Danish physicist Henrik Svensmark lays out his long-running hypothesis that variations in galactic cosmic rays, modulated by solar activity, influence low cloud formation and therefore climate. He describes the proposed microphysical pathway: ionisation stabilises tiny molecular clusters, which can grow into aerosols and eventually cloud condensation nuclei. Svensmark points to correlations between measured cosmic rays and satellite-era low-cloud data, and argues that coronal mass ejections provide a ‘natural experiment’ in which short-lived drops in cosmic rays are followed by measurable dips in aerosols and cloudiness days later.
He extends the argument to geological and astrophysical timescales, claiming reconstructed supernova rates and the Solar System’s passage through Milky Way spiral arms align with glaciations and long-term temperature proxies. He also discusses work using carbon isotope ratios in sediments as a proxy for marine biomass and organic carbon burial, which he links to oxygen production over deep time.
Alongside the science, Svensmark gives a detailed account of professional conflict. He says he faced aggressive institutional and public criticism after early publications, struggled to secure public funding, and alleges attempts to undermine private support. He claims DTU tried to dismiss him, later restricted his lab access, and that he was removed from CERN’s CLOUD collaboration. He argues that climate research has become politicised, and that mainstream modelling and IPCC assessments underweight solar and cosmic-ray influences while relying heavily on cloud and water-vapour feedback assumptions.
Svensmark argues that coronal mass ejections, which temporarily reduce incoming galactic cosmic rays, can be used to test whether ionisation affects aerosols and cloudiness. He says satellite datasets show dips in aerosols and cloud measures days after such events, consistent with time needed for particles to grow into cloud condensation nuclei.
Read this moment →Svensmark says he helped initiate CERN’s CLOUD project on cosmic rays and aerosols but was later expelled from the collaboration and removed from its steering group. He suggests internal concerns about funding and political fallout from public attention to his work were factors, though CERN and the collaboration are not represented in the episode.
Read this moment →Henrik Svensmark tells the podcast that IPCC assessments attribute negligible recent warming influence to the Sun, arguing this downplays mechanisms involving cosmic rays and clouds. He estimates cosmic-ray-related forcing at roughly 1.5 watts per square metre, which he says would be comparable to the overall anthropogenic estimate shown in the report graphic discussed on air.
Read this moment →Henrik Svensmark says DTU removed his access to a laboratory he had used for experiments on ionisation and aerosol formation. He attributes the decision to broader institutional pressure around his climate-related research, while also noting an official deficit explanation was given elsewhere in the discussion.
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