Bruxley - on the surface, variation in solar forcing is at least a plausible alternate explanation. The sun is what warms the earth, and so variation in its output could be the cause of the warming observed over the last century. But, the problem is, there is little credible evidence that it is an important mechanism.
Those very pages you linked, which are not current (2001), and which don't represent the views of most scientists, go no further than to say that solar forcing may be responsible for 1/3 or 1/2 of global warming:
Some say that as much as 1/3 of the global warming may be the result of an increase in solar energy.
http://www.research.noaa.gov/spotlite/archive/spot_sunclimate.html
Many researchers believe the steady rise in sunspots and faculae since the late seventeenth century may be responsible for as much as half of the 0.6 degrees of global warming over the last 110 years
http://earthobservatory.nasa.gov/Library/SORCE/sorce_04.html
That minority of scientists who 5 years ago were even trying to assert that variation in solar output was causing a significant part of global warming, has largely faded, as the result of critical looks at those publications by other scientists.
Those pages cite the IPCC report of 2001 as evidence for their claim - because that at least gives some credulity to the notion. Without even going there, lets look at precisely what the 2005 IPCC report says in it's conclusions?
Here is a chart of the various contributions to global warming from the 2005 IPCC AR4 report, Summary for Policymakers - "anthropogenic" means caused by man, btw. You will note that solar irradiance is considered to be a minor contributor:
See, page 4, IPCC AR4 report, Summary for Policymakers:
http://ipcc-wg1.ucar.edu/wg1/wg1-report.html
Don't like complex charts? How about text? Again, from the 2005 IPCC AR4 report, Summary for Policymakers
2005 IPCC AR4 Report Summary said:
Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.12 This is an advance since the TAR’s conclusion that “most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations”. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns (see Figure SPM.4 and Table SPM.2). {9.4, 9.5}
See conclusion in section entitled "Understanding and Attributing
Climate Change" on page 10, IPCC AR4 report, Summary for Policymakers:
http://ipcc-wg1.ucar.edu/wg1/wg1-report.html