24 October 2003
Prof. J. Beer, EAWAG, ETH Zürich
Prof. U. Cubasch, Institute of Meteorology, Berlin
Prof. O. Eugster, Space science and planetology, Bern
Dr. C. Fröhlich, World Radiation Centre, Davos
Prof. G. Haug, Geoforschungszentrum, Potsdam
Dr. F. Joos, Climate and Environmental Physics, Bern
Prof. M. Latif, Ocean Circulation and Climate, Kiel
Dr. U. Neu, ProClim, Swiss Academy of Sciences
Prof. C. Pfister, Institue of History, Bern
Prof. S. Rahmstorf, Potsdam Institute for Climate Impact Research
Dr. R. Sartorius, Protection of Earth's atmosphere, UBA, Berlin
Prof. C.D. Schönwiese, Institute of Meteorology and Geophysics, Frankfurt
Prof. W. Seiler, Meteorology und Climate Science, Garmisch-Partenkirchen
Prof. T. Stocker, Climate and Environmental Physics, Bern
According to a group of leading climate scientists, a recently published paper by Nir Shaviv and Jan Veizer which claims to show a correlation between the temperature evolution on Earth and cosmic rays is based on highly questionable methods. The influence of CO2 on climate estimated in the article refers only to geologic time scales of hundreds of millions of years. Even if the analyses were correct, no conclusions for the ongoing global warming could be drawn, since on the time scale of decades different factors play a crucial role.
In July 2003, the journal GSA Today published a paper by astrophysicist Nir Shaviv and geologist Jan Veizer which claimed to present a relation between cosmic rays and the long-term temperature evolution on Earth over hundreds of millions of years. The authors claim in addition that the influence of CO2 on climate is much weaker than previously thought. Because this paper was heralded by lobbyists and 'climate sceptics' and was widely cited in the media and in politics, some clarification by climatologists has become necessary.
According to many climate scientists, the analyses made by Shaviv and Veizer and especially their conclusions are scientifically not well founded. The data of cosmic rays and temperature so far in the past are extremely uncertain. The reconstruction of cosmic rays is only based on about 50 meteorites, which are interpreted in a completelyvery very different way by other experts. In addition, the two curves only show a statistical correlation because the time scale of the cosmic data was arbitrarily stretched to obtain a match - the unadulterated data show no significant correlation.
Even more questionable is the attempt of Shaviv and Veizer to estimate the effect of CO2 on climate from the reconstructed data. Since always several forcing factors are acting together, it is not possible to isolate the magnitude of the influence of a single factor by a simple comparison of curves. The application of this method to the much more reliable Antarctic ice core data for temperature and CO2 of the last 420,000 years would result in a warming of more than 10°C for a doubling of CO2 concentration. This is however not a feasible method to estimate the effect of CO2.
On the basis of the well-known radiative effects of CO2, climatologists have concluded that a doubling of CO2 would lead to a warming of 1,5-4,5°C.
Even if the analyses by Shaviv and Veizer were methodologically correct, it would yield no new insights about the ongoing climate change. The authors emphasize themselves that the correlation found is only valid for time scales of several million years. The current climate warming, however, occurs over a time scale of a few hundred years, for which completely different mechanisms are relevant. It is well known that on different time-scales different external forcing factors influence climate. Over millions of years this is, e.g., the shifting of continents, while over hundreds of thousands of years there are changes in orbital parameters. The latter e.g. initiate or terminate ice ages. But for time periods of years, decades or centuries these processes are irrelevant. Volcanic eruptions, changes in solar activity or the concentration of greenhouse gases, as well as internal oscillations of the climate system, are crucial here. Nevertheless, in someone german media reports (WirtschaftsBild, 6 Oct. 2003) the suggested cycle of Shaviv and Veizer has been cited as the cause of the warming of the past two decades. Even if this cycle were real, with 3 degrees C warming over 70 million years it would at most cause a warming of a few millionths of a degree over 250 years.
The strong increase of CO2 (and some other greenhouse gases) in the atmosphere due to anthropogenic emissions is most probably the main cause of the global warming of the last few decades. This conclusion is based on hundreds of scientific studies, and the most important physical processes are well understood. Model calculations as well as data analyses both come to the conclusion that the anthropogenic contribution to the global warming of the 20th century is dominant.
This well-founded scientific knowledge is not called into question in any way by a single, speculative publication based on uncertain data and questionable methods.
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Adresses:
Prof. Jörg Beer, EAWAG, Ueberlandstrasse 133, CH-8600 Döbendorf, e-mail: juerg.beer@eawag.ch - Prof. Ulrich Cubasch, Freie Universität Berlin, Institut für Meteorologie, Carl-Heinrich-Becker-Weg 6-10, D-12165 Berlin, e-mail: cubasch@zedat.fu-berlin.de - Prof. O. Eugster, Abteilung fuer Weltraumforschung und Planetologie, Sidlerstr. 5, CH-3012 Bern, e-mail: otto.eugster@phim.unibe.ch - Dr. Claus Fröhlich, PMOD/WRC, Dorfstrasse 33, CH-7260 Davos Dorf, e-mail: cfrohlich@pmodwrc.ch - Prof. Gerald Haug, Geoforschungszentrum Potsdam , Telegrafenberg, D-14473 Potsdam, e-mail: haug@gfz-potsdam.de - Dr. Fortunat Joos, Klima- und Umweltphysik, Universität Bern, Sidlerstr. 5, CH-3012 Bern, e-mail: joos@climate.unibe.ch - Prof. Mojib Latif, Institut für Meereskunde, Universität Kiel, D�sternbrooker Weg 20, D-24105 Kiel, e-mail: mlatif@ifm.uni-kiel.de - Dr. Urs Neu, ProClim, Bärenplatz 2, CH-3011 Bern, e-mail: neu@sanw.unibe.ch - Prof. Christian Pfister, Historisches Institut, Universität Bern, L�nggassstrasse 49, CH-3000 Bern 9, e-mail: pfister@hist.unibe.ch - Prof. Stefan Rahmstorf, Potsdam Institut für Klimafolgenforschung, PO Box 60 12 03, D-14412 Potsdam, e-mail: rahmstorf@pik-potsdam.de - Dr. Rolf Sartorius, Umweltbundesamt, Schutz der Erdatmosphäre, Postfach 33 00 22, D-14191 Berlin, e-mail: rolf.sartorius@uba.de - Prof. Christian-D. Schönwiese, J.W. Goethe-Universität, Institut für Meteorologie und Geophysik, Postfach 11 19 32, D-60054 Frankfurt a.M., e-mail: schoenwiese@meteor.uni-frankfurt.de - Prof. Wolfgang Seiler, Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe GmbH, Kreuzeckbahnstr. 19, D-82467 Garmisch-Partenkirchen, e-mail: wolfgang.seiler@imk.fzk.d - Prof. Thomas Stocker, Klima- und Umweltphysik Universität Bern, Sidlerstr. 5, CH-3012 Bern, e-mail: stocker@climate.unibe.ch
