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Professor Henrik Svensmark argues that cosmic rays have a strong influence on cloud formation and hence climate change. In 2016 he predicted global warming was over. He was recently the lead author of an article: Increased ionization supports growth of aerosols into cloud condensation nuclei.

He was recently quoted as saying:

The logical consequence is that the climate sensitivity of CO2 is smaller than what climate models suggest which is 2-4 deg C for each doubling of CO2, since both CO2 and solar activity has had an impact.

Is this true?

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This question is basically the same as Do the recent CLOUD results have significant implications for global warming? but with reference to a new Svensmark's new study.

It somewhat relates to

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    Welcome to Skeptics! The claim in this question isn't clear. You link to a new paper about the role of cosmic rays in cloud formation, which appears to support a previous set of experiments. Is there a claim in this paper you want us to evaluate? If you want us to assess the claims about Climate Change, find someone making the claim that the two are related. (Note: The theory of AGW is based on a number of strands of evidence, and is unlikely to be overturned by the results of a single experiment.)
    – Oddthinking ♦
    Feb 11, 2018 at 2:56
  • Thank you, @Oddthinking! I added a paragraph to the end of the post that tries to find the two claims in the post being made in a popular forum and to justify the relevance of the paper I linked. I hope that my question won't be understood merely as asking "how much does this new paper change the validity of these claims," but also "what, in broad terms, is the current state of the evidence for these claims."
    – capet
    Feb 11, 2018 at 16:22
  • Thank you @Nat! I want this question to be a bit less pure science so I revised the claims in the above post to be more polemical claims based on the press and my personal interactions. What do you think?
    – capet
    Feb 11, 2018 at 22:35
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    I made a rather substantial edit to allow it to be reopened. It is now on-topic here, but I agree with @Nat that Earthscience.SE may well be a better fit.
    – Oddthinking ♦
    Feb 12, 2018 at 1:18
  • @Oddthinking: I'm happy with your edits, thank you! Is a question like this likely to be identified as duplicative of other non-solar radiation-related questions about temperature sensitivity to CO2?
    – capet
    Feb 12, 2018 at 1:59

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Doubling CO2 will cause approximately 2-4°C increase in annual mean surface air temperature. That means that Svensmark's claim that the increase would be less than this is false.

The last two assessment reports from the Intergovernmental Panel on Climate Change (IPCC) agree that the temperature increase of doubling CO2 is something around the alleged. Model runs from their fourth assessment report (AR4) estimates it at 2.1-4.4°C while the fifth assessment report (AR5) estimates it at 2.1-4.7°C. The ensemble mean is 3.2°C in both cases. (AR5, Section 9.7.1)

The overall assessment of the Equilibrium Climate Sensitivity (ECS) according to (AR5, Section 10.8.2) is:

...there is high confidence that ECS is extremely unlikely less than 1°C and medium confidence that the ECS is likely between 1.5°C and 4.5°C and very unlikely greater than 6°C.

However this value is not for each doubling of CO2, but for the first doubling from the pre-industrial levels. For the next doubling the value will probably be different.

The AR5 says specifically about the proposed link (AR5, Section 7.4.6):

Correlations between cosmic ray flux and observed aerosol or cloud properties are weak and local at best, and do not prove to be robust on the regional or global scale. Although there is some evidence that ionization from cosmic rays may enhance aerosol nucleation in the free troposphere, there is medium evidence and high agreement that the cosmic ray-ionization mechanism is too weak to influence global concentrations of CCN or droplets or their change over the last century or during a solar cycle in any climatically significant way.

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  • Thank you @Communisty! It seems like this statement is probably "robust" to the new paper. I will add information if I find any credible-looking statements that specifically address this paper.
    – capet
    Feb 12, 2018 at 14:38
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    The IPCC said at the end of AR5 Section 10.8.2 was "In conclusion, estimates of the Equilibrium Climate Sensitivity (ECS) based on multiple and partly independent lines of evidence from observed climate change, including estimates using longer records of surface temperature change and new palaeoclimatic evidence, indicate that there is high confidence that ECS is extremely unlikely less than 1°C and medium confidence that the ECS is likely between 1.5°C and 4.5°C and very unlikely greater than 6°C."
    – Henry
    Feb 12, 2018 at 14:49
  • So your quotation of a CIMP5 model run rather than the full IPCC evidence seems rather selective, and your statement that an increase less than 2°C would be false does not appear to be what the IPCC is saying either in detail or in its Synthesis report
    – Henry
    Feb 12, 2018 at 14:53
  • Thank you @Henry, I wasn't trying to overlook anything. I'll add a quotation from section 10 to address the ECS.
    – Communisty
    Feb 12, 2018 at 15:12

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