Organisation profile

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The Center for Sun-Climate Research at the National Space Institute investigates the connection between solar activity and climatic changes on Earth.

Climate changes have both a scientific and a social perspective. The social perspective is associated with the range of climate change that can be attributed to the increasing human induced contribution. The scientific perspective is an endeavour to understand the full complex system of the various sources of climate change and their mutual interactions. 

The National Space Institute (NSI) comprises the country's largest collected expertise in the scientific disciplines that play a major and documented role in the understanding of climate change both in geological and historical time, namely variations in solar activity. The NSI regards it as essential that this collected expertise is being used in an attempt to understand the natural causes of climate change in order to evaluate the contribution of natural causes to global change.

The NSI is basing its effort in this area on its own scientific results – observational, experimental, and theoretical. The scientific results have been published internationally and indicate that the varying activity of the Sun is indeed the largest and most systematic contributor to natural climate variations.

The significant contribution from solar activity variations to global temperature increase does not, however, exclude other contributions to the rising global temperature, natural as well as human. However, taking into account the large uncertainty associated with the estimated human contribution, a good research-based estimate of the range of natural climate variations is an essential information. The NSI is focused on establishing the best possible and scientifically based evaluation of the size of solar induced effects on climate.

Connection between Solar Activity and Climate Changes

Climate is subject to influences by both natural and human forces, including greenhouse gases, aerosols, solar activity, and land use change.

Changes in the sun contribute to climate change. Solar activity has been exceptionally high in the 20th century compared to the last 400 years and possibly compared to the past 8,000 years. When solar activity is high, the flux of galactic cosmic rays is reduced due to increased magnetic shielding by the Sun. The cosmic rays may influence the Earth's climate through the formation of low-lying clouds.

In 1996 a surprising discovery was announced that the intensity of cosmic rays incident on the earth's atmosphere correlates closely with variations of global cloud cover [Svensmark and FriisChristensen 1996]. Clouds both reflect incoming and trap outgoing radiation, and they thus play an important role in the Earth’s radiation budget.

A connection between solar activity and the Earth’s cloud cover could have immense implications in our understanding of climate and the relevance of processes in space. The evidence from observations warrants a scientific investigation into a possible link between solar activity and the Earth’s cloud cover. The reported variation of cloud cover was approximately 2% over the course of a sunspot cycle. This may appear to be quite small; however, the possible consequences on the global radiation (energy) budget are not. Simple estimates indicate that the resultant global warming could be comparable to that presently attributed to greenhouse gases from the burning of fossil fuels. The hypothesis is that solar variability is linked to climate variability by a chain that involves the solar wind, cosmic rays and clouds.

The weak link is the connection between cosmic rays and clouds. Recent work has directed attention to a mechanism involving aerosol production and the affects on low clouds. This idea suggests that ions and radicals produced in the atmosphere by cosmic rays could influence aerosol production and thereby cloud properties.

Cosmic rays ionize the atmosphere, and an experiment performed at the National Space Institute has found that the production of aerosols in a sample atmosphere with condensable gases (such as sulphuric acid and water vapor) depends on the amount of ionization. Since aerosols work as precursors for the formation of cloud droplets, this is an indication that cosmic rays influence cloud formation.

Climate models only include the effects of small variations in direct solar radiation (infrared, visible and UV). The effects of cosmic rays on clouds are not included in models, and the models do a rather poor job of simulating clouds in the present climate. Since cloud feedbacks are a large source of uncertainty, this is a reason for concern when viewing climate model predictions. The climate system is extremely complex and any estimate of the human contribution to climate change is very uncertain. 

Cosmoclimatology is the term coined for this research, involving a range of disciplines from space physics to atmospheric science to cloud microphysics.

Review of the field of Cosmoclimatology

Summary of a review article on cosmoclimatology by Henrik Svensmark, National Space Institute, published in Astronomy & Geophysics, February 2007.

In this article, the leader of sun-climate research at the National Space Institute, Henrik Svensmark, puts together the findings reported by him and his colleagues in a dozen scientific papers, to show how the climate is governed by atomic particles coming from exploded stars. These cosmic rays help to make ordinary clouds. High levels of cosmic rays and cloudiness cool the world, while milder intervals like the warming in the 20th Century occur when cosmic rays and cloud cover diminish. Here are some of the salient points of the article: 

A secure scientific base  

For more than 20 years, satellite records of low-altitude clouds have closely followed variations in cosmic rays (diagram 1). Just how cosmic rays take part in cloud-making appeared in the SKY experiment, conducted in the basement of the National Space Institute. Electrons set free in the air by passing cosmic rays help to assemble the building blocks for cloud condensation nuclei on which water vapour condenses to make clouds (Note 1: See also NSI press release, 4 October 2006. Getting closer to the cosmic connection to climate). 

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1. Red depicts cosmic-ray intensities and blue, the low-level cloudiness (click for larger image).

Evidence that the Earth’s climate really responds to variations in cloud cover comes from Antarctica. When the rest of the world warms up, the southern continent tends to cool down, and vice versa (diagram 2). This contrary behaviour is predictable, because clouds have an unusual warming effect over Antarctica.  

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2. Upper curve shows temperature changes in the Northern Hemisphere, and the lower curve, changes in the Antarctic region (click for larger image).

Cosmic ray intensities – and therefore cloudiness – keep changing because the Sun’s magnetic field varies in its ability to repel cosmic rays coming from the Galaxy before they can reach the Earth. Radioactive carbon-14 and other unusual atoms made in the atmosphere by cosmic rays provide a record of how cosmic-ray intensities have varied in the past. They explain repeated alternations between cold and warm periods during the past 12,000 years. Whenever the Sun was feeble and cosmic-ray intensities were high, cold conditions ensued, most recently in the Little Ace Age that climaxed 300 years ago.

Events around 40,000 years ago posed a difficulty for the new theory. Cosmic rays intensified dramatically then, because the Earth’s magnetic field weakened, but no perceptible cooling resulted. Calculations now show that the cosmic rays involved in cloud formation at low altitudes are produced by particles coming from the Galaxy with such high energy that the Earth’s magnetism scarcely affects them.  

‘From this secure base,’ Dr Svensmark writes in Astronomy & Geophysics, ‘we can broaden the horizons of space and time to consider the relevance of cosmic rays to climate change since the Earth was young.’

Climate change during the Earth’s history

On long timescales the intensity of cosmic rays varies more emphatically because the influx from the Galaxy changes. During the past 500 million years, the Earth has passed through four ‘hothouse’ episodes, free of ice and with high sea levels, and four ‘icehouse’ episodes like the one we live in now, with ice-sheets, glaciers and relatively low sea levels.

Nir Shaviv of the Hebrew University in Jerusalem, together with Ján Veizer of the Ruhr University and the University of Ottawa, links these changes to the journey of the Sun and the Earth through the Milky Way Galaxy. They blame the icehouse episodes on encounters with bright spiral arms, where cosmic rays are most intense (diagram 3).
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3. The red curve shows changes in tropical sea-surface temperatures over the past 500 million years. In the blue curve, drawn upside down to match, cosmic-rays intensities have varied according to our position relative to the Galaxy’s spiral arms. After Shaviv and Veizer (click for larger image)

More frequent chilling events, every 34 million years or so, occur whenever the solar system passes through with the mid-plane of the Galaxy. Dr Svensmark has used the climate records to improve our knowledge of important astronomical details about the Milky Way (Note 2: See also NSI press release, 15. November 2006: The microscopic hitchhiker’s guide to the Galaxy). 

In Snowball Earth episodes around 700 and 2300 million years ago, even the Equator was icy. At those times the birth-rate of stars in the Galaxy was unusually high, which would have also meant a large number of exploding stars and intense cosmic rays. Earlier still, the theory of cosmic rays and clouds helps to explain why the Earth did not freeze solid when it was very young. The Sun was much fainter than it is now, but also more vigorous in repelling cosmic rays, so the Earth would not have had much cloud cover.

While calculating the changing influx since life began about 3.8 billion years ago, Dr Svensmark discovered a surprising connection between cosmic-ray intensities and a variability of the productivity of life (Note 3: See also NSI press release 15. November 2006: The Milky Way shaped life on Earth). 

Near the end of his review Dr Svensmark writes: ‘The past 10 years have seen the reconnaissance of a new area of research by a small number of investigators. The multidisciplinary nature of cosmoclimatology is both a challenge and an opportunity for many lines of inquiry.’ Even the search for alien life is affected, because it should now take into account of the need for the right magnetic environment if life is to originate and survive on the planets of other stars.


‘Cosmoclimatology: a new theory emerges’, Henrik Svensmark, Astronomy & Geophysics, Vol. 48, Issue 1, pages 1.18-1.24, February 2007.


Click to read about the CLOUD og SKY experiments.

Contact person

Head of division Henrik Svensmark.



Contact information

Kgs. Lyngby
  • Phone: (+45) 4525 9500
  • Fax: (+45) 4525 9575
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