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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.

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What's the link between cosmic rays and climate change?

What the science says...

Select a level... Basic Intermediate Advanced

Cosmic ray counts have increased over the past 50 years, so if they do influence global temperatures, they are having a cooling effect.

Climate Myth...

It's cosmic rays

"When the Sun is active, its magnetic field is better at shielding us against the cosmic rays coming from outer space, before they reach our planet. By regulating the Earth’s cloud cover, the Sun can turn the temperature up and down. ... As the Sun’s magnetism doubled in strength during the 20th century, this natural mechanism may be responsible for a large part of global warming seen then."  (Henrik Svensmark)

At a glance

Space may often be regarded as dark, cold, empty and lifeless but there is plenty going on out there, including the streams of high-energy particles that whizz around at close to the speed of light. These are galactic cosmic rays, discovered by Austrian-American physicist Victor Hess in 1912. Galactic cosmic rays is a catch-all term that includes high-energy particles with sources ranging from the Sun to remnants of ancient supernovae – exploded stars – in other galaxies.

When cosmic rays hit the top of our atmosphere, a highly crowded environment compared to deep space, they interact with the atoms up there producing showers of charged particles known as ions. The ions then head on down towards the surface, where they make up just over ten percent of our typical yearly radiation dose. That's approximately equivalent to three chest x-rays.

The Solar wind protects Earth from cosmic rays and Earth's strong magnetic field in turn shields us from both. In this respect we are fortunate: if you want to find out what happens to a planet that has lost its strong magnetic field, go and take a look at Mars. As a result of these planetary defences, the amount of cosmic rays reaching the lower atmosphere and surface of Earth is minimised.

Direct recording of the cosmic ray flux has been possible since the beginning of the satellite era, since the satellites can carry particle detectors. We therefore have over a half-century worth of data on the changes in the intensity of the flux.

The idea that changes in the cosmic ray flux could drive the observed global warming has a small but determined number of fans. In short, their hypothesis suggests that the ions produced by cosmic rays can 'seed' clouds. That means more cloudiness - and clouds reflect sunlight, reducing the energy reaching Earth's surface. So, it imagines, if there are fewer cosmic rays reaching Earth, there will be fewer clouds, more sunlight reaching the Earth's surface, and thus more global warming. In a sense, this is a variant of the “It's the Sun” argument, because the cosmic ray flux falls when the Sun is in the active phase of its 11-year sunspot cycle and the Solar wind is typically stronger.

In 2017, the Cosmics Leaving Outdoor Droplets (CLOUD) experiment reported in. It had been created to test the link between cosmic rays and climate and was specifically looking for any connection between ions resulting from cosmic rays and cloud-seeding. The CLOUD experiment succeeded in unlocking many of the mysteries of cloud formation and growth in our atmosphere. That greatly improved our understanding of human influences on climate. In particular the study concluded that the effect of changes in cosmic ray flux intensity on the cloud condensation process is small. To quote its authors, it is, "unlikely to be comparable to the effect of large variations in natural primary aerosol emissions" - things like volcanic eruptions, wildfires and so on. So no, cosmic rays do not have much of an effect at all.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

It was at one time hypothesised that galactic cosmic rays (GCRs) may play a part in helping form clouds. A leading proponent of this idea was Danish scientist Henrik Svensmark. If this hypothesis were correct, an increase in the GCR flux, creating an increase in cloud condensation nuclei, would lead to more cloud cover in our lower atmosphere, reflecting more sunlight and resulting in a global cooling effect. Conversely, a decrease in GCR flux would lead to a reduction in cloudiness, warming the planet by letting more sunlight through.

People look at new hypotheses in order to test them. In order to calculate the maximum possible role of GCRs in recent warming, global temperatures have been compared to variations in the GCR flux, as measured by particle detectors aboard satellites and by neutron monitors at the Earth's surface. We'll take a look at some of these studies, for there are many, mostly reaching the same conclusion but with improvements in methodology through time, as always tends to occur in scientific research.

A 2003 paper observed that while there was some correlation between GCR levels and temperature prior to 1970, the correlation breaks down sharply after that point. The analysis concluded that "between 1970 and 1985 the cosmic ray flux, although still behaving similarly to the temperature, in fact lags it and cannot be the cause of its rise. Thus changes in the cosmic ray flux cannot be responsible for more than 15% of the temperature increase" (Krivova & Solanki 2003).

Krivova & Solanki (2003) 

Figure 1: Reconstructed cosmic radiation (solid line before 1952) and directly observed cosmic radiation (solid line after 1952) compared to global temperature (dotted line). All curves have been smoothed by an 11 year running mean (Krivova & Solanki 2003).

Another analysis from the 2000s scrutinises the link between GCRs and cloud cover and finds several discrepancies. As GCR flux shows greater variation in magnitude in high latitudes, one would expect larger changes in cloud cover in polar regions. This, the authors found, was not observed. They also examined the aftermath of the nuclear reactor accident at Chernobyl: the thinking was that if the GCR hypothesis was correct then since the Chernobyl site was a strong source of ionising radiation, some effect on cloud formation might be expected. They found none (Sloan & Wolfendale 2008).

The chance to soundly test the Svensmark hypothesis came up again in a particularly extended Solar minimum (between solar cycles 23 and 24, 2008-2010). This minimum was associated with a record high level of GCR flux, but at the same time, there was a record low level of cloudiness in the lower atmosphere – the bottom ~10 kilometres of the atmosphere where most of the weather occurs. If the GCR/cloud seeding idea was correct, the reverse should have been the case (Agee et al. 2012; fig. 2). Also in 2012, an important review paper, covering the past 35 years of research, concluded, “it is clear that there is no robust evidence of a widespread link between the cosmic ray flux and clouds.” (Laken et al. 2012).

GCR vs. Temp

Figure 2: Annual average GCR counts per minute (blue - note that numbers decrease going up the left vertical axis, because lower GCRs should mean higher temperatures) from the Neutron Monitor Database vs. annual average global surface temperature (red, right vertical axis) from NOAA NCDC, both with second order polynomial fits.

More recently, the Cosmics Leaving Outdoor Droplets (CLOUD) experiment, created to systematically test the link between GCRs and climate, reported in. It had been specifically looking for any connection between ions resulting from GCRs and aerosol nucleation and thus cloud condensation nuclei and cloud formation. The CLOUD experiment succeeded in unlocking many of the mysteries of nucleation and cloud growth in our atmosphere, greatly improving our understanding of human influences on climate. In particular the study concluded that the effect of changes in GCR flux intensity on the cloud condensation process is small and “unlikely to be comparable to the effect of large variations in natural primary aerosol emissions” - things like volcanic eruptions, wildfires etc (Gordon et al. 2017).

This story illustrates nicely how science proceeds. Someone thinks up a hypothesis and it is repeatedly put to the test and is found to be wanting. In turn that leads to further research and important discoveries, providing progressively better understanding into the details of how certain processes work – atmospheric ones in this case. In recent years, Svensmark has been associated with the likes of the Global Warming Policy Foundation, the Heartland Institute and other such organisations who prefer their own version of reality (details at DeSmog). Meanwhile, the science has moved on and left him behind.

Update June 17, 2023 - Based on feedback received, added a concluding paragraph to the at-a-glance section.

Last updated on 18 June 2023 by John Mason. View Archives

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Further reading

A team of scientists from 17 countries have found the most likely origin of galactic cosmic rays - the centres of distant galaxies (Active Galactic Nuclei) powered by supermassive black holes. This discovery is not particularly pertinent to the global warming debate but it is cool :-)

Further viewing

This video published on Nov. 17, 2019 by "Have a think" provides some more and current explanations of why GCRs do not play a role in current global warming.



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Comments 26 to 50 out of 61:

  1. #25 muoncounter, thanks for transferring my comment to a more appropriate thread. As I tried to explain in the other thread, I am not "chasing" anything, only mentioning a few of the causes of variations in sensitivity that are not in any paleo studies or current era models. Dana shows the non-match to 30 years of temperature in his threads and John shows recent non-match above as well. So the CO2 warming theory holds well for 30 years of precise GCR data and warming data. GCR is a much more variable effect both with clouds as above, and SSW and other effects. Those effects come and go and may balance out over time. They won't explain any inexorable monotonic warming or cooling because of their measured variability and because their effects are nonlinear and indirect through cloud and weather pattern changes. Events like SSW are modulated by solar activity but celestial in origin so their source is unpredictable but their intensity will be more or less diminished by higher solar activity. The most interesting aspect is the effect on climate sensitivity, not direct warming or cooling. A blocking event such as created by SSW or by other mechanisms will tend to diminish the water vapor feedback part of sensitivity. Another aligned effect is what Mizimi mentions in #10 above, that the water vapor feedback is diminished by CGR cloud formation. It is not an energy effect as correctly pointed out in #12, but it is an important change in sensitivity. A long term change in solar activity and thus GCR flux is a very important consideration in potential paleo and model analysis. Part of the higher sensitivity to CO2 warming measured in the late 20th century may be due to solar activity which diminished both the intensity of SSW events and the low clouds from GCR. Likewise part of the recent decrease in sensitivity may be be explained with solar activity decreases. This last paragraph is speculative, and I welcome critiques and counter evidence.
  2. This paper seems appropriate to post here to compliment the new thread where it is postulated that the ends of ice ages are caused by increased solar forcing and CO2 feedback (with presumably the reverse for starting ice ages). GCR considered as a forcing is much more speculative and Kirkby et al say so. But what would be most interesting to me is to combine the orbital variation forcing (which is notably weak) with the GCR "sensitivity" modulation (see my post 26) to help explain rapid changes especially into ice ages. I put quotes around sensitivity since it is not the usual definition but a broader one in which any forcing (orbital solar or current CO2 increase) can be amplified or deamplified (or lowered forcing can also be amplified or deamplified).

    This may help explain some of the changes that are not explained by the orbital curves in the other thread, especially the initiation of warming and cooling since the GCR effects can be much more powerful (partly forcing as postulated in the paper, but also amplification of the weaker orbital forcing).

  3. Eric (skeptic) scientists study all the reasonable hypothesis, it's their job afterall. Problem is when people take those hypothesis as valid alternative theories. Note that the paper, although submitted, didn't show up. Here you can find a nice review which includes the same data shown in the arxiv paper.
  4. Riccardo, thanks for the link. That paper is broad, not deep, but I agree with their conclusion that it is inconclusive especially without good GCR proxy data earlier than the present and 130k interglacials. What is a little weird about the paper is that they keep going back to solar and use GCR proxies as a solar proxy. Although GCR's are modulated by solar activity, they are their own animal. My guess looking at fig 8 is that an interglacial requires both low GCR plus northern hemisphere sun (the other thread), it can't happen with just one or the other.
  5. #29: "they keep going back to solar and use GCR proxies as a solar proxy." Because solar magnetic activity is what causes the observed variation in GCR frequency. Are you reading anything in this thread and following links that we provide? "Although GCR's are modulated by solar activity, they are their own animal." What animal? GCRs are infrequent and random; they come from times long ago and places far away. The only important feature of GCRs in this context is that they "are modulated by solar activity", which is what you questioned one sentence earlier in your comment #29! #26: "Events like SSW" OK, so I looked at the website you linked, which is pretty nice. It would be interesting if you would try to match up the dates of their SSW events with anything else. However, there are fewer than 30 events in their entire database, which runs back to 1958. How much warming are we supposed to be getting from fewer than one event per year? I suppose if you pile up enough of these disparate straws, you will indeed be able to put together a modest-sized strawman. But it is stunning that one can cling to 'this might' and 'that could', when the answer, my friend, is blowing in the increasingly CO2-enriched wind (apologies to Mr. B. Dylan). Unless it is some preconceived idea that it just can't be CO2. In which case, you should be reading more here at SkS.
  6. muoncounter, the low frequency is a good point. I don't what the frequency history is, but likely it varies between zero and small. But it is not random since the galactic structure was not random over time and our transit through the galaxy does not take us into random amounts of leftover particles. It is quite unpredictable, although not completely unpredictable. They are modulated by solar activity and of course solar activity itself has other effects, so it is worth studying and using GCR as a proxy. But this thread is about cosmic rays and (my addition) the effect they could have on sensitivity. The fact of CO2 warming is not in question here (or at all), just sensitivity and I would greatly appreciate your opinion on my final sentence in #29.
  7. Den siste mohikanen said: "For sure, neither the sun nor CO2 nor the two together make up for the only climate forcing. So your argument is a bit weak by itself, but even if we assume that all the unexplained difference is due to CO2, that doesn't give as much room for IPCCs +6°C forecast that you seem to imply." This is only true if there are no "tipping points" involved. However, if the current temperature rise is sufficient to bring trapped methane out of solution in the oceans, permafrost, clathrates etc, then there could be a feedback effect. Or there are other possibilities that could have the same effect. Here is a comment from Michael Benton, a paleontologist at Bristol University. He says that evidence points to the cause of the Permian extinction being prolonged and violent eruptions from the Siberian traps, a huge region of volcanic rock. In this scenario, mass eruptions triggered environmental catastrophe by belching an overwhelming quantity of gas into the atmosphere for half a million years. "The main follow on was a flash warming of the Earth. That caused stagnation in the oceans, as normal circulation shut down. On land, the consequence of all the carbon dioxide and other gases appears to have been massive acid rain that killed the forests and stripped the landscape bare," Benton said. "This was the greatest of all mass extinctions, the time when life was most nearly completely wiped out."
    Response: [muoncounter] Good points. See the discussions on methane hydrates here and here.
  8. This recent "The Register" article about a recently published paper on the influence of cosmic rays on climate presents a very different picture than the one shown here. I would be interested in seeing a assessment of the paper if anyone has time.
  9. #33: "a recently published paper" Thanks for finding the appropriate thread. It appears that this is one from the 'here-we-go-again' department (I note that W^W# has already picked it up). I won't bother with snide editorialisms in the Register article or any of the other newspaper feeds that parrot it. Here is a link to the Rao paper (it's actually a 'research communication') to be published in the IAS publication 'Current Science'. I see several points in a quick look: a. No new data are presented b. The most recent reference is 2007 c. He relies on the 'well-established excellent correlation' between GCRs and low-level clouds. The CERN CLOUD experiment, designed specifically to test this, has yet to produce any such results d. He pulls a graph from a 2005 paper (Veizer) to show 'correlation' between surface neutron monitors and 'low-level cloud intensity'. Although this is supposed to be a heat-trapping mechanism and the conclusion of the 'paper' is that this is a significant part of global warming, he curiously omits any temperature data. e. He makes the usual hash of GCRs-> more high level clouds -> cooling due to increased albedo and GCRs->low level clouds -> warming due to heat trapping. Same old song, not even a new verse.
  10. pdt, I wouldn't get your information from sources like THE REGISTER. Having done a search for this 'paper', I have only found links to the usual sources of denial (like the one you found), most of them having copied a report from THE HINDU. In that original report, it states that Dr. Rao's findings were released "as a discussion paper" by the Indian Environment Minister. Another Indian source calls it a "scientific review...[of] recent studies..." There is also a report in the HINDUSTAN TIMES, with a reaction from V Ramanathan of the SCRIPPS Institute. None of the reports I found, actually had a link to the paper or, even, any information about the title or where it was to be published. Seems like they were just interested in copying and pasting something that sounded to them like an anti-AGW paper. Strange, eh ? Doesn't appear to be that hoped-for 'final nail in the coffin of AGW' that some are hoping for (yet again).
  11. @35 (and others) I finally found the paper: It includes a reaction by prof Rawanathan (Scripps San Diego)
  12. #36, I just read through that paper and I don't see a quantification of the low cloud changes. I don't think the author divided the TSI fluctuation by 4. His figure (2) comes from this paper The Veizer paper references this paper which states "Correlation of our total irradiance time series with T accounts statistically for 80% of the variance in global temperature over that period, although the irradiance variation amplitude is insufficient to influence global warming in present-day climate models." I'm skeptical about this last paper, correlations are suspect and I have not seen correlations of TSI and GAT.
  13. #37: Eric, Thanks for the link to the Veizer 2005 paper; it certainly is unusual. He's mixed together what we've called GCRs with solar cosmic rays into a 'cosmic ray flux' (CRF). The CRF, in turn, is believed to correlate with the low altitude cloud cover. The postulated causation sequence is therefore: brighter sun ⇒ enhanced thermal flux + solar wind ⇒ muted CRF ⇒ less low-level clouds ⇒ lower albedo ⇒ warmer climate. Who among us will argue with the endpoints of that logical chain? Yes, brighter sun -> warmer climate. However, nothing in either Veizer or Rao substantiate the postulated mechanism of the steps in between.
  14. The 'its cosmic rays' crowd should be poised for a big couple of weeks. The recent solar flares (coronal mass ejections or CMEs) have arrived on earth, bringing "waves of ionization." More are coming, as the active far side of the sun rotates our way in the next week or two. See 19 Feb and 20 Feb for a recap. The WUWT crowd is excited by this news, responding with such gems as "Global Temps should go up? (as middle height tropical clouds do not form droplets see svensmark). Interesting to record the time it takes…" So far, some very nice auroral displays. -- spaceweather's Aurora gallery Surface neutron monitors are unimpressed, example here, (approx a -4% change), which is not much compared to this 3x larger event from September 2005. Note these are links to dynamically generated images from the online neutron monitor at Oulu, Finland. These are known as Forbush decreases, as ionizing radiation shields the earth's surface from the normal CR flux. Yes, in another of nature's apparent paradoxes, a solar flare can decrease ground level cosmic ray counts. And the effect on clouds is ...
  15. Which came first, the chicken or the egg? Can it be proven that the CO2 rose before the warming in the past or did it rise as a result of the warming of the oceans which then gave off the CO2? I have understood that the rising CO2 levels do not proceed the observed warming and actually lag it in most cases where tests are done with core samples etc. I keep an open mind myself.
  16. electroken - please see CO2 lag temperature for answer. This has nothing to do with cosmic rays. Short answer CO2 can be both forcing and feedback.
  17. A new study could heat up the discussion again, I think.
  18. There is a discussion on Roy Spencer's blog (hope I got the link process right) on a recent though as yet non peer reviewed study by Svensmark. Any comments?
  19. Philip Shehan given that gcr count didn't change in the last 70 years or so, Spencer's hypothesis can not change the picture of the last decades. It might only have an effect before then, if any.
  20. Thanks Riccardo
  21. JoeRG and anyone reading this discussion might be interested in the new topic posted at RC
  22. THanks for that link Arch. The real problems with the GCR hypothesis are unresolved. The effect is weak at best, whatever particles are created are too small, and no possibility for a particle growth process has been put forth yet.
  23. Philippe - I was hoping you would notice this since we had this discussion on another thread a few weeks ago. You were right. ;-)
  24. Denier sites are all over this recent interview with Rolf-Dieter Heuer, claiming it means scientists are being "censored" and "gagged". Here's the quote in question as rendered by Google Translate: "The results will be published shortly. I asked the colleague to make the results clear, however, not to interpret. This would go immediately into the highly political arena of the climate change debate. One has to be clear that it is the cosmic radiation is only one of many parameters." It's in reference to the CLOUD project's investigations on the influence of cosmic rays on cloud formation. Since his remarks have already become a denier talking point, you might consider addressing them in this article. At the end, you say: "Even if these difficulties can be resolved and the causality link between cosmic rays and cloud formation is proven, this would mean cosmic rays would have been imposing a cooling influence on climate over the last few decades." So for the sake of argument, let's say the latest CLOUD results find that "causality link." Why the "do not interpret" instruction from Heuer? What does it actually mean, in the context of scientific research and publication of results?
  25. 49, pixelDust, The interview is awfully thin, and only touches on CLOUD for one very brief question. Refer to this post on RealClimate about some other recent, similar research. Basically, in order to build the GCR theory into something remotely respectable, they have to first create all of the building blocks that are similar to an understanding of greenhouse gas theory. In the case of GCR's, they need a precise mechanism, with all of the intermediate steps needed, to show how GCR's actually, mechanically affect cloud formation. Now, his one comment about not interpreting the results probably (I expect) speaks to the fact that the CERN research can only show some very small pieces of the puzzle. They need lots and lots of pieces to compile it into a workable, fully functioning GCR theory. So to over interpret any one step, in either direction, would be foolish. Some of the proposed experiments (to show their level and nature... I took this from some rather old documents on the CLOUD website):
    • We will investigate the clustering of trace gas molecules onto ions. The trace gas molecules will either be formed by the ionising particle beam in pure artificial air or be introduced directly into the chamber.
    • Aerosols of a well defined size and with typical atmospheric composition (NaCl, (NH4)2SO4, H2SO4) can be produced with standard aerosol generation techniques (Section 4.4.3). Experiments will be performed to examine the activation of these aerosols into cloud droplets.
    • These experiments will first quantify the poorly-known production rates of a) nitric oxide (NO) and b) hydroxyl radicals (OH) by cosmic radiation. The former will involve pure artificial air and the latter will involve argon and water vapour. Subsequently we will investigate the effects of these vapours on the nucleation and growth of aerosols.
    • These studies concern the formation of ice nuclei in supercooled vapours at low temperatures. The expansion chamber will be used to create a supercooled cloud by expansion and growth of drops at temperatures below 260 K. ... In addition to experiments with supercooled liquid droplets already present(freezing nucleation),we will also investigate ice nucleation without pre-existing droplets(deposition nucleation).
    • These experiments concern the deposition nucleation of nitric acid and water vapours onto ion clusters to form nitric acid hydrates. Particles composed of such hydrates are thought to be the principal component of the polar stratospheric clouds that initiate the destruction of ozone.
    So, in a nutshell, nothing earth shattering in any direction can come from current CLOUD experiments. They are necessary first baby steps to creating a plausibly complete GCR theory (which in and of itself shows how sicence-fictiony the theory is at its current stage). Which means, in a nutshell, that the deniers are getting their undies all in a bunch over nothing, just because they love to cry wolf conspiracy.

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