Cosmic rays boost cloud cover, according to new research.

As cosmic rays rain down from space, they increase the number of ions in Earth's atmosphere, which, according to scientists at the Technical University of Denmark, encourages the formation and growth of cloud condensation nuclei.

The amount of cloud condensation nuclei in the atmosphere directly impacts cloud formation, which affects Earth's climate.

Cloud condensation nuclei are often formed by aerosols, clusters of molecules suspended in the air. Smog, haze, fog, dust and smoke are all common aerosols.

Until now, scientists assumed the smallest aerosols couldn't form cloud condensation nuclei. But a combination of lab experiments and theoretical modeling suggest interactions between ions and aerosols encourage small groups of molecules to become big and heavy enough to form cloud condensation nuclei.

"Finally, we have the last piece of the puzzle explaining how particles from space affect climate on Earth," Henrik Svensmark, a researcher at the Technical University of Denmark, said in a news release. "It gives an understanding of how changes caused by solar activity or by supernova activity can change climate."

It's not the first time Svensmark has made such claims. His previous attempts to link cosmic ray fluctuation with climate change has garnered serious criticism. While several studies have undermined Svensmark's conclusion that cosmic rays can explain global warming, most scientists agree with his claim that cosmic rays can alter cloud formation.

The latest work shows electromagnetic interactions between aerosols and ions encourages aerosols to accumulate particles. The ions trigger nucleation, which prevents molecular clusters from being evaporated. Ions also help the clusters increase their mass.

Models suggest the growth rate of aerosols at low ion levels is only 5 percent. But when a nearby supernovae rains cosmic rays on Earth's atmosphere, ion-inspired aerosol growth jumps to 50 percent.

Researchers tested their models in a large cloud chamber. The results -- detailed this week in the journal Nature Communications -- confirmed their theoretical predictions.

Scientists at DTU claim cosmic rays encourage the formation of low clouds, which are most effective at blocking the suns energy and cooling the planet.

Authors of the latest study have previously argued that changes in solar activity can impact the levels of cosmic rays streaming into the atmosphere, and thus alter cloud formation rates, affecting climate.

"Earth is under constant bombardment by particles from space called galactic cosmic rays," Jacob Svensmark said last year. "Violent eruptions at the Sun's surface can blow these cosmic rays away from Earth for about a week."

"When the cosmic rays are reduced in this way there is a corresponding reduction in Earth's cloud cover," he said. "Since clouds are an important factor in controlling the temperature on Earth our results may have implications for climate change."

The work of Jacob and Henrik Svensmark has been championed by climate science deniers, some of whom believe cosmic variability, not rapidly increasing levels of atmospheric CO2, accounts for long-term changes in Earth's climate.

While many scientists agree that cosmic rays can influence the formation of low-lying clouds, few believe the mechanism can explain global warming. Solar activity and cosmic ray levels have varied predictably -- up and down, up and down -- while global temperatures have consistently increased.

When a pair of British scientists looked at the correlation between cosmic ray flux and temperature change, they found solar activity and its influence on cosmic ray concentrations could be responsible for only a small percentage of temperature variation.

"We conclude that the level of contribution of changing solar activity is less than 10 percent of the measured global warming observed in the 20th century," Terry Sloan, from the University of Lancaster, told the Scientific American. "As a result of this and other work, the Intergovernmental Panel on Climate Change state that no robust association between changes in cosmic rays and cloudiness has been identified."

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