Of course transpiration increases with increasing plant growth. The point is that it all results in more carbon being extracted from the atmosphere. The Dutch tomato growers know all about this. They increase the carbon dioxide in their greenhouses by a factor of two by burning kerosene. This results in four crops of tomatoes instead of three per season.
Solar radiance is discussed, but only direct changes are considered.
The Nature paper of 3rd June 1999, page 437 by Lockwood, Stamper and Wild reports that the Sun's coronal magnetic field has doubled during the last century. They show a graph of the variations in the total solar magnetic flux emanating through the coronal source sphere from 1870 to date. It deserves to be taken very seriously and casts further doubt upon the current IPCC methods of forecasting the future climate. Heinz Hug and I are currently making progress with our separate studies of the spectroscopy underlying the radiative transfer merchants. There are grave discrepancies between theory and experimental measurements of simple spectra which must be explained before the radiative theorists are to be believed.