Heat and light from the sun drives Earth’s climate. The sun’s output varies with time, and this will obviously affect Earth’s climate, but by how much? Over the eleven year sunspot cycle, total solar irradiation varies by about 0.1%, a small forcing compared to the effect of greenhouse gasses. The sun’s output also varies on longer time-scales, but the amplitude of these fluctuations is less well constrained. It is also possible that small changes in total solar irradiation are amplified through the impact of uv radiation on stratospheric ozone or cosmic radiation on cloud formation. These affects are discussed in more detail by Engels and van Geel (2012).
Past solar activity can be reconstructed from cosmogenic isotopes records; radiocarbon (14C) in tree-rings and Beryllium-10 (10Be) in ice cores. These isotopes are formed by interactions between cosmic radiation and gas in the atmosphere. When the sun is active, more incoming cosmic radiation is deflected by the solar wind, and less cosmic radiation reaches the atmosphere, so less 14C and 10Be are created.
The availability of long records of solar activity, showing more variability than found in the instrumental record, and uncertainty about the effect of solar variability on climate presents an opportunity for palaeo-sciences. Many palaeoclimatologists have taken advantage of this opportunity, and there are many papers purporting to find a relationship between solar activity and climate proxies. These papers are loved by fake climate sceptics, giving them the opportunity to say that it’s the Sun wot done it. I have always been sceptical of these solar forcing-palaeoclimate papers: palaeoclimate data are noisy, autocorrelated and the chronology is always uncertain, and these problems are often underplayed. At least some papers find links on the basis of poor application of statistics.
Engels and van Geel (2012) present a useful review of the palaeoclimatological evidence for the impact of solar variability on climate. However, the evidence is presented rather than critically assessed: it shows that there are many palaeoclimate papers reporting a solar-climate link, but does not determine if this evidence is robust. I want to address this deficit and subject the papers cited in Engels and van Geel (2012), and perhaps others I find that interest me, to critical review. The results will be posted on this blog.