More on sun-climate relations

Four new papers discuss the relatiosnhip between solar activity and climate: one by Judith Lean (2010) in WIREs Climate Change, a GRL paper by Calogovic et al. (2010), Kulmala et al. (2010), and an on-line preprint by Feulner and Rahmstorf (2010). They all look at different aspects of how changes in solar activity may influence our climate.

The paper by Judith Lean (2010) has the character of a review article, summarizing past studies on the relationship between solar forcing and climate. The main message from her article is that the solar forcing probably plays a modest role for the global warming over the last 100 years (10% or less). It’s a nice overview, but I miss treatment of uncertainties.

Her analysis is based on the HadCRUT3 data, and I wonder if she would get similar results if she chose the GISTEMP or NCDC instead. The choice may in particular be relevant for the discussion of the temperatures after 1998.

Personally, I regard the data on solar activity before 1900 as quite uncertain too. The reason is that there are strange things happening to the solar cycle length in the shift from the 19th to the 20th century. Hence, any analysis based on the past centuries is uncertain because of suspect data quality in the early part of the record. Lean mentions that proxy-based records are uncertain, however.

Another source of uncertainty stems from the analysis itself – a regression analysis with chaotic data can easily yield misleading results. Gavin and I showed in a recent paper that multiple regression can produce strange results when applied to the global mean temperature and a number of forcings.

In other words, I think the reader may get the wrong impression from Lean (2010) that the link between solar activity and climate is better established than the data and methods suggest. Especially when she discusses forecasts for the near future (eg. for year 2014) – I fear that such a discussion can be misinterpreted and misused. However, that’s my view, and it does not necessarily mean that her paper is incorrect – quite the opposite, I think her main conclusions are sound (Her estimate of the solar contribution to the global warming over past century – 10% or less – is in good agreement with the figure Gavin and I got in our analysis).

The positive side is that the paper is probably clearer and more accessible without all these caveats. I also think she makes an interesting point when she discusses ‘fundamental puzzles’ associated with claims of strong solar role in terms of the past warming. She puts this into the context of climate sensitivity, arguing that it would imply that Earth’s climate be insensitive to well-measured increases in GHG concentrations and simultaneously excessively sensitive to poorly known solar brightness changes. Furthermore, Lean argues that it would also require that the Sun’s brightness increased more in the past century than at any time in the past millennium – a situation not readily supported by observations.

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