What was done
The author reviews what we know about past climatic responses to solar forcing and their geographical coherence based upon proxy records of temperature and the cosmogenic radionuclides ¹⁰Be and ¹⁴C, which provide a measure of magnetized plasma emissions from the sun that impact earth's exposure to galactic cosmic rays, thereby altering cloud formation and climate, as described in the companion review of de Jager (2005) and in many of the other items we have archived in the Cosmic Rays section of our Subject Index.

What was learned
Versteegh finds that "proxy records provide ample evidence for climate change during the relatively stable and warm Holocene," and that "all frequency components attributed to solar variability re-occur in proxy records of environmental change," notably mentioning, in this regard, the ~90 years Gleisberg and ~200 years Suess cycles in the ¹⁰Be and ¹⁴C records," as well as "the ~1500 years Bond cycle which occurs in several proxy records [and] could originate from the interference between centennial-band solar cycles."

What it means
Versteegh reports that "the contribution of solar forcing to these changes is highly debated and reinforced by the present-day combination of global warming and an exceptionally active sun." Nevertheless, he concludes from his review of the pertinent scientific literature that "long-term climate change during the preindustrial seems to have been dominated [our italics] by solar forcing," and that the long-term response to solar forcing "greatly exceeds [our italics] unforced variability." Hence, there is ample reason to question both the open-mindedness and the sincerity of people who adamantly declare, without equivocation, that anthropogenic-induced increases in atmospheric greenhouse-gas concentrations - and especially that of carbon dioxide - are the primary cause of 20th-century global warming.