Introduction

One could write an interesting essay on the twists and turns in Sun–Earth climate science, addressing both the instrumental record and the much longer interval of paleoclimate records. The conclusion at the time of this writing with respect to the importance of low-frequency solar variability in the most recent decades, and perhaps up to centuries, might be “Perhaps, but probably small”. The main reasons why uncertainties persist regarding this issue include:

(i) The ˜ 150-year instrumental record is too short to draw definitive statistical conclusions about the connection of any relation existing on the multi-decadal time scale.

(ii) Forcing from anthropogenic greenhouse gases represents a significant overprint on trends since about 1850 CE. Because to first order the trends in proxies for solar activity indices and in greenhouse gas concentrations are similar, there is a statistical degeneracy that leads to ambiguous, and thus potentially misleading, conclusions unless great care is taken.

(iii) A similar problem of statistical degeneracy applies to the Little Ice Age interval of cool conditions during the last millennium (main phase about 1450–1850 CE), when mountain glaciers advanced in many regions and planetary temperatures were about 0.5°C lower (e.g. Jones and Mann, 2003; Hegerl et al., 2007). During the Little Ice Age, solar activity, as inferred from changes in radiogenic isotopes such as 14C and 10Be, appears to have varied similarly to pulses in volcanism and slightly lower carbon dioxide (CO2) levels (further discussed below).