Climate Science Glossary

Term Lookup

Enter a term in the search box to find its definition.


Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup


All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Home Arguments Software Resources Comments The Consensus Project Translations About Donate

Twitter Facebook YouTube Pinterest

RSS Posts RSS Comments Email Subscribe

Climate's changed before
It's the sun
It's not bad
There is no consensus
It's cooling
Models are unreliable
Temp record is unreliable
Animals and plants can adapt
It hasn't warmed since 1998
Antarctica is gaining ice
View All Arguments...

Keep me logged in
New? Register here
Forgot your password?

Latest Posts


Climate Hustle

What do the CERN experiments tell us about global warming?

What the science says...

Select a level... Basic Intermediate

The CERN experiment only tested one-third of one out of four requirements to blame global warming on cosmic rays. At least two of the other requirements (strengthening solar magnetic field, fewer cosmic rays reaching Earth) have not been met over the past 50 years. The lead scientist in the CERN CLOUD experiment explicitly stated that the experiment "actually says nothing about a possible cosmic-ray effect on clouds and climate."  Many other studies have concluded that cosmic rays play a minor role in cloud formation, and have not contributed in any significant way to the global warming over the past 50 years.

Climate Myth...

CERN CLOUD experiment proved cosmic rays are causing global warming

"The new [CERN] findings point to cosmic rays and the sun — not human activities — as the dominant controller of climate on Earth...CERN, and the Danes, have in all likelihood found the path to the Holy Grail of climate science" [Lawrence Solomon]

Clouded “Reporting”

Many climate "skeptics" have claimed that CLOUD at CERN has “proven that cosmic rays drive climate change”, or something to that effect. Unfortunately for “skeptics” (and if we don’t reign in greenhouse emissions, everyone else), it’s not true. While cosmic rays may have some influence on cloud formation, they are not responsible for the present, human-driven climatic change or alleged changes in the geologic past.

What’s the deal?

Although seemingly out of fashion for a while until recently, the “cosmic rays are driving climate” myth has long been one of the mainstays of the self-contradictory climate “skeptic” argument stable, and it’s something covered fairly often at The Way Things Break (previous posts here, here, here, here, here, and here). And as with any good falsehood, it starts with a kernel of truth.

It is completely accepted in mainstream science that galactic cosmic rays (GCRs) might be able to influence the nucleation process of potential cloud condensation nuclei (CCN), and that it’s conceivable that this could influence cloud behavior at some level. As the IPCC AR4 noted,

"By altering the population of CCN and hence microphysical cloud properties (droplet number and concentration), cosmic rays may also induce processes analogous to the indirect effect of tropospheric aerosols. The presence of ions, such as produced by cosmic rays, is recognised as influencing several microphysical mechanisms (Harrison and Carslaw, 2003). Aerosols may nucleate preferentially on atmospheric cluster ions. In the case of low gas-phase sulphuric acid concentrations, ion-induced nucleation may dominate over binary sulphuric acid-water nucleation."

While a plausible mechanism exists, real world verifications are necessarily difficult to undertake. The CLOUD project at CERN is seeking to do exactly that. The “skeptic” and right wing blogospheres are abuzz because Jasper Kirkby, et al. have just published the first results in Nature (Kirkby 2011).

RealClimate has a good rundown of what Kirkby et al.’s results do and do not mean. The short version is that Kirkby et al. do find increased aerosol nucleation under increased ionization (i.e. “more cosmic rays”), particularly in the mid-troposphere, but the effect is smaller at warmer, lower levels where the cosmic ray-climate myth proponents claim it has its greatest climatic effect. Lead author Jasper Kirkby has tried to set the record straight, stating (emphasis added):

"[The paper] actually says nothing about a possible cosmic-ray effect on clouds and climate, but it’s a very important first step."

More recently, in May 2013 Kirkby reiterated this point.

"At the present time we can not say whether cosmic rays affect the climate"

While the CLOUD results provide some confirmation of the potential mechanism by which GCRs might induce cloud nucleation, they in no way demonstrate that GCRs do significantly promote cloud formation in the real world, let alone support the myth that GCRs drive significant climatic change.

This point was driven home by Erlykin et al. (2013), who examined the preliminary CLOUD results in the context of climate change. The authors found: 

Taken at face value the CLOUD results would indicate an increase in nucleation rate of about 3 orders of magnitude in going from the equator to a latitude of 80. Even allowing for various reductions due to ‘sinks’ (Kirkby 2012, private communication), a big change should surely follow.

A search for the latitude dependence of the [cosmic ray, low cloud] correlation, or the related dependence on the [cosmic ray] vertical rigidity cut-off (VRCO), gave negative results (Sloan and Wolfendale 2008), and indeed, this was one of the first demonstrations of the lack of a genuine  [cosmic ray, low cloud] correlation. A latitude dependence of the correlation was not detected at any altitude, in fact. Thus, the expected big change with latitude for H2SO4 nucleation anywhere is not observed.

“But wait!” I’m sure some of you may be thinking, “the CLOUD results themselves don’t disprove GCRs drive significant climatic changes.” And that’s true enough.

How Do We Know That Cosmic Rays Aren’t Driving Significant Climatic Change?

In reference to the present anthropogenic climatic changes that we’re driving through alteration of the planetary energy balance notably through greenhouse gas emissions, we can theorize what certain “fingerprints” of enhanced greenhouse warming should look like, and examine observational data to see whether those fingerprints show up. And they do.

Moreover, we can examine the claims made by Svensmark, Shaviv, and others who proclaim GCRs drive climate and see whether or not they hold up. They don’t.

We can look at the paleoclimatic record during periods of significant changes in GCR activity, and there is no corresponding change in climate, e.g. the Laschamp excursion ~40kya (Muscheler 2005).

We can examine the change in GCRs in response to solar variability over recent decades or the course of a solar cycle, and find there is no or little corresponding change in climate (Lockwood 2007, Lockwood 2008, Kulmala 2010).

We can look at alleged correlations between GCRs and climate in the geologic past due to our sun passing through galactic spiral arms, and find that these “correlations” were based on an unrealistic, overly-simplified model of spiral structure and are not valid (Overholt 2009). Standard climatic processes (like CO2) more parsimoniously explained the climatic changes even before taking the flawed spiral model into account (Rahmstorf 2004).

We can examine the specific mechanisms by which Svensmark and others have claimed GCRs influence climate via cloud behavior and show that alleged correlations between GCRs and clouds were incorrectly calculated or insufficiently large, proposed mechanisms (e.g. Forbush decreases) are too short lived, too small in magnitude, or otherwise incapable of altering cloud behavior on a large enough scale to drive significant climatic change (Sloan 2008, Erlykin 2009, Erlykin 2009a, Pierce 2009, Calogovic 2010, Snow-Kropla 2011, Erlykin 2011).

Basically, what’s actually been demonstrated by Kirkby, et al. isn’t at odds with the IPCC. What is at odds with the IPCC hasn’t been demonstrated by Kirkby, et al. And the claims by Svensmark, Shaviv, and other ‘GCRs drive climate’ proponents have been debunked at pretty much every step of the way. GCRs may have some influence on cloud behavior, but they’re not responsible for significant climatic changes now or in the geologic past.

To Be Continued?

The CLOUD project at CERN is essentially just getting started. Its preliminary findings will help aerosol modelers, and hopefully it will continue to provide useful results. After the initial furor of “skeptic” blog-spinning dies down, cosmic rays will probably find themselves falling out of favor once again. But there’s no such thing as too debunked when it comes to myths about climate change, and there’s little chance this will be the last time cosmic rays will be trotted out to claim that we don’t need to reduce greenhouse gas emissions.


  • Calogovic, J., et al. (2010): Sudden cosmic ray decreases: No change of global cloud cover. Geophysical Research Letters, 37, L03802, doi:10.1029/2009GL041327.
  • Erlykin, A.D., et al (2009): Solar activity and the mean global temperature. Environmental Research Letters, 4, 014006, doi:10.1088/1748-9326/4/1/014006.
  • Erlykin, A.D., et al (2009a): On the correlation between cosmic ray intensity and cloud cover. Journal of Atmospheric and Solar-Terrestrial Physics, 71, 17-18, 1794-1806, doi:10.1016/j.jastp.2009.06.012.
  • Erlykin, A.D., and A.W. Wolfendale (2011): Cosmic ray effects on cloud cover and their relevance to climate change. Journal of Atmospheric and Solar-Terrestrial Physics, 73, 13, 1681-1686, doi:10.1016/j.jastp.2011.03.001.
  • Erlykin, A. D., T. Sloan, and A. W. Wolfendale (2013), A review of the relevance of the “CLOUD” results and other recent observations to the possible effect of cosmic rays on the terrestrial climate, Meteorol Atmos Phys, 1–6, doi:10.1007/s00703-013-0260-x.
  • Kirkby, J., et al. (2011): Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation. Nature, 476, 429–433, doi:10.1038/nature10343.
  • Kulmala, M., et al. (2010): Atmospheric data over a solar cycle: no connection between galactic cosmic rays and new particle formation. Atmospheric Chemistry and Physics, 10, 1885-1898, doi:10.5194/acp-10-1885-2010.
  • Lockwood, M., and C. Fröhlich (2007): Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature. Proceedings of the Royal Society: A. 463, 2447- 2460, doi:10.1098/rspa.2007.1880.
  • Lockwood, M., and C. Fröhlich (2008): Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature. II. Different reconstructions of the total solar irradiance variation and dependence on response time scale. Proceedings of the Royal Society: A, 464, 1367-1385, doi:10.1098/rspa.2007.0347.
  • Muscheler, R., et al. (2005): Geomagnetic field intensity during the last 60,000 years based on 10Be and 36Cl from the Summit ice cores and 14C. Quaternary Science Reviews, 24, 16-17, 1849-1860, doi:10.1016/j.quascirev.2005.01.012.
  • Overholt, A.C., et al. (2009): Testing the link between terrestrial climate change and galactic spiral arm transit. The Astrophysical Journal Letters, 705, 2, L101, doi:10.1088/0004-637X/705/2/L101.
  • Pierce, J.R., and P.J. Adams (2009): Can cosmic rays affect cloud condensation nuclei by altering new particle formation rates? Geophysical Research Letters, 36, L09820, doi:10.1029/2009GL037946.
  • Rahmstorf, S., et al. (2004): Cosmic Rays, Carbon Dioxide, and Climate. Eos Transactions AGU, 85(4), doi:10.1029/2004EO040002.
  • Sloan, T., and A.W. Wolfendale (2008): Testing the proposed causal link between cosmic rays and cloud cover. Environmental Research Letters, 3, 024001, doi:10.1088/1748-9326/3/2/024001.
  • Snow-Kropla, E.J., et al. (2011): Cosmic rays, aerosol formation and cloud-condensation nuclei: sensitivities to model uncertainties. Atmospheric Chemistry and Physics, 11, 4001-4013, doi:10.5194/acp-11-4001-2011.

Last updated on 31 May 2013 by thingsbreak. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page


Comments 1 to 6:

  1. Try as hard as I might, I cannot see why proving that climate change is human in origin should have any bearing on the need to take action. We know that atmospheric CO2 acts as a greenhouse gas and we know how to reduce the amount of it we humans release into the atmosphere. So let's just get on with reducing it.

    Every time we argue about whether the change in the climate that we are experiencing is human in origin or not gives the politicians, who have an urgent need to protect their job in elections that come by every four or five years, an excuse to procrastinate. And heaven knows they have been excellent procrastinators when one looks at what has actually been achieved since Kyoto. It sickens me to think that Monckton and his ilk are winning hands down as things are. Though I doubt they will enjoy the prize they earn.

    I have given the example elsewhere on this site that you would not refuse to change direction or speed because the iceberg dead ahead is not human in origin.
    Response: [muoncounter] You're wildly off-topic for this thread (which should have links to all of the existing it's not cosmic rays threads).
  2. I was looking for this argument on the "Climate myths sorted by taxonomy" page, but I could not find it. Is there a reason it isn't on that page, and if not, would someone who can consider adding it there? I imagine it should be added under "It's cosmic rays".

  3. For a recent example where this experiment is in fact cited by a climate denier, check out the Letters to the Editor of the Lynchburg, VA News and Advance on 06/26/2014 (2nd Paragraph):

    It is sadly amazing how such things take a life of their own.  However the letter did send me looking for what the experment was doing.  As is often the case, some folk put words into scientists' mouths without first asking them.


    [JH] Activated link.

  4. I commend you for your strictness in interpreting the results of the Cloud Experiment. As you quite rightly say, the lead author (Jasper Kirby) has been very cautious in his claims, limiting himself to the results of the CERN experiments. You can see this very clearly in the recent paper that reports the experimental results.

    In his lecture available via Youtube, Dr Kirby was careful to warn his audience concerning the uncertainties in the putative mechanism relating GCR to climate via cloud formation. There is a big ? mark in the graphic and he points it out to the audience.

    In an earlier paper Dr Kirby was likewise cautious about what was expected from the Cloud Experiment, together with the uncertainties in relation to climate.

    He stated,

    "Although recent observations support the presence of ioninduced
    nucleation of new aerosols in the atmosphere, the possible contribution of such new particles to changes in the number of cloud condensation nuclei remains an open question." Page 32.

    Cosmic Rays, Clouds, and Climate (CERN-PH-EP/2008-005, 26 March 2008. Surveys in Geophysics 28, 333-375.


    Only the most intrepid readers will wish to study the full paper, however the Youtube video contains the gist of the paper and several of the graphics.

    Dr Kirby's presentation is clear and I believe accessible to non-physicists. 

    There is a reference to protons and muons at one point, but readers of your blog will know that Wikipedea has good explanations of these.


    [RH] Shortened link.

  5. Quick question: What would the cosmic ray hypothesis say about the effect on upper atmosphere temperature change?

  6. jd_germany, assuming we are talking about the 'cosmic ray hypothesis' which holds that 'a decrease in cosmic rays penetrating the atmosphere could lead to decreased cloud formation and thus increased solar radiation reaching the surface' (there are others) then there would be no change in the 'greenhouse effect' and we wouldn't expect to see the cooling of the upper atmosphere (i.e. stratosphere) which is characteristic of greenhouse warming (yet, we do).

    Similarly, if global warming were being driven by increased solar radiation (introduced by cosmic rays or otherwise) then we would expect to see the greatest warming increases during the day (we don't), in summer (nope), and near the equator (wrong again).

    In short, this 'hypothesis' is sort of the opposite of scientific progress... multiple lines of evidence all converge to show that it is false.

Post a Comment

Political, off-topic or ad hominem comments will be deleted. Comments Policy...

You need to be logged in to post a comment. Login via the left margin or if you're new, register here.

Link to this page

The Consensus Project Website



(free to republish)



The Scientific Guide to
Global Warming Skepticism

Smartphone Apps


© Copyright 2015 John Cook
Home | Links | Translations | About Us | Contact Us