Cosmic rays interacting with the Earth's atmosphere producing ions than can turn small aerosols into cloud condensation nuclei – seeds on which liquid water droplets form to make clouds. It is cascades of secondary particles that ionize molecules when traveling through the air. (Illustration: H. Svensmark/DTU)

New study connect exploding stars, clouds and Earth climate

Cosmic radiation The Solar system Climate change

New research from DTU Space provides better understanding of how cosmic rays can influence Earth´s cloud cover and climate.

A DTU lead team of scientists have found a missing link between exploding stars, clouds and the Earth's climate. The findings have just been published in the journal Nature Communications. The scientists call the new discoveries 'a breakthrough' in the understanding of how cosmic rays from supernovae can influence Earth´s cloud cover and thereby climate on Earth.

The study conducted by Dr. Henrik Svensmark, from DTU Space,  senior researcher Martin Bødker Enghoff, from DTU Space, professor Nir Shaviv at the Hebrew University of Jerusalem, and Ph.D. student Jacob Svensmark at the University of Copenhagen reveals how atmospheric ions, produced by the energetic cosmic rays raining down through the Earth's atmosphere, helps the growth and formation of cloud condensation nuclei – the seeds necessary for forming clouds in the atmosphere.

This is a change in the understanding of how cosmic rays affect climate on Earth. Until now it has been assumed that additional small aerosols - the precursors to cloud condensation nuclei - would not grow to sizes relevant for cloud formation, since no mechanism was known to achieve this. 

The new results reveal, both theoretically and experimentally, 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,” says Henrik Svensmark who is the lead author of the new Nature-study.

A fundamental new approach

The fundamental new idea in the study is to include a contribution to growth of aerosols by the mass of the ions. 

Although the ions are not the most numerous constituents in the atmosphere, the electro-magnetic interactions between ions and aerosols compensate for the scarcity and make fusion between ions and aerosols much more likely. Even at low ionization levels the study shows that about 5 percnt of the growth rate of aerosols is due to ions.

"Finally we may have the last piece of the puzzle explaining how particles from space affect climate on Earth"
Martin Bødker Enghoff, senior researcher DTU Space.

In the case of a nearby super nova the effect can be more than 50 percent of the growth rate, which will have an impact on the clouds and the Earth’s temperature. 

2 years and 3100 hours of testing

To achieve the results a theoretical description of the interactions between ions and aerosols was formulated along with an expression for the growth rate of the aerosols. 

The ideas were then tested experimentally in a large cloud chamber.

Due to experimental constraints caused by the presence of chamber walls, the change in growth rate that had to be measured was of the order of 1 percent, which poses a high demand on stability during the experiments. The experiments were repeated up to 100 times in order to obtain a good signal relative to unwanted fluctuations.

Data was taken over a period of 2 years with total 3100 hours of data sampling. The results of the experiments agreed with the theoretical predictions.           

“Finally we may have the last piece of the puzzle explaining how particles from space affect climate on Earth,” says study coauthor Martin Bødker Enghoff from DTU Space.

“It gives an understanding of how changes caused by Solar activity or by super nova activity can affect climate.” 

The hypothesis explained in short:

• Cosmic rays, high-energy particles raining down from exploded stars, knock electrons out of air molecules. This produces ions, that is, positive and negative molecules in the atmosphere. 

• The ions help aerosols - clusters of mainly sulphuric acid and water molecules - to form and become stable against evaporation. This process is called nucleation. The small aerosols need to grow nearly a million times in mass in order to have an effect on clouds.

• The second role of ions is that they accelerate the growth of the small aerosols into cloud condensation nuclei - seeds on which liquid water droplets form to make clouds. The more ions the more aerosols become cloud condensation nuclei. It is this second property of ions which is the new result published in Nature Communications.

• Low clouds made with liquid water droplets cool the Earth’s surface.

• Variations in the Sun’s magnetic activity alter the influx of cosmic rays to the Earth.

• When the Sun is lazy, magnetically speaking, there are more cosmic rays and more low clouds, and the world is cooler. When the Sun is active fewer cosmic rays reach the Earth and with fewer low clouds the world warms up.


Click here for a film explaining the project (a short film by science journalist Lars Oxfeldt Mortensen).