Mrs. Haigh - in 2.1 and 2.2 I stumbled about your dealing with the famous Hockey Stick graph. Why did you omitt the McIntyre/Wegman discussion of the graph? Or do you still consider this curve to be a valid representation of the last 1.000 years' temperatures?

Also the differences and the discussions between Svensmark and Laut (2.2) do not end in 2000 and with the more interesting part happening after 2000. Given the fact that you are personally involved (1999) in the discussions of Frijs-Christensen/Svensmark data and interpretations this may be a bit difficult for you. But some more clarity would be appreciated.

But I also appreciate your work very much. Thank you. E.G.Pohlhausen

I am grateful to Mr Pohlhausen for his interest in my paper and his kind remarks about my work.

The so-called “hockey-stick” graph which I present in Figure 2 of my article is based on the work of Mann, Bradley and Hughes (1998, 1999). The papers by McIntyre and McKitrick (2003, 2005) found an error in the statistical technique used to derive this reconstruction, and their work was validated by statistician Wegman, but implementation of the correction makes essentially no difference to the curve. A detailed assessment of the input data, the statistical techniques and impacts of various methods/approximations is presented by Wahl and Ammann (2007) who conclude: “Our examination does suggest that a slight modification to the original Mann et al. reconstruction is justifiable for the first half of the 15th century (∼+0.05–0.10), which leaves entirely unaltered the primary conclusion of Mann et al. (as well as many other reconstructions) that both the 20th century upward trend and high late-20th century hemispheric surface temperatures are anomalous over at least the last 600 years.” Given these conclusions I did not feel it necessary to cover such detail in my review article.

I am not aware of any work since 2000 which has validated the approaches used by Friis-Christensen and Lassen. However, there have been some developments in issues related to ionisation of aerosol and its potential development into cloud condensation nuclei: scientists in Helsinki (Laakso et al, 2007) have designed observational techniques for studying neutral and ion-induced nucleation and suggest there may be a real difference in growth rates; experiments are planned at the CERN accelerator to simulate the effects of cosmic radiation on cloud condensation; theoretical models of cloud droplet formation are also making considerable advances (Spracklen and Carslaw, University of Leeds, UK). While many questions still remain, these developments will bring new and quantitative data to assess the possibility of solar-modulation of climate through ionisation of aerosol by galactic cosmic rays.

Laakso L, Gagne S, Petaja T, Hirsikko A, Aalto PP, Kulmala M and Kerminen VM (2007) Detecting charging state of ultra-fine particles: instrumental development and ambient measurements, Atmos. Chem. and Phys., 7, 1333-1345.

Mann ME, Bradley RS, Hughes MK (1998) Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392:779–787.

Mann ME, Bradley RS, Hughes MK (1999) Northern hemisphere temperatures during the past millennium: inferences, uncertainties, limitations. Geophys Res Lett 26:759–762.

Mclntyre S, McKitrick R (2003) Corrections to the Mann et al. (1998) proxy data base and Northern Hemispheric average temperature series. Energy and Environment 14:751–771.

Mclntyre S, McKitrick R (2005) Hockey sticks, principal components, and spurious significance. Geophys Res Lett 32:L03710, doi: 10.1029/2004GL021750.

Wahl ER and Ammann CM (2007) Robustness of the Mann, Bradley, Hughes reconstruction of Northern Hemisphere surface temperatures: Examination of criticisms based on the nature and processing of proxy climate evidence. Climatic Change, 85, 33–69.