FILM SYNOPSIS:

THE CLOUD MYSTERY is a scientific detective story. It tells how a Danish scientist, Henrik Svensmark, through pioneering experiments in Copenhagen, solved the puzzle of how supernova explosions in our Galaxy and variations in the Sun govern climate changes on the Earth.

Scarcely audible above the noise about global warming, Svensmark has reported a new kind of aerial chemistry triggered by events in our Galaxy that shower our planet with atomic particles – the cosmic rays. This celestial mechanism determines cloudiness and temperatures on Earth. It is so powerful that we now have to re-evaluate the causes of global warming.
Henrik Svensmark linking the Sun and the clouds with climate change.
STEPS IN THE STORY
Strong evidence that changes in the Earth's climate follow changes in the Sun's magnetic behaviour came from the Copenhagen scientists Eigil Friis-Christensen and Knud Lassen in 1991. In principle it might explain most of the warming in the 20th Century, but no one knew how the Sun could affect the climate so much.

One effect of solar changes is to vary the number of cosmic rays reaching the Earth from the Galaxy. In 1995 Henrik Svensmark, also in Copenhagen, began to wonder if the cosmic rays could affect cloud cover. When he compared satellite observations of clouds with the varying counts of cosmic rays from year to year, he found an amazing link. A stronger Sun and fewer cosmic rays meant fewer clouds and a warmer world. Friis-Christensen agreed with this explanation for the Sun's role.

Scientists favouring carbon dioxide as the driver of global warming rejected the discovery. Yet ample evidence reveals Sun-driven climate change, long before human industry could have been a factor. The astronomer Nir Shaviv of Jerusalem takes the viewer to cliffs by the Dead Sea, where layers of sediment show alternations of wet and dry periods over centuries and millennia. The changes matched solar-driven variations in the intensity of cosmic rays.
Dark and light bands by the Dead Sea tell of ever-changing wet and dry periods
Continuing his investigations, with Nigel Marsh, Svensmark found that the clouds most affected by variations in the cosmic rays are those at low altitude. Although surprising, this fitted very well with the emerging theory of cloud-driven climate change. The next question was, How could the cosmic rays affect cloud formation? As explained in the film by Richard Turco from Los Angeles, water vapour condenses to make cloud droplets only in the presence of invisible particles floating in the air as aerosols, or cloud condensation nuclei. Cosmic rays might help to make the aerosols.

Svensmark wanted to carry out a laboratory experiment on that possible influence of cosmic rays on cloud condensation nuclei. But he ran into strong opposition to the very idea of an experiment, from scientists who dismissed the possibility of any connection. As a result, Svensmark and his team were short of funds. The work of building an experimental chamber in the basement of the Danish National Space Center was painfully slow.

Astronomer Nir Shaviv, Jerusalem, explains our position in the Milky Way.
Meanwhile, in 2002, unexpected support came from Nir Shaviv in Jerusalem, who carried the story of cosmic rays and climate back over hundreds of millions of years, from an astronomer's point of view. Shaviv realised that whenever the Sun and its planets visit the bright spiral arms of the Milky Way Galaxy, they are exposed to stronger cosmic rays, which create 'icehouse' conditions on the Earth. Visiting MØns Klint in Denmark, Shaviv points out the chalk cliffs dating from the 'hothouse' world of the dinosaurs, and the bulldozing action of glaciers in the present 'icehouse' era. His interpretations turned out to fit beautifully with the discovery of alternations of warm and cool oceans, by the geologist Ján Veizer of Toronto.
Geologist Jan Veizer, Ottawa, shows a brachiopod, used for estimating Earth's temperatures the past 500 million years

By 2005, the experiment in Copenhagen was at last running well. It gave very clear evidence for the role of cosmic rays in helping to build small aerosols that grow into cloud condensation nuclei. But the next problem was to get the report published in a scientific journal. Svensmark and his team faced rejection after rejection, for no very good reason. As the eminent physicist Eugene Parker comments in the film, politically incorrect ideas about global warming face this kind of resistance in the scientific community.

At long last, the paper was accepted for publication by the Royal Society of London in 2006. While the champagne flowed, Svensmark said the cloud mystery was solved but wondered when the climate community would catch up with his discovery. The film ends with Shaviv pointing out that we are part of a galactic ecosystem, and Svensmark saying we must no longer think of the Earth as an island in space.
Henrik Svensmark and Jens Olaf Pepke Pedersen with the basement experiment 
Conclusion: 
Svensmark built the world’s largest cloud chamber at the Danish National Space Centre(Now DTU Space) in Copenhagen. They worked five years in the laboratory before they had the results. During the fall of 2006 the epoch making results were publishing world-wide in science literature. The results concluded that the climate of Earth is decisively influenced by exploding stars and additionally: This mechanism literally turns the Earth’s thermostat upside down. This means that Cosmic rays produces aerosols in our atmosphere, which are necessary for the formation of clouds. Without these aerosols water vapour cannot condense into droplets and form clouds. Remember that water vapour is by far the dominating greenhouse gas, and even very small changes in the global cloud cover will change the Earth radiation budget significantly.
Natural events in our Galaxy and on the manic-depressive Sun decide whether the Earth’s cloud cover is letting the Sun heat or cool the surface of the Earth. The clouds, obedient to the cosmic rays, are the dominant driver of climate change. As a result we have to re-evaluate our understanding of the climate. A new field in climate research called cosmoclimatology is progressing.
Past climate variations
Using this discovery we are now able to explain how the feeble Sun allowed more clouds to form and cool the world 300 years ago during the Little Ice Age. During the 20th Century the Sun doubled its magnetic strength and warmed the Earth. And just recently Henrik Svensmark together with astronomers and geologists have documented an extraordinary relationship between these celestial events and the evolution of biosphere's millions of years back in time. 

The discovery introduces a new fascinating perspective that we are indeed taking part in a intergalactic eco system where the evolution of biosphere's are linked to celestial forces through climate change on all time scales.