'Cosmic rays from exploding stars affect Earth's climate'

Cosmic rays from supernovae - or exploding stars - can influence Earth's cloud cover and its climate, scientists have found.

The findings by researchers from University of Copenhagen in Denmark and Hebrew University of Jerusalem explain climate changes observed during the 20th century as well as the coolings and warmings of around two degree Celsius that have occurred repeatedly over the past 10,000 years, as the Sun's activity and the cosmic ray influx have varied.

Atmospheric ions, produced by the energetic cosmic rays raining down through the atmosphere, helps the growth and formation of cloud condensation nuclei - the seeds necessary for forming clouds in the atmosphere.
 

When the ionisation in the atmosphere changes, the number of cloud condensation nuclei changes affecting the properties of clouds.

More cloud condensation nuclei mean more clouds and a colder climate, and vice versa.

Since clouds are essential for the amount of solar energy reaching the surface of Earth the implications can be significant for our understanding of why climate has varied in the past and also for future climate changes.

Cloud condensation nuclei can be formed by the growth of small molecular clusters called aerosols.

It has until now been assumed that additional small aerosols would not grow and become cloud condensation nuclei, since no mechanism was known to achieve this.

The new results reveal how interactions between ions and aerosols can accelerate the growth by adding material to the small aerosols and thereby help them survive to become cloud condensation nuclei.

It gives a physical foundation to the large body of empirical evidence showing that solar activity plays a role in variations in Earth's climate.

For example, the Medieval Warm Period around year 1000 AD and the cold period in the Little Ice Age 1300-1900 AD both fits with changes in solar activity.

"Finally we have the last piece of the puzzle explaining how particles from space affect climate on Earth. It gives an understanding of how changes caused by Solar activity or by super nova activity can change climate," said Henrik Svensmark, from the Technical University of Denmark.