According to this Science Daily summary of a recent paper published in Nature and the abstract at Nature, a recent result indicates that cosmic rays play a larger role in cloud formation than was previously credited.

This has been claimed to "convincingly validate the Danes’ groundbreaking theory [that cosmic rays and the sun hold the key to the global warming debate]."

Does it? (Or, more reasonably, how "big a deal" are these results? If the findings are duplicated, would they imply dramatic changes in the models that predict global warming?)

Edit: To be clear, the claim I am interested in is this studies' import, not the legitimacy of the study itself. The claim, as I see it, is that this study overturns an assumption that is known to be foundational in the field; if the study is, in fact, strictly incremental and whose import is only parseable by experts in the field, that would be an answer. ( I suppose I could change the question to "... do results overturn foundational assumptions of ...")

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    I don't know whether this is appropriate here. Research level climate science?
    – Sklivvz
    Commented Aug 31, 2011 at 8:50
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    @Sklivvz -- Everything related to AGW is research level right now. I don't disagree with you, but if that's our criteria, we have a lot of questions to go back and close. Commented Aug 31, 2011 at 12:02
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    @Rus: that's just factually incorrect.
    – Sklivvz
    Commented Aug 31, 2011 at 17:41
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    @Russell Oh please keep the discussion serious. I have explicitly said that I cannot comment on the study’s relation to climate, merely on the interpretation that Larry linked to. And nowhere did I say that the sun doesn’t affect temperature. That would be ridiculous. But the theory that variations in the sun are responsible for the global climate change is demonstrably false, variations in the sun’s activity cannot alone account for global long-term upwards trends in our climate, there’s a complete mismatch. Finally, why didn’t you provide a link to that CERN statement if it was relevant? Commented Aug 31, 2011 at 20:49
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    @rus: i meant, it's factually incorrect that all AGW is all research level. It's clearly not.
    – Sklivvz
    Commented Sep 1, 2011 at 7:27

5 Answers 5


Low-level clouds form around particles called "cloud condensation nuclei" (CCN). CCNs, themselves form around aerosols. According to the the CLOUD paper (Kirkby et. al), cosmic rays increase aerosol nucleation. If increasing aerosol nucleation increases CCN formation, and CCN formation increases cloud formation, then increasing cosmic rays would increase cloud cover, cooling the Earth.

Some of the media reporting around this story, such as the linked article in the Financial Post, claim that the paper says that cosmic rays are the causing of global warming. To get from this paper to this result, it would require:

  1. That an increase in aerosol nucleation increases CCNs
  2. That an increase in CCNs increases low-level cloud cover
  3. That there be a downwards trend in cosmic rays.

There is no such trend in cosmic rays.

There is also another hitch. It's not yet clear that aerosol nucleation is a limiting factor for the formation of CCNs. Snow-Kropla et. al. (2011) found that the difference in concentration of CCNs between the solar minimum and maximum (which are the high and low points for cosmic rays, respectively) was less than 0.2%.

The Kirkby et. al. paper does provide part of the evidence you'd need for establishing a link between cosmic rays and global temperatures, but you'd also need evidence of a stronger link between nucleation and cloud condensation nuclei. So I'd definitely say it is relevant to climate research - but it's relevance is currently in the technical literature. Meanwhile, without a negative change in the rate of cosmic rays being received, they cannot be responsible for the observed change in temperatures. observed temperature change

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    Note: none of this is against the paper itself. The paper's conclusions are that cosmic rays trigger aerosol nucleation, and none of what I have cited here disputes that. This content here concerns the link from aerosol nucleation to temperatures, which is not claimed in the paper or the CERN press release (but has been claimed by, say, the opinion columnist linked in the question).
    – Jivlain
    Commented Sep 1, 2011 at 5:08
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    They're only looking at 1nm particles, whereas you need >100nm particles to trigger cloud formation. They find the observed increase in 1nm particles, but didn't attempt to study whether this increased the formation of >100nm CCNs, which are large enough for water to condense around. I'm sure it'll be an interesting study in the future though. Here's a interview with the first author: media.nature.com/download/nature/nature/podcast/v476/n7361/… (starts from 21:45), which I found interesting.
    – Jivlain
    Commented Sep 3, 2011 at 11:46
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    @elliotsvensson: It's right there in his answer, including links.
    – DevSolar
    Commented Feb 1, 2019 at 9:23
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    @elliot: ...which you are making up from whole cloth...?!?
    – DevSolar
    Commented Feb 4, 2019 at 16:29
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    @elliotsvensson: That is in no way or form a "seeming-decrease in climate sensitivity to CO2". Don't make up claims from information you haven't understood.
    – DevSolar
    Commented Feb 4, 2019 at 17:10

"Do the recent CLOUD results have significant implications for global warming?"


Global atmospheric particle formation from CERN CLOUD measurements

Eimear M. Dunne, Hamish Gordon, Andreas Kürten, João Almeida, Jonathan Duplissy, Christina Williamson, Ismael K. Ortega, Kirsty J. Pringle, Alexey Adamov, Urs Baltensperger, Peter Barmet, Francois Benduhn, Federico Bianchi, Martin Breitenlechner, Antony Clarke, Joachim Curtius, Josef Dommen, Neil M. Donahue, Sebastian Ehrhart, Richard C. Flagan, Alessandro Franchin, Roberto Guida, Jani Hakala, Armin Hanse, Martin Heinritzi, Tuija Jokinen, Juha Kangasluoma, Jasper Kirkby, Markku Kulmala, Agnieszka Kupc, Michael J. Lawler, Katrianne Lehtipalo, Vladimir Makhmutov, Graham Mann, Serge Mathot, Joonas Merikanto, Pasi Miettinen, Athanasios Nenes, Antti Onnela, Alexandru Rap, Carly L. S. Reddington, Francesco Riccobono, Nigel A. D. Richards, Matti P. Rissanen, Linda Rondo, Nina Sarnela, Siegfried Schobesberger, Kamalika Sengupta, Mario Simon, Mikko Sipilä, James N. Smith, Yuri Stozkhov, Antonio Tomé, Jasmin Tröst, Paul E. Wagner, Daniela Wimmer, Paul M. Winkler, Douglas R. Worsnop, Kenneth S. Carslaw

Science 27 Oct 2016: DOI: 10.1126/science.aaf2649


Fundamental questions remain about the origin of newly formed atmospheric aerosol particles because data from laboratory measurements have been insufficient to build global models. In contrast, gas-phase chemistry models have been based on laboratory kinetics measurements for decades. Here we build a global model of aerosol formation using extensive laboratory-measured nucleation rates involving sulfuric acid, ammonia, ions and organic compounds. The simulations and a comparison with atmospheric observations show that nearly all nucleation throughout the present-day atmosphere involves ammonia or biogenic organic compounds in addition to sulfuric acid. A significant fraction of nucleation involves ions, but the relatively weak dependence on ion concentrations indicates that for the processes studied variations in cosmic ray intensity do not significantly affect climate via nucleation in the present-day atmosphere.

[emphasis mine]

I suspect this received a lot of attention on climate skeptic blogs ;o)

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    @elliotsvensson The question is "Do the recent CLOUD results have SIGNIFICANT implications for global warming?" the scientists say " do not SIGNIFICANTLY affect climate". In what way is my answer more confident than the scientists?
    – user18604
    Commented Jan 30, 2019 at 9:40
  • OK, you were right. \-1. But I'm going to start bringing up the more recent CLOUD results... Commented Jan 30, 2019 at 14:42
  • @elliotsvensson Right, so I give a quote from a paper from CLOUD that explicitly answers the question almost as exactly as posed, and the best you can bring yourself to do is give it 0 rather than -1? LOL.
    – user18604
    Commented Jan 31, 2019 at 7:34
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    The system won't let me undo my down-vote until the answer changes somehow. I proposed a neutral edit to enable this, but that didn't come through... sorry. Commented Jan 31, 2019 at 16:53
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    @elliotsvensson LOL, that really is pathetic.
    – user18604
    Commented Feb 1, 2019 at 8:59

Bad Astronomer (Phil Plait) discusses this in a recent blog post of his. In short, while cosmic rays do increase the amount of aerosol particles, it is not significant enough to cause cloud formation. The post can be found here.

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    Discovery magazine is totally a more reputable source than CERN. I can't believe we didn't go there first. Commented Sep 2, 2011 at 19:13
  • The Discovery blog post utilizes CERN as a source. Takes the information from a PhysOrg article. Commented Sep 2, 2011 at 22:38
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    @Russell Perhaps not more reputable, but we’ve already seen on Skeptics that institution press releases are often worthless (this CERN press release looks very good, though). Discovery Magazine, on the other hand, in general has a very high standard of reporting and cites relevant sources for most claims. So in general I would actually say that DM is more valuable than a press release. Commented Sep 3, 2011 at 11:03

It's premature to draw any conclusion (and i voted to close), as this research has just been released. However if you must draw a conclusion, CERN is a reliable source, has confirmed the research, and as they generally support AGW theory can be said to be not biased against AGW.

According to CERN

[I]t is clear that the treatment of aerosol formation in climate models will need to be substantially revised, since all models assume that nucleation is caused by these vapours and water alone.

It's up to you to draw a conclusion, but I do not logically see how one can hold the position that climate models need to be substantially revised, but this will have no impact on the results .... unless the opinions, I mean results, are predetermined.

Also of note here is that CERN isn't saying that it will have a significant effect, or what direction (in regards of supporting AGW) that effect will take. All they are saying is that things need to be substantially revised. Substantial revision = significant implications. What those implications are will not be known until more research is performed. What was thought to be a "known" just became a very large "unknown" and that shakes things up.

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    However, according to the very same press release you quoted, "However, it is premature to conclude that cosmic rays have a significant influence on climate until the additional nucleating vapours have been identified, their ion enhancement measured, and the ultimate effects on clouds have been confirmed." There are a number of assumptions that have to be made to get from these results to "cosmic rays affect the climate".
    – Jivlain
    Commented Sep 2, 2011 at 5:25
  • Yes, it is premature to conclude that. I would hope you understand the difference between a significant impact on climate modeling and a significant impact on climate. I also make it extremely clear in my post that it's too early to draw conclusions. But again, if you wish conclude in advance that they will have the same results despite needing substantial revision, that is your prerogative. Otherwise we should all be waiting to see what new models, which take this into account, reveal. Commented Sep 2, 2011 at 19:13
  • In principle, yes, the models would have to take into account the results showing the lack of a relationship between cosmic rays and cloud condensation nuclei (CCN) (this paper only considers the relationship from CRs to aerosol nucleation, which is a stage in CCN formation). So one would have to figure that it'll make a significant difference in something which doesn't significantly affect anything else.
    – Jivlain
    Commented Sep 2, 2011 at 20:57
  • "So one would have to figure that it'll make a significant difference in something which doesn't significantly affect anything else" -- Welcome to chaos theory. Commented Sep 5, 2011 at 15:57
  • -1: seven years have passed and it's no longer premature. Commented Jan 29, 2019 at 17:03


TL;DR - Several experiments at the CLOUD lab show that cosmic and human contributions are inadvertently reducing the impact of CO2 emissions.

  1. Hydrocarbon emissions - Experiments show that sulfuric acid in the atmosphere increases cloud cover much more than we thought before. Therefore the use of cleaner fuels may well be warming the earth.

  2. Cosmic rays - Experiments show that many places on the earth's surface are subject to the formation of clouds by cosmic rays. Therefore astronomic cycles like the sunspot cycles must not be ignored when making climate models.

    Bonus: Low tide - Researchers know that algae that dries out during low tide releases chemicals that cause the formation of clouds. Therefore the moon's cycles (such as supermoons) must not be ignored when making climate models.

Long Form

Back in August 2011, a study conducted at CERN in Switzerland published its first findings, which found that...

ionisation from cosmic rays significantly enhances aerosol formation. Precise measurements such as these are important in achieving a quantitative understanding of cloud formation, and will contribute to a better assessment of the effects of clouds in climate models.

The IPCC 2013 report repeated this concern:

Changing cloud amount of properties modify the Earth's albedo and therefore affect climate. It has been hypothesized that cosmic ray flux create atmospheric ions which facilitates aerosol nucleation and new particle formation with a further impact on cloud formation (Dickinson 1975; Kirkby, 2007). ... However, there is high confidence (medium evidence and high agreement) that the cosmic ray-ionization mechanism is too weak to influence global concentrations of cloud condensation nuclei or their change over the last century or during a SC in a climatically significant way...

We asked at the time:

how "big a deal" are these results? If the findings are duplicated, would they imply dramatic changes in the models that predict global warming?

The answers that came were in the "wait and see" category, since there was still too little that we were sure about. Now, after seven years have passed (and the CLOUD laboratory has not slowed down) let's see what the scientists are saying:

Sulfuric Acid

New particle formation in the sulfuric acid–dimethylamine–watersystem: reevaluation of CLOUD chamber measurements and comparison to an aerosol nucleation and growth model

Andreas Kürten & Chenxi Li et al, Jan 23, 2018

New particle formation rates [i.e. cloud cover effects] from CLOUD chamber measurements for the sulfuric acid-DMA-water system were re-analyzed. It was found that the previously published rates by Almeida et al. (2013) underestimate the NPF rates [i.e. cloud cover effects] by up to a factor of ~50 at high sulfuric acid concentrations (~1 X 10^7 cm^-3).


... Only one study has so far reported sulfuric acid–amine nucleation (Zhao et al., 2011). The nucleation of sulfuric acid–amines could, however, occur more often than currently thought.

This correction means that previous estimates of the cloud-forming effects of sulfuric acid may have been way too low, and that it would have been incorrect to dismiss such effects from major climate models. Another recent CLOUD study has this to say:

Measurement–model comparison of stabilized Criegee intermediate and highly oxygenated molecule production in the CLOUD chamber

Nina Sarnela & Tuija Jokinen et al, Feb 19, 2018

The results indicate that the CLOUD experiments on α-pinene ozonolysis support the recently published chemistry of HOM and sCI [i.e. stabilized Criegee intermediates] formation, thus making the experimentally determined yield and loss terms more reliable for modelling and theoretical use.

So what do HOMs and sCIs tell us? Well:

Elucidating the molecular mechanisms of Criegee-amine chemistry in the gas phase and aqueous surface environments

Manoj Kumar & Joseph S. Francisco, Oct 24, 2018

Though the Criegee-ammonia reactions have been found to be tropospherically insignificant, the facile nature of Criegee-dimethylamine reactions suggests that these chemistries may play a role in the new particle forming events [i.e. cloud cover effects] under certain conditions [such as in California's central valley, New York City and Paris] and thus, need to be updated in the existing atmospheric models.

(emphasis by Elliot)

So since Kumar et al (2018) was published, there have been about four months during which climate scientists could have been updating their existing atmospheric models to account for a newly-verified source of sulfuric acid cloud-forming events, which as Kürten et al (2018).

Cosmic Rays

Ion-induced nucleation of pure biogenic particles

Jasper Kirkby, Jonathan Duplissy & Kamalika Sengupta et al, May 26, 2016

Atmospheric aerosols and their effect on clouds are thought to be important for anthropogenic radiative forcing of the climate, yet remain poorly understood. ... We find that ions from Galactic cosmic rays increase the nucleation rate by one to two orders of magnitude compared with neutral nucleation. Our experimental findings are supported by quantum chemical calculations of the cluster binding energies of representative HOMs. Ion-induced nucleation of pure organic particles constitutes a potentially widespread source of aerosol particles in terrestrial environments with low sulfuric acid pollution.

This was reviewed by various other researchers... as of today, it has been cited 189 times according to Google Scholar. V. Faye McNeill wrote in 2017 that "Significant progress has been made over the past two decades in understanding atmospheric aerosol chemistry and its connections to climate.". Here's one follow-up paper:

Reduced anthropogenic aerosol radiative forcing caused by biogenic new particle formation

Hamish Gordon, Kamalika Sengupta & Alexandru Rap et al, October 25, 2016

A mechanism for the formation of atmospheric aerosols via the gas to particle conversion of highly oxidized organic molecules is found to be the dominant aerosol formation process in the preindustrial boundary layer over land. The inclusion of this process in a global aerosol model raises baseline preindustrial aerosol concentrations and could lead to a reduction of 27% in estimates of anthropogenic aerosol radiative forcing.

Note that biogenic refers to the particles of the mechanism that's dominant for cleaner air mixtures in places like the Amazon rain forest and the southern hemisphere, and also pre-industrial unpolluted air.

How does aerosol forcing compare with greenhouse gas forcing? According to Climate Forcing by Anthropogenic Aerosols, Charlson & Schwartz et al, 1992, "Current climate forcing due to anthropogenic sulfate is estimated to be ... is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign."

For those of you in the audience who are keeping score, this means that any reduction in an estimate of anthropogenic aerosol forcing increases the effect of anthropogenic greenhouse gas forcing. But that says nothing of the effect of cosmic rays.

Finally, here we read:

Unexpected player in particle formation

Chris Cappa, May 26, 2016

"Three studies find that a family of organic compounds affects the formation and initial growth of atmospheric aerosol particles in clean air — with implications for our knowledge of the climate effects of aerosols."

Tidal Effects

Measurement–model comparison of stabilized Criegee intermediate and highly oxygenated molecule production in the CLOUD chamber

Nina Sarnela, Tuija Jokinen and Jonathan Duplissy et al. Aug 22, 2017

Field studies suggest that iodine oxides could be the key compounds for new particle formation in coastal areas during periods when high tidal movements expose algae beds to sunlight (O'Dowd et al., 2002; Sipilä et al., 2016). However, these iodine oxides do not appear as abundantly in the atmosphere as sulfuric acid or low-volatility organic vapours, so their importance seem to be limited to coastal areas.

Final Comments

Beyond Equilibrium Climate Sensitivity

Reto Knutti, Maria A. A. Rugenstein & Gabriele C. Hegerl

Nature Geoscience volume 10, pages 727–736 (2017)

Equilibrium climate sensitivity characterizes the Earth's long-term global temperature response to increased atmospheric CO2 concentration. It has reached almost iconic status as the single number that describes how severe climate change will be. The consensus on the 'likely' range for climate sensitivity of 1.5 °C to 4.5 °C today is the same as given by Jule Charney in 1979, but now it is based on quantitative evidence from across the climate system and throughout climate history. The quest to constrain climate sensitivity has revealed important insights into the timescales of the climate system response, natural variability and limitations in observations and climate models, but also concerns about the simple concepts underlying climate sensitivity and radiative forcing, which opens avenues to better understand and constrain the climate response to forcing. Estimates of the transient climate response are better constrained by observed warming and are more relevant for predicting warming over the next decades. Newer metrics relating global warming directly to the total emitted CO2 show that in order to keep warming to within 2 °C, future CO2 emissions have to remain strongly limited, irrespective of climate sensitivity being at the high or low end.

In summarizing experiments such as the CLOUD experiments in 2017, Knutti et al acknowledge that CO2 is not the only important factor for climate change, and also other mitigating facts that should hedge extrapolations of future climate change. These mitigating facts include "timescales of the climate system response, natural variability and limitations in observations and climate models" as well as "concerns about the simple concepts underlying climate sensitivity and radiative forcing". Considering that radiative forcing is human-caused or cosmic-caused cooling cloud cover, these researchers acknowledge that YES: recent CLOUD experiments do have significant implications for global warming.

In the judgment of Knutti et al, limiting CO2 is a good goal to set irrespective of how bad it is for the climate. That's why they say that CO2 emissions have to remain strongly limited. But this is a judgment call rather than an experimental result.

  • Deleted a bunch of not-so-nice comments. Please avoid back and forths.
    – Sklivvz
    Commented Feb 1, 2019 at 22:54

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