A central topic within the climate debate of the recent years was whether, or to what an extent, the observed changes in climate can be attributed to anthropogenic emission of greenhouse gases or are due to natural variability. This extensively conducted discussion can not be fully summarized here; however, some aspects could be emphasized.

One central argument of the so-called "climate sceptics" is based on observed changes in the sun's activity and a rise in solar radiation intensity. The question arises: to what extent are changes in the sun responsible for the recent warming? Although the rarity of the current episode of high average sunspot numbers (Solanki et al. 2004) may indicate that the sun has contributed to the unusual climate change during the twentieth century, the work of Solanki and Krivova (2003) reveals that solar variability is unlikely to have been the dominant cause of the strong warming observed during the past three decades. A model-based exhaustive study by Allen et al. (2006) concludes that increasing anthropogenic gas concentrations produced a 0.3-0.5 K per century warming from 1906 to 1996 and are the dominant cause of global warming after 1976. In contrast, Scafetta and West (2006, 2007) report that empirical analyses suggest that solar variability accounts for as much as 69% of the twentieth century warming and 25 to 35% of recent warming, globally. The IPCC report (AR4) compares observed continental- and global-scale changes in surface temperature with results simulated by climate models using natural and anthropogenic forcings. One relevant conclusion is (IPCC 2007a): "The observed widespread warming of the atmosphere and ocean, together with ice mass loss, support the conclusion that it is extremely unlikely that global climate change of the past fifty years can be explained without external forcing, and very likely that it is not due to known natural causes alone".

A multivariate analysis using the most reliable estimates of the relevant parameters together with the observed surface temperature record from 1889 to 2006 enabled Lean and Rind (2008) to assess the global and regional responses to different forcings. Their conclusion with respect to the solar influence is that solar forcing contributed negligibly to long-term warming over the past 25 years and about 10% of the warming over the last century. This result is in strong contrast to the 69% estimate of Scafetta and West (2006).

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