There is a claim that different patches of the galaxy have different climate effects due to variations in cosmic ray strength. I've occasionally heard this used to explain global warming but more often ice ages.

This article is an example of both claims via the cosmic ray mechanism.

If [solar cycles affect temperature], then one should expect climatic variations while we roam the galaxy. This is because the density of cosmic ray sources in the galaxy is not uniform. In fact, it is concentrated in the galactic spiral arms (it arises from supernovae, which in our galaxy are predominantly the end product of massive stars, which in turn form and die primarily in spiral arms). Thus, each time we cross a galactic arm, we should expect a colder climate. Current data for the spiral arm passages gives a crossing once every 135 ± 25 Million years. [...]

A record of the long term variations of the galactic cosmic ray flux can be extracted from Iron meteorites. It was found in the present work that the cosmic ray flux varied periodically (with flux variations greater than a factor of 2.5) with an average period of 143 ± 10 Million years. This is consistent with the expected spiral arm crossing period and with the picture that the cosmic ray flux should be variable.

So, what's the current state of our scientific knowledge on the subject? Is there an effect? Is it of comparable magnitude to atmospheric gas composition?

  • The "effective sky temperature" is a fictional temperature used to describe how much heat we're radiating out, assuming black body conditions (if I recall). I've seen other claims that different patches of the galaxy are warmer or cooler; which is equivalent to a different effective sky temperature. I can probably find a notable claim of that too, if needed. I only mentioned it because I don't actually care about the mechanism; I just want to know if galactic year affects climate. May 25, 2013 at 3:34
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    I don't see anything in the claim that something that happens every >100 million years contradicts the scientific consensus on global warming. If we speak of global warming we are speaking about decades or centuries and not millions of years.
    – Christian
    May 25, 2013 at 15:24
  • When I skimmed the article, I caught a bit that claimed humans were still partially the cause of global warming; implying that these galactic effects were of the same order of magnitude. May 25, 2013 at 19:31
  • 3
    "the scientific consensus about global warming", I was unaware that a consensus on the cause of global warming had been reached. What consensus are you referring to? The one where they agree implicitly agree that CO2 is a green-house gas, the one which we agree the earth has warmed since the little ice age and humans have had a 'significant' effect without defining the term, or some other study.
    – user1873
    May 26, 2013 at 2:18
  • "Is it of comparable magnitude to atmospheric gas composition?" Do we even know the cumulative effect of water vapor? I have heard that climate models have estimated that cloud reflectivity reduces water vapors warming by 1/3, but since climate models have been a poor predictor of future climates, I don't know how much faith we should put in those.
    – user1873
    May 26, 2013 at 2:22

1 Answer 1


"So, what's the current state of our scientific knowledge on the subject?"

The evidence doesn't look good. The EU funded a very substantial project at CERN, called the CLOUD project, to look into the basic physics of cloud formation resulting from nucleation by cosmic rays. This project produced some very good science and some interesting results, but were not able to demonstrate nucleation in conditions representative of the actual atmosphere.

There is a recent survey of GCR - climate links by Laken et al, suggests there isn't much evidence for a relationship between cloud cover and galactic cosmic rays.

B.A. Laken, E. Pallé, J. Čalogović, and E.M. Dunne, "A cosmic ray-climate link and cloud observations", J. Space Weather Space Clim., vol. 2, pp. A18, 2012. (www)

Despite over 35 years of constant satellite-based measurements of cloud, reliable evidence of a long-hypothesized link between changes in solar activity and Earth’s cloud cover remains elusive. This work examines evidence of a cosmic ray cloud link from a range of sources, including satellite-based cloud measurements and long-term ground-based climatological measurements. The satellite-based studies can be divided into two categories: (1) monthly to decadal timescale analysis and (2) daily timescale epoch-superpositional (composite) analysis. The latter analyses frequently focus on sudden high-magnitude reductions in the cosmic ray flux known as Forbush decrease events. At present, two long-term independent global satellite cloud datasets are available (ISCCP and MODIS). Although the differences between them are considerable, neither shows evidence of a solar-cloud link at either long or short timescales. Furthermore, reports of observed correlations between solar activity and cloud over the 1983–1995 period are attributed to the chance agreement between solar changes and artificially induced cloud trends. It is possible that the satellite cloud datasets and analysis methods may simply be too insensitive to detect a small solar signal. Evidence from ground-based studies suggests that some weak but statistically significant cosmic ray-cloud relationships may exist at regional scales, involving mechanisms related to the global electric circuit. However, a poor understanding of these mechanisms and their effects on cloud makes the net impacts of such links uncertain. Regardless of this, it is clear that there is no robust evidence of a widespread link between the cosmic ray flux and clouds.

[emphasis mine]

There is also the problem that there is no long term trend in Galactic Cosmic Rays to cause the trend in clod cover for which there is no robust evidence:

Cosmic Rays from Neutron Monitors (via RealClimate)

Note that the first line of the quote in the original question ("If [solar cycles affect temperature], then one should expect climatic variations while we roam the galaxy.") is a bit odd, as the strongest cycle in solar activity is the 11-year cycle, and that leaves barely a trace on global mean surface temperatures!

Solar Cycles vs Global Mean Surface Temperature (via WoodForTrees.org)

Note I have used a five year running mean, which leaves the 11-year cycle in the SIDC sunspot numbers, but as you can see there is little sign of an 11 year cycle in the HadCRUT4 global mean surface temperature data.

"Is there an effect?"

Not that can be reliably detected (see above).

"Is it of comparable magnitude to atmospheric gas composition?"

No, see Rahmstof et al (citation below), which discusses the problems with a paper by Shaviv (the source of the article in question) and Veizer, and also describe the estimation of the sensitivity of the climate to changes in CO2 from analysis of ice core data, which shows the effects of GCR are not plausibly of comparable magnitude.

Stefan Rahmstorf et al., "Cosmic rays, carbon dioxide, and climate", Eos, Transactions American Geophysical Union, Volume 85, Issue 4, pages 38–41, 27 January 2004 (www)

See also this article from Scientific American, which discusses a paper by Sloan and Wolfendale, Sloan being quoted as saying:

"We conclude that the level of contribution of changing solar activity is less than 10 percent of the measured global warming observed in the 20th century. As a result of this and other work, the Intergovernmental Panel on Climate Change state that no robust association between changes in cosmic rays and cloudiness has been identified."

Note here solar activity includes both direct effects (e.g. changes in total solar irradiance) and indirect effects, such as the possible influence of Galactic Cosmic Rays on cloud formation (for which there is no robust evidence - see above).

It does however show that mainstream science is not hostile to alternative explanations of climate change, the EU spent over 10 million Euro on the CLOUD project!

  • This is a great answer and covers two mechanisms well. How about the third mechanism of different parts of space being warmer than others? Jul 21, 2014 at 13:47
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    I can't see any mention of that in the article.
    – user18604
    Jul 21, 2014 at 13:55
  • Bit late, but it occurs to me that the temperature of space would have a negligible effect because it is a near vacuum, it would take very little energy to warm it, so it could impart very little energy to the climate system.
    – user18604
    Nov 16, 2021 at 18:46

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