Scientists Trace Exploding Stars’ Effect on Earth’s Climate

Cosmic rays interacting with the Earth’s atmosphere producing ions that helps turn small aerosols into cloud condensation nuclei — seeds on which liquid water droplets form to make clouds. (H. Svensmark/DTU)

(CN) – The byproducts of exploding stars passing through Earth’s atmosphere may be responsible for extreme weather events of the past, according to a new study.

The findings, published Tuesday in the journal Nature Communications, show how atmospheric ions – produced by energetic cosmic rays raining down through the atmosphere – stimulate the formation and growth of cloud condensation nuclei, the building blocks for clouds.

Changes in the atmosphere’s ionization correlated to changes in cloud condensation nuclei, which affect the properties of clouds. More cloud condensation nuclei leads to more clouds and a chillier climate.

As clouds regulate the level of solar energy that reaches Earth’s surface, the impact of cosmic rays on cloud formation can provide insight into why climate has varied in the past – and how it will do so in the future.

Cloud condensation nuclei are formed by the growth of small molecular clusters known as aerosols. Until now, scientists had assumed additional small aerosols would not grow and become cloud condensation nuclei as no mechanism to facilitate this process was known. The new findings, however, reveal both experimentally and theoretically how interactions between aerosols and ions can accelerate the former’s growth by adding material to the molecular clusters, enabling them to survive and become cloud condensation nuclei.

The results provide a physical foundation for the significant body of empirical evidence that show how solar activity contributes to variations in Earth’s climate. For example, the cold period during the Little Ice Age from 1300 to 1900 and the Medieval Warm Period around the year 1000 both align with fluctuations in solar activity.

“Finally we have the last piece of the puzzle explaining how particles from space affect climate on Earth,” said lead author Henrik Svensmark, director of the Center for Sun-Climate Research in Denmark. “It gives an understanding of how changes caused by solar activity or by supernova activity can change climate.”

The key concept of the study is to incorporate the contribution of the mass of the ions to the growth of the aerosols. While there are relatively few ions in the atmosphere, the electromagnetic interactions between ions and aerosols compensate for their limited presence and increase the likelihood of fusion between ions and aerosols.

Even at low ionization levels, roughly 5 percent of the growth of aerosols is attributable to ions. A nearby supernova can contribute to more than 50 percent of the molecules’ growth rate, which will produce a major impact on both clouds and Earth’s temperature.

For the study, the scientists created a theoretical description of the interactions between aerosols and ions, as well as an expression for the growth rate of the aerosols. The ideas were then tested in a massive cloud chamber.

The experimental constraints presented by the walls of the chamber forced the team to measure the change in growth rate as of the order of 1 percent, which presents a high demand on stability during the experiments. The tests were repeated up to 100 times to obtain a clear signal amid unwanted fluctuations.

The findings were captured over the course of two years, with a total of 3,100 hours of data sampling.

The results of the experiments supported the study’s theoretical predictions.


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