Introduction

--Several recent papers have applied correlation analysis to climate-related time series in the hope of finding evidence for causal relationships. For a critical discussion of correlations between solar variability, cosmic rays and cloud cover see [Laut, 2003].

Marsh and Svensmark [2003] provide the state of the art summary of the solar activity $\rightarrow$ cosmic-ray $\rightarrow$ cloud-cover relationship that disclaims the statements of Laut [2003] concerning an older dataset (see also http://www.dsri.dk/response).

--A prominent new example is a paper by Shaviv & Veizer [2003] (henceforth called SV03), which claims that fluctuations in cosmic ray flux reaching the Earth can explain 66% of the temperature variance over the past 520 million years (520 Myr), and that the sensitivity of climate to a doubling of CO$_2$ is smaller than previously estimated.

Shaviv and Veizer's paper was accompanied by a press release titled ``Global warming not a man-made phenomenon", in which Shaviv is quoted stating: ``The operative significance of our research is that a significant reduction of the release of greenhouse gases will not significantly lower the global temperature, since only about a third of the warming over the past century should be attributed to man''.

In our view, public relation releases should not be a part of, or justification for, publications in serious scientific literature. If Rahmstorf et al. believe we have made mistakes in our scientific analyses, they should concentrate on specific scientific points, and not on PR statements. The professional practitioners of PR proclamations should take into account that they too live in glass buildings.

The above notwithstanding, a recent analysis by Shaviv [2004] which includes the comparison between the change in the radiative forcing and temperature change over 6 different time periods (The Phanerozoic, the Cretaceous, Eocene, Last Glacial Maximum, Past Century and the Solar Cycle) yields that all time scales are consistent with a sensitivity of $1.15\pm0.25$ $\mathrm{^\circ C}$  ($0.62$ to $1.86$ $\mathrm{^\circ C}$ at 99% confidence), which is lower than the values obtained in GCMs ($1.5-5$ $\mathrm{^\circ C}$). The estimated $1.4\pm 0.4$ W/m$^2$ of warming attributable to the increased solar luminosity and reduced CRF since 1900 should have therefore contributed about $0.32\pm 0.11$ $\mathrm{^\circ C}$, or roughly half of the observed global warming.

The low sensitivity obtained over different time scales is clearly below the large range obtained in Global Circulation Models. This implies that (a) Earth has shown us that the GCMs do not predict the global sensitivity accurately (This is most likely because of our poor understanding of cloud feedback [Cess et al., 1989]), and (b) Even if we halved the CO$_2$ output, and the CO$_2$ increase by 2100 would be, say, a 50% increase relative to today instead of a doubled amount, the expected reduction in the rise of global temperature would be less than 0.5 $\mathrm{^\circ C}$. This is not significant. Thus, the secondary role of CO$_2$ and lower implied climate sensitivity, as shown by SV03 and corroborated with more research does imply that a ``significant reduction of greenhouse gases will not significantly lower the global temperature".

We should point out that we fully support the effort to cut in the emissions and accelerate the development of alternative energy sources, simply because of real pollution and resource conservation considerations, but this effort should be rational and based on sound scientific research. As openly admitted in the German/Swiss ``manifesto", publicly released by the Potsdam-Institut für Klimafolgenforschung (24.10.2003), the attack on SV03 is motivated mostly by political considerations.