What was done
The authors review reports of numerous solar radiation measurement programs from around the world to determine if there has been any trend in the mean amount of solar radiation reaching the surface of the earth over the past half-century.  They then review a number of studies that describe potential causes of the observed insolation trend, as well as studies that describe the likely agricultural consequences of that trend.

What was learned
The authors detect a significant 50-year trend in solar radiation reception at the earth's surface: a reduction that "has globally averaged 0.51 ± 0.05 W m^-2 per year, equivalent to a reduction of 2.7% per decade, and now totals 20 W m^-2."  This stunning decrease in surface-available solar radiation is large by any standard, and greater by far than that of most negative feedback mechanisms typically suggested to be at work in countering the ongoing rise in the air's CO2 content.

After reviewing materials pertaining to several possible causes of this phenomenon, the authors conclude "the most probable is that increases in man made aerosols and other air pollutants have changed the optical properties of the atmosphere, in particular those of clouds."  With respect to the agricultural consequences of this phenomenon, they say that "while model studies indicate that reductions in productivity and transpiration will be proportional to those in radiation, this conclusion is not supported by some of the experimental evidence."

What it means
The data reported in this study suggest that a huge negative feedback factor - or combination of factors - has been working overtime to stabilize earth's near-surface air temperature in the face of the impetus for warming provided by observed increases in atmospheric CO2 and other greenhouse gases over the past half-century.  These data also call into question the validity of the highly-corrupted near-surface air temperature data set used by climate alarmists to support their claim that the planet has recently experienced unprecedented warming; for how could the earth have warmed at all over the past half-century in the face of such a large reduction in surface-available solar radiation?  Also, in a testament to the resiliency of earth's vegetation, especially when helped along by the ongoing rise in the air's CO2 content (which provides an important aerial fertilization effect), the planet's vegetation - and mankind's crops - have not missed the "missing" solar radiation, showing no signs of any decrease in productivity.