species, such as water vapor and NH3, a process called homogeneous nucleation. They can also condense on existing particles and contribute to them growing in size, referred to as condensation. Furthermore, aerosol particles can undergo evaporation, deposition, and coagulation, whereby two particles collide to form one larger particle. All these processes influence the evolution of the aerosol population in time.



Atmospheric aerosol particles span several orders of magnitude in diameter, from a few nanometer to hundreds of micrometer. Cloud droplets and raindrops can grow even larger, but are usually treated separately. Though not all particles are spherical, they are usually characterized according to their equivalent spherical diameter. Most of the properties, and thus effects, of particles depend on their size.



The tropospheric number concentration of aerosol particles ranges from several tens or hundreds per cubic centimeter of air in remote locations, to more than hundred thousand a million per cubic centimeter in polluted environments or after a strong nucleation event. The number concentration as a function of particle size is described by a size distribution. An important feature of atmospheric aerosol size distributions is their multimodal character: commonly found are the aitken mode, the accumulation mode, and the coarse mode. The nucleation mode consists of freshly nucleated particles and usually occupies diameters below a few tens of nanometers. The accumulation mode can vary in size between 50 nanometer and 1 micrometer, and its name comes from the fact that particles tend to accumulate in this size range, because they bear the longest lifetime (~2 weeks): smaller particles are efficiently removed by deposition and coagulation, and larger particles are efficiently removed by gravitational settling. The coarse mode usually refers to particles larger than 1 micrometer diameter. Sometimes an Aitken mode is defined as the mode that could exist between the nucleation and accumulation mode.

The number concentration of the atmospheric aerosol is usually dominated by sub-micrometer particles, while the total particle surface area and volume (and thus mass) are more influenced by larger particles. The exact shape of the size distribution depends on the environment, as it reflects the sources, sinks, and transformations of the particles.



Atmospheric particles have both natural and anthropogenic sources.

Examples of natural (primary) particle emissions are the shattering of sea-spray into tiny droplets, that evaporate before gravitating back towards the water surface, particles caused by forest fires, soil dust, pollens, etc. Examples of natural emissions of precursors to (secondary) aerosol are SO2 and H2S from volcanoes, dimethylsulfide (DMS) from phytoplankton, volatile organic compounds such as monoterpenes from vegetation, etc.

Examples of anthropogenic (primary) particle sources are vehicular emissions, industrial emissions, and some natural sources that are significantly enhanced by human activities, such as biomass burning and soil dust (due to e.g. erosion). Anthropogenic precursors to (secondary) aerosol are emitted by industry, burning of fossil fuels, usage of solvents, etc. Primary emissions contribute relatively more to the aerosol volume (because they’re often large), while gas to particle conversion (secondary aerosol) contributes relatively more to the aerosol number.


Cloud formation

Aerosol particles are necessary for clouds to form: Dependent on their size, composition and hygroscopic properties, they can act as cloud condensation nuclei (CCN) on which the available water vapor condenses, when it reaches a certain critical super-saturation of a few tenths of a percent. This process is called (cloud droplet) activation. Without these substrates to condense on, the relative humidity would have to exceed several hundred percent in order for the water vapor to overcome the Kelvin effect and homogeneously nucleate into droplets.

Similarly, some aerosol particles act as ice nuclei on which water can freeze into an ice crystal. Aerosol particles also provide a surface for specific heterogeneous reactions to take place, and thus affect the chemistry of the atmosphere.


Health effects

Atmospheric aerosols have potentially far reaching effects on health, ecosystems, and climate. Aerosol particles are a major component of smog, and act to reduce visibility. Their health effects are mainly due to their adverse effects on the respiratory function, and some also act as irritants for the eyes. Small particles can enter deep into the lungs, reaching the alveoli, where the transfer of O2 and CO2 takes place. H2SO4 is often a major component of these small particles, and dependent on their history other toxic compounds, such as heavy metals and PAH’s, could also have partitioned into the particle phase. Due to their low volatility, these species would otherwise not have reached that deep into the lungs. Atmospheric particles have been found to play a major role in excess mortality in epidemiological studies in several cities. Through their acid composition, aerosol particles also contribute to acidification of ecosystems.


Climate effects

Atmospheric particles contribute to climate change directly by scattering or, in the case of black carbon, absorbing sunlight, and indirectly by changing the radiative properties and lifetime of clouds. Both direct and indirect radiative forcing are influenced by particle size, composition and relative humidity. These processes are responsible for the largest amount of uncertainty in assessing climate change. The upper end of the estimated (direct and indirect) negative forcing of atmospheric aerosols could -at least on a global and annual average- balance the positive forcing of CO2. This does not necessarily mean that on a regional scale the opposing forcing mechanisms will balance each other. Aerosols are not homogeneously mixed throughout the globe, whereas the atmospheric lifetime of most greenhouse gases is much larger, so they will get distributed equally over the globe, despite large spatial differences in emissions.



The short lifetime (days to weeks) of aerosols is an important reason for the uncertainty in their role in climate change. It causes their concentration to be highly variable in time and space, and it’s hard to even know what the global concentration is, let alone what it was in the pre-industrial era. Add to that their variability in size and chemical composition, and the poorly understood role of clouds, and it’s clear that the uncertainty in aerosol radiative forcing will remain a steady feature of climate science for some time to come.


Other introductory explanations about aerosols are here, here, and here (German) and good presentation slides here.


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