Earth's recent warming trend might in part be due to a lack of starlight reaching our planet, a new study suggests. But other scientists are not so sure.
According to a theory proposed a decade ago, when a star explodes far away in the Milky Way, cosmic rays — high-speed atomic particles — go through the Earth's atmosphere and produce ions and free electrons.
The released electrons act as catalysts and accelerate the formation of small clusters of sulfuric acid and water molecules, the building blocks of clouds. Therefore, cosmic rays would increase cloud cover on Earth, reflecting sunlight and keeping the planet relatively cool.
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However, because the Sun's solar wind — which shields the Earth from these rays — doubled in intensity during the last century, there has been a reduction in cloudiness, a possible contributor to Earth's warming.
Scientists at the Danish National Space Center mimicked chemistry of the lower atmosphere in a large reaction chamber. They created a mixture that contained gases at realistic concentrations and used an ultraviolet lamp to act as the Sun.
Microscopic droplets, precursor to clouds, started floating in the air of the reaction chamber.
"We were amazed by the speed and efficiency with which the electrons do their work of creating the building blocks for the cloud condensation nuclei," said team leader Henrik Svensmark, Director of the Center for Sun-Climate Research at the Danish National Space Center. "This is a completely new result within climate science."
The results however, may not transfer to natural conditions outside the controlled laboratory environment.
"Studies that have evaluated the claims that global cloud cover is related to changes in cosmic rays find that if you re-examine this matter outside of the brief period which they used, the relationship falls apart," said Raymond Bradley director of the Climate System Research Center at the University of Massachusetts.
Bradley was not involved with the study.
The researchers agree that further study is needed to estimate the contribution of this mechanism to the recent warming of the Earth's atmosphere and oceans.
This work does not mean that there is no human influence on climate, Svensmark told LiveScience. "But it might be necessary to revaluate the climate sensitivity to carbon dioxide."
The study was detailed online this month in the Proceedings of the Royal Society A.
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