TCS Daily

Cosmic Ray Days

By Kenneth Silber - July 14, 2003 12:00 AM

The global warming debate has been complicated in recent years by a growing body of evidence that the sun's variability is a major factor in climate change. Some recent research affirms this emphasis on the sun, but also suggests that the galaxy at large needs to be taken into account in assessing the nature and risks of global warming.

The research, performed by physicist Nir Shaviv of Hebrew University in Jerusalem and geologist Ján Veizer of the University of Ottowa and Ruhr University in Germany, indicates that Earth's climate is profoundly affected by cosmic rays, high-energy particles emitted by stars throughout the galaxy. Such rays evidently have a cooling effect on the Earth's surface, by causing increased low-level cloud formation. However, heightened solar activity diminishes the cosmic rays reaching Earth. According to Shaviv and Veizer, this blocking of cosmic rays has been the dominant cause of global warming during the past century.

In an article [PDF] in the July issue of GSA Today, a publication of the Geological Society of America, the scientists correlate cosmic ray flux with Earth's climate history over the past 550 million years. They conclude that cosmic ray changes account for at least 66 percent of the temperature variation during that period. Indeed, there is a correlation between ice ages on Earth and the passage of our solar system through our galaxy's spiral arms, where the sources of cosmic rays are congregated. Such a passage occurs roughly every 135 million years.

Moreover, using climate models that take cosmic ray flux into account, Shaviv and Veizer estimate carbon dioxide's impact on present-day global warming, putting the impact of a CO2 doubling at a maximum of 1.9 degrees Celsius, and the expected impact at about 0.5 degrees. This is in notable contrast to models used by the Intergovernmental Panel on Climate Change, which estimated the impact of a CO2 doubling at between 1.5 and 5.5 degrees. As Shaviv and Veizer note, their own estimate accords well with that of MIT meteorologist Richard Lindzen, who has put the range at 0.6 to 1.6 degrees.

It should be noted that Shaviv and Veizer's estimate applies to sea temperatures at low latitudes, near the equator, and it is expected that temperature change would be greater in high latitudes. However, according to the IPCC models, the globally averaged warming would typically be only 1.5 times higher than the warming in the tropics. Hence, Shaviv and Veizer's estimate still points to a far lesser warming than that estimated by the IPCC.

Are Shaviv and Veizer right about the role of cosmic rays in driving climate change? The empirical evidence is considerable but, as is often the case in climate research, somewhat open to interpretation. Iron meteorites provide data on cosmic ray flux, sea fossils give indications of carbon dioxide levels, and sediments give clues to ancient temperatures. Airborne and ground-based experiments suggest that cosmic rays encourage cloud formation, but exactly how this happens is unclear. It is hoped that particle accelerator experiments that mimic cosmic ray radiation will elucidate the mechanisms involved.

What is clear is that there remain large scientific uncertainties about both the causes and effects of global warming. Shaviv and Veizer's research further undermines the already shaky supposition that certitude and consensus have been achieved. It is also clear that Shaviv and Veizer's findings do not stem from any particular desire to champion fossil fuels. On his website, Shaviv states that human activity may still be responsible for a third to a half of present-day global warming, and expresses a hope to see fossil fuel use reduced, "even though global warming is not the main reason."

In addition, it is noteworthy that an esoteric astronomy topic, such as the solar system's motion through the galaxy, could end up affecting a debate over environmental problems and policy. It shows the importance of keeping an eye on the really big picture.

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