What was done
The author reviews what we know about the role of the sun in orchestrating climate change over the course of the Holocene or current interglacial period, including changes that occurred during the 20th century, focusing on (1) the direct effects of solar irradiance variations and (2) the indirect effects of magnetized plasma emissions.

What was learned
With respect to the first phenomenon, i.e., solar irradiance variations, de Jager writes that "the fraction of the solar irradiance that directly reaches the earth's troposphere is emitted by the solar photosphere [and] does not significantly vary." The variable part of this energy flux, as he continues, is emitted by chromospheric parts of centers of solar activity, and "it only directly influences the higher, stratospheric terrestrial layers," which "can only influence the troposphere by some form of stratosphere-troposphere coupling."

With respect to the second phenomenon, de Jager concludes that (1) "the outflow of magnetized plasma from the sun and its confinement in the heliosphere influences the earth's environment by modulating the flux of galactic cosmic radiation observed on earth," (2) "cosmogenic radionuclides are proxies for this influence," and (3) "the variable cosmic ray flux may influence climate via variable cloudiness," as described by the many items archived in the Cosmic Rays section of our Subject Index.

Of these two phenomena, the author seems to lean toward the latter as being the more significant, noting that the Northern Hemispheric temperature history developed by Moberg et al. (2005) "runs reasonably well parallel to" reconstructions of past solar variability derived from cosmogenic radionuclide concentrations, which are proxies for the outflow of magnetized plasma from the sun. Perhaps most interesting of all, in this regard, is de Jager's observation that "never [our italics] during the past ten or eleven millennia has the sun been as active in ejecting magnetized plasma as during the second half of the twentieth century."

What it means
De Jager notes that "a topical and much debated question is that of the cause of the strong terrestrial heating in the last few decades of the twentieth century," and that "it is usually ascribed to greenhouse warming." The weight of his review, however, gives credence to the view that solar activity, especially that associated with the effects of ejected magnetized plasma on the galactic cosmic ray flux incident on the earth's atmosphere, could well be responsible for the bulk of 20th-century global warming, as well as most of the major temperature swings (both up and down) of the entire Holocene.

Reference
Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617.