Discussion

The attribution of climate change to potential causes is a key issue in climate research. It is very important to be able to separate climate effects due to human activities from the response due to natural climate factors and internal climate variability. Climate feedbacks dominate the hydrologic cycle due to the large amounts of water on our planet and its strong impact on the energy balance (van Dorland 2006). Climate change caused by freshwater flux may have been restricted to the early Holocene (Clarke et al. 2003; Wiersma and Renssen 2006).

Since the atmospheric greenhouse gas concentration did not change much before the start of the fossil fuel combustion by people, greenhouse gas forcing was not an important forcing on climate during the Holocene, with the exception of the past few decades. In spite of the fact that single volcanic eruptions are of short duration the combination of many eruptions can have significant climatic effects on annual to decadal time-scales (Crowley 2000b; Robock 2000; Ammann et al. 2003).

On longer time-scales there is an increasing amount of evidence for solar forcing of climate change (van Geel et al. 1999; Renssen et al. 2000; Bond et al. 2001; Mauquoy et al. 2002; Hu et al. 2003; van der Plicht et al. 2004; van Geel et al. 2004a,b; Maasch et al. 2005; Versteegh 2005; Xiao et al. 2006). Climate proxy data indicate that variations in solar activity are an important driving force of variations in climate. The apparently large sensitivity of the climate system to small changes in solar activity indicates that amplification by feedback processes in the climate system has to be taken into account. Two mechanisms have been proposed for this: cosmic ray-cloud correlations and UV-ozone atmospheric circulation changes.

Variation in solar UV radiation controls stratospheric ozone production and may trigger climate change. Haigh (1994, 1999) performed simulations with climate models to study the relation between the 11-year solar activity cycle, ozone production, and climate change. A chemical-atmospheric model showed that a 1 percent increase in UV radiation at the maximum of a solar cycle generated 1-2 percent more ozone in the stratosphere. This increased amount of ozone used as input in a climate model resulted in the warming of the lower stratosphere by the absorption of more UV radiation. In addition, stratospheric winds were strengthened, and the tropospheric westerly jet streams were displaced poleward. The position of these jets determines the latitudinal extent of the Hadley Cell circulation and, therefore, the poleward shift of the jets resulted in a similar displacement of the descending parts of the Hadley Cells. The change in circulation ultimately caused a poleward relocation of mid-latitude storm tracks. The opposite effect to that described by Haigh may have played a role during the climate shift of 850 bc. Reduced solar activity, as indicated by the observed strong increase of atmospheric 14C and 10Be, may have resulted in a decrease in stratospheric ozone. A decrease in latitudinal extent of the Hadley Cell circulation with equatorward relocation of mid-latitude storm tracks would consequently follow (Haigh 1994, 1999; van Geel et al. 1998).

The second, and more controversial, potential amplification factor involves changes in cosmic ray flux (as a consequence of a fluctuating solar wind) and its relationship with global cloud cover. According to Pudovkin and Raspopov (1992), and Raspopov et al. (1997), ionization by cosmic rays positively affects aerosol formation and cloud nucleation. Svensmark and Friis-Christensen (1997) found indications for the possible importance of this process. They recorded a correlation between the variation in cosmic ray flux and observed global cloud cover over the solar cycle from 1983 to 1995. An increase in global cloud cover was thought to cause global cooling. The increase in cloudiness and accompanying cooling corresponds with reconstructed wetter and cooler conditions at mid-latitude in Europe at around 850 bc, when there was an abrupt and steep increase of cosmogenic radiocarbon in the atmosphere, but according to van Geel et al. (2001) the UV-ozone amplification mechanism is more probable than the cosmic-ray-cloud hypothesis. Wagner et al. (2001) also did not find evidence for a cosmic ray climate connection.

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