The Svensmark hypothesis
Sequence of discoveries
The connection between cosmic rays and the cloud cover observed by satellites was announced at a space science meeting in 1996 and published in the following year (Henrik Svensmark & Eigil Friis-Christensen 1997). The report’s title called the discovery the “missing link” because it solved a big puzzle for climate researchers, as to how the Sun could exert an impact on climate that empirically exceeded by a wide margin the effect of variations in solar brightness measured by satellites.
In 1998 Svensmark was able to show that climate follows low-altitude counts of cosmic-ray muons more closely than other indicators of solar activity. By 2000, Nigel Marsh and Svensmark had established that the greatest effect of cosmic ray variations is paradoxically in the low cloud cover.
In 2002 strong support for the Svensmark hypothesis came from Nir Shaviv, an astrophysicist at the Racah Institute of Physics in Jerusalem. He found that over hundreds of millions of years the cold episodes in Earth history were linked to passages of the Sun and Earth through the spiral arms of the Milky Way Galaxy, where cosmic rays are most intense. In later papers, Shaviv explored this cosmic connection further back in time, and in collaboration with Ján Veizer of the Ottawa-Carleton Geoscience Centre he drew conclusions about the limited influence of carbon dioxide over geological time.
Beginning in 2001, Svensmark had initiated in Copenhagen the world’s first laboratory experiment on the role of cosmic rays in aerosol formation. Problems of funding this controversial project, called SKY, were not fully resolved until Svensmark managed to win for his Center for Sun-Climate Research a special line in the Danish national budget. By 2005 Svensmark’s team had obtained surprising and very positive results, eventually published (after long delays) by the Royal Society of London in 2007.
In 2006, the team delivered to Geneva an improved copy of SKY, for a 6-week experimental run using a beam of pion particles at CERN to simulate cosmic rays. The results of what was called the CERN pilot CLOUD experiment were inconclusive because of unresolved technical problems.
A scaled-down version of the Danish experiment called SKY@Boulby then operated in partnership with the Boulby Dark Matter Laboratory, 1.1 km underground in a deep mine in the UK. With the equipment wrapped in lead, both cosmic rays and radioactivity were effectively excluded from the experiment. That allowed the team to investigate aerosol formation both in the absence of ionization and with controlled ionization using radioactive sources, while experimenting with various concentrations of vapours. The Boulby experiment was completed satisfactorily in 2009. The equipment then went to Aarhus University in 2010 for further investigations.
In parallel with these experimental projects, the astrophysical study of climate change on long geological timescales became an important part of Svensmark’s work. Making the first application of geological data as a tool for galactic astrophysics, he used climatic variations to date the cosmic ray variations associated with the Solar System’s oscillation across the mid-plane of the Milky Way Galaxy. Svensmark was then able to derive key parameters describing the Galaxy’s spiral arms, published in 2006.
In another recent contribution, Svensmark and his colleagues have pioneered the use of global cosmic ray data, combined with heliospheric and atmospheric models of cosmic radiation, to infer variations in muon counts at low altitudes. This enabled them to go beyond the state of the art in tracing big effects on clouds of “Forbush decreases” in cosmic rays lasting only a few days, which other investigators had failed to find – as reported at a conference in 2008 and published in 2009.
Another line of enquiry is the impact on the Earth’s biosphere of climate change driven by comic rays from supernovae over the past billion years, pursuing in more detail a link first detected over the whole history of life on Earth, in a paper that Svensmark published in 2006.
“Finding connections between cosmic rays and climate over timescales from hours to billions of years,” Svensmark says, “has been the most exhilarating and persuasive feature of the cloud hypothesis.”