Role of Cosmic Rays in Climate Change Refuted
A group of Earth and space scientists say that a recent paper attributing most climate change on Earth to cosmic rays is incorrect and based on questionable methodology. Writing in the 27 January issue of Eos, Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research and colleagues in Canada, France, Germany, Switzerland, and the United States challenge the cosmic ray hypothesis.
In July 2003, astrophysicist Nir Shaviv and geologist Jan Veizer wrote in GSA Today that they had established a correlation between cosmic rays and temperature evolution over hundreds of millions of years. They said that cosmic rays-high-energy particles that bombard Earth from space-normally play a part in cloud formation near Earth’s surface, thus leading to cooler temperatures. But, they theorized, during times of intense solar activity, fewer cosmic rays reach Earth, leading to warmer temperatures (such as during the twentieth century). Shaviv and Veizer also claimed that current global warming is not primarily caused by human emissions of carbon dioxide. Their findings have been widely reported.
According to the authors of the Eos paper, Shaviv and Veizer’s analyses-and especially their conclusions-are scientifically ill founded. The data on cosmic rays and temperature from so far in the past are extremely uncertain, he says. Further, their reconstruction of ancient cosmic rays is based on only 50 meteorites, and most other experts interpret their significance in a very different way, he says. He adds that two curves presented in the article show an apparent statistical correlation only because the authors adjusted the data, in one case by 40 million years. In short, say the authors of the Eos article, Shaviv and Veizer have not shown that there is any correlation between cosmic rays and climate.
As for the influence of carbon dioxide in climate change, many climatologists were surprised by Shaviv and Veizer’s claim that their results disproved the belief that current global warming was caused by human emissions, Rahmstorf says. Even if their analysis was methodologically correct, their work applied to time scales of several million years.
The current climate warming has, however, occurred over just 100 years, for which completely different mechanisms are relevant, he says. For example, over millions of years, the shifting of continents influences climate, while over hundreds of thousands of years, small changes in Earth’s orbit can initiate or terminate ice ages. But for time periods of years, decades, or centuries, these processes are irrelevant. Volcanic eruptions, changes in solar activity, and the concentration of greenhouse gases, as well as internal oscillations of the climate system, are crucial on this scale.
The 11 authors of the Eos article affirm that the strong increase of carbon dioxide and some other greenhouse gases in the atmosphere due to manmade emissions is most probably the main cause of the global warming of the last few decades. The most important physical processes are well understood, they say, and model calculations as well as data analyses both come to the conclusion that the human contribution to the global warming of the twentieth century was dominant.
Copyright American Meteorological Society Apr 2004