WASHINGTON — Eleven 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 January 27 issue of Eos, published by the American
Geophysical Union, 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 also claimed that current global warming is
not primarily caused by human emissions of carbon dioxide. Their
findings have been widely reported in international news media.

According to Rahmstorf, Shaviv and Veizer’s analyses–and
especially their conclusions–are scientifically ill-founded. The data
on cosmic rays and temperature 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 that current global warming was caused by
human emissions, Rahmstorf says. Even if their analysis were
methodologically correct, their work applied to time scales of
several million years.

The current climate warming has, however, occurred during just a
hundred 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 20th
century was dominant.