Physics: Cosmic Rays

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Physics: Cosmic Rays


Cosmic rays are naturally occurring high-energy particles—protons, helium nuclei, and electrons—that travel at nearly the speed of light. Some scientists argue that cosmic rays may cause cloud droplets to form in Earth's atmosphere, increasing cloudiness when the sun emits more cosmic rays or when the solar system passes through a part of the galaxy where cosmic rays are more abundant. Increased cloud cover might, in turn, affect Earth's climate. As of 2007 climate scientists disagreed about whether cosmic rays are a significant influence on Earth's climate. The large hadron collider, an experimental device under construction at the European Organization for Nuclear Research (CERN) particle accelerator laboratory in Switzerland, due to be completed in 2010, might decide the question.

Historical Background and Scientific Foundations

Scientists have long known that slight changes in the sun's energy output might affect Earth's climate. American astronomer Jack Eddy (1932–) pointed out in the 1970s that the Little Ice Age, a cold period in Earth's climate from 1500–1850, coincided with a historic low point in the number of sunspot numbers known as the Maunder minimum. In 1997, Danish scientists Hen-rik Svensmark (c.1958–) and Eigil Friis-Christensen noted that Earth's global cloudiness had decreased by 3% from 1987–1990, at the same time that cosmic rays had decreased by 3.5% due to the regular cycle of solar activity.

They proposed that cosmic rays might increase cloud cover in this way: Because cosmic rays have high energy, they can strip electrons from atoms when they strike Earth's atmosphere. These stripped atoms, now with positive electric charges, might cause water to condense out of humid air into cloud droplets. This, in turn, might increase the number of clouds, their density, or both.

The cosmic ray theory conflicts with another scientific hypothesis that global warming is caused by increased carbon dioxide in Earth's atmosphere, not by changing levels of solar activity. In 2003 two other scientists, Nir J. Shaviv and Ján Veizer, took the dispute to a new level by arguing that over the last 545 million years, about two-thirds of Earth's climate changes could be attributed to highs and lows in the number of cosmic rays that strike Earth from outside the solar system. In addition to fluctuations in cosmic ray flow from our own sun, cosmic ray changes have occurred throughout Earth's existence over millions of years as the solar system's orbital path around the center of the galaxy take it into and out of the Milky Way's spiral arms.

Modern Cultural Connections

Evidence from several independent sources indicates that changing solar energy output has affected Earth's climate in the past. Moreover, the solar hypothesis and the carbon dioxide hypothesis do not necessarily refute each other: Earth's climate might be influenced by multiple factors. As of 2007, however, disagreement about the climatic effects of solar rays was still a source of controversy.

In 2004 German scientist Stefan Rahmstorf (1960–) and a group of 10 other scientists from the United States, Switzerland, and other countries published a paper disputing Shaviv and Veizer's 2003 cosmic ray climate theory. In 2007 British scientists Mike Lockwood and Claus Fröhlich (1936–) published a study indicating that “the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability,” no matter what mechanism for solar influence on climate is invoked or how that influence might be amplified, as, for example, by cosmic rays. The solar influence on climate, they assert, has decreased over the last 20 years while temperature has increased.

Present-day climate change, the authors contend, is due to human activity, not solar influence.

Nevertheless, the possibility of a cosmic-ray connection to climate during other periods, whether slight or great, remains open. At the CERN particle accelerator laboratory in Geneva, an experiment called CLOUD (Cosmics Leaving Outdoor Droplets) is under construction. Due to be completed in 2010, CLOUD will enable scientists to observe the effects of artificial cosmic rays on air and water vapor and may settle the question of whether cosmic rays can influence cloud formation.

See Also Earth Science: Atmospheric Science; Earth Science: Climate Change.



Kanipe, Jeff. “Climate Change: A Cosmic Connection.” Nature 443 (September 14, 2006): 141–143.

Rahmstorf, Stefan, et al. “Cosmic Rays, Carbon Dioxide, and Climate.” Eos: Transactions of the American Geophysical Union 53 (2004):38–40.

Schiermeier, Quirin. “No Solar Hiding Place for Greenhouse Skeptics.” Nature 448 (July 5, 2007):8–9.

Shaviv, Nir J., and Ján Veizer. “Celestial Driver of Phanerozoic Climate?” GSA Today (July 2003):4–10.

Web Sites

American Geophysical Union. “Cosmic Rays Are Not the Cause of Climate Change, Scientists Say.” (accessed August 8, 2007).

Proceedings of the Royal Society. “Recent Opposite Directed Trends in Solar Climate Forcings and the Global Mean Surface Air Temperature.” (access November 6, 2007).

Larry Gilman

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Physics: Cosmic Rays

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