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

(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.”

Rebuttal

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

Rebuttal

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.

Conclusion

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.

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The NIPCC report: don’t be fooled

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

The new ammunition put forward by “skeptics” seems to be the Heartland Institutes’ NIPCC 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.

Risk
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.

Feedbacks
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.

Aerosols
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.

Het NIPCC rapport: Laat je niks wijsmaken

(English version here)

Het nieuwe NIPCC rapport (2009) is uit, en Nederlands bekendste “skepticus” (en co-auteur) Hans Labohm zwaait er al mee als zijnde de doodsteek voor de anthropogene klimaatverandering (AGW). Het is opgesteld door een reeks oude bekenden uit de “skeptische” wereld, aangevoerd door Fred Singer, en uitgegeven door een conservatieve denktank, het Heartland Instituut. Volgens Labohm wordt er niets heel gelaten van de AGW theorie. Dat is nogal een sterke uitlating, die door een sterke bewijsvoering gestoeld moet worden. Laten we eens kijken…

Waar komt het op neer?
Het rapport (niet te verwarren met het IPCC rapport) ademt een grenzeloos en ongefundeerd vertrouwen in negatieve feedbacks (terugkoppelingsmechanismen) van de natuur. Het bestaan van deze feedbacks is volkomen hypothetisch met vaak conflicterende en soms nauwelijks of geen aanwijzingen dat ze op globale schaal werkzaam zijn. De relatieve onzekerheid van verschillende factoren die bij het klimaat een rol (zouden kunnen) spelen wordt volkomen inconsistent benaderd: Als het afkoeling veroorzaakt, dan is geen onzekerheid te groot (en doet de afwezigheid of zwakte van enige bewijsvoering er niet toe); Als het daarentegen opwarming veroorzaakt, dan is de onzekerheid bij voorbaat te groot (en is geen berg van aanwijzingen hoog genoeg).
Daarnaast passeren een hoop van de veelgehoorde –en al even vaak weerlegde– argumenten de revue. Oud nieuws, en bij lange na niet afdoende om de sterke uitlating zoals door Labohm gedaan te staven. (De Engelse versie geeft een uitgebreidere discussie, met name over feedbacks en aerosols.)

Laten we eens kijken hoe het rapport zich verhoudt tot mijn lijst met tips om het kaf van het koren te scheiden in de informatiejungle:
– Het rapport ziet duidelijk door de bommen het bos niet meer; de context ontbreekt of wordt verdraaid.
– Het is onwaarschijnlijk dat de gangbare klimaatwetenschap er zo ver naast zit als dit rapport beweert; De sterke beschuldiging aan het adres van de gangbare klimaatwetenschap wordt niet door een afdoende sterke bewijsvoering ondersteund; De waarschijnlijkheid van afkoelende effecten (feedbacks) wordt schromelijk overdreven, terwijl opwarmende effecten worden gebagatelliseerd, ontkend, of verzwegen.
– De economische risico’s van emissiereductie worden schromelijk overdreven, terwijl de risico’s van ongelimiteerde klimaatverandering gebagatelliseerd worden.
– Het geeft een verkapte argumentatie voor de consensuspositie in het algemeen (namelijk door de belangrijkheid van een “second opinion” te onderstrepen), maar verdraait dat dan in de door hun bevoorrechte richting.
– De beschrijving van het IPCC proces heeft de zweem van een complottheorie en is vol stropop argumenten.
– Een deel van de auteurs heeft (of had) relevante expertise; een deel ook niet. In de lijst met ondertekenaars aan het eind van het rapport is relevante expertise dun gezaaid.
– Het Heartland Instituut is een conservatieve denktank met een afkeer van overheidsingrijpen, en geen betrouwbare bron van wetenschappelijke informatie.
– Tijdsschalen worden –voor zover ik heb gezien- niet incorrect gehanteerd. Bijna verbazingwekkend, gezien het feit dat Labohm continue weer en klimaat door elkaar haalt op diverse fora.
– Interne consistentie ontbreekt, bijvoorbeeld wat betreft de mate van onzekerheid en de gevolgen van een overschatting van de aerosolkoeling (die juist op een hogere klimaatgevoeligheid duidt).
– Sommige redeneringen kloppen logisch gezien niet. Zo wil het feit dat het klimaat in het verre verleden ook aan verandering onderhevig was helemaal niet zeggen dat menselijke activiteit er nu ook niets mee van doen heeft. Een pyromaan kan zichzelf ook niet zomaar vrij pleiten door te wijzen op het feit dat bosbranden altijd al van nature hebben plaatsgevonden.

Climate “skeptics” out of touch with reality

 

This is a reply to a Dutch article which quotes Fred Singer as claiming that CO2 has hardly any effect on climate; that climate change is due to the sun; that global warming stopped in 1998; and that sealevel won’t rise faster than it has done in the past thousands of years. I wrote the following reply.
(Nederlandse versie hier.)

 

The premise that CO2 has hardly any influence on climate, as argued by Fred Singer, can not be sustained. These so-called “critical” opinions may fare well in the media, but in the current scientific debate they don’t play any role. They have been proven wrong long ago.

 

The current media don’t pay much attention anymore to claims that smoking wouldn’t have adverse health effects (something Singer argued in the past). Or to claims that CFC’s wouldn’t destroy the ozone layer (Singer also argued that, until just before the Nobel Prize for Chemistry was awarded to those who showed that they do). It’s about time that the media also start portraying climate change in line with current scientific knowledge.

 

CO2 and global warming: hypothesis or fact?

It has been known for over 100 years that CO2 absorbs infrared radiation, and thus has a warming effect. This effect causes the Earth to have a hospitable climate, and Venus to be boiling hot. Of course there are uncertainties in climate science, but our knowledge has definitely progressed a few stations beyond what Singer and a handful of other (ex-) scientists claim.

 

The observed change in temperature over the past 100 years is reasonably well reproduced by climate models. The past 10 years do not change that picture, although it has to be noted that temperature changes over a decade (or less) are heavily influenced by the changeable weather, large volcanoes and the El Niño (1998) / La Niña (2007) cycle.

 

There are a few things that are now beyond reasonable doubt:

–     Global climate has warmed over the past ~100 years, with the largest increase in temperature from about 1975.

–     The concentration of CO2 and other greenhouse gases has increased due to human activity.

–     Basic physics and numerous observations point to a causal relationship.

–     Further increase in greenhouse gas emissions will lead to further warming.

 

Other factors

Of course there are other factors besides greenhouse gases that can influence the climate, such as aerosols, land use, solar activity, etc. The sun is often put forward by “skeptics” as being the main reason for the current warming. But although the increase in solar activity in the first half of the 20^th century indeed contributed to the warming, the solar activity (and related quantities such as cosmic rays) has remained more or less constant since the sixties. The strong increase in temperature from 1975 onwards can therefore not be explained by varying output of the sun.

 

An alternative explanation of current climate change can not just put aside the accumulated body of knowledge and observations; they have to be integrated into a total picture. The infrared absorbing ability of greenhouse gases can not just be denied by pointing to the sun. You don’t deny gravity either when you see a bird fly.

 

Sea level rise

Sea level is rising faster than it has in the past (before 1900), and also faster than predicted by climate models. The possibility of (mechanically) enhanced melt of land ice is an active research area where knowledge is still lacking. But this uncertainty should not be a comfort, because the risks of a substantial sea level rise are large. Many of the large population centers are located in the vicinity of the sea at only small elevation.

 

Discussion

Singers unfounded opinion about CO2 is not relevant for the discussion about energy options, just as his opinion about smoking is not relevant for the discussion of anti-smoking laws. About policy options, for example concerning energy, there will always be different opinions. And different opinions should be heard. But please leave out scientifically proven untruths. They don’t contribute to the debate. To the contrary.