According to the IPCC (2007), changes in climate occur as a consequence of variations in the Earth’s radiation budget (solar energy absorbed by versus leaving the surface). Changes in the Earth’s radiation budget occur for 3 primary reasons; two of those three reasons involve solar forcing.
“Global climate is determined by the radiation balance of the planet. There are three fundamental ways the Earth’s radiation balance can change, thereby causing a climate change:
(1) changing the incoming solar radiation (e.g., by changes in the Earth’s orbit or in the Sun itself),
(2) changing the fraction of solar radiation that is reflected (this fraction is called the albedo – it can be changed, for example, by changes in cloud cover, small particles called aerosols or land cover), and
(3) altering the longwave energy radiated back to space (e.g., by changes in greenhouse gas concentrations).”
Reason (3) is, of course, the one that gets nearly all the attention from those who wish to characterize climate changes as primarily influenced by — or caused by — human activity. That’s where the 100 parts per million change in atmospheric CO2 concentration since 1900 comes in. According to the latest IPCC report, the total amount of radiative forcing attributed to changes in atmospheric CO2 concentrations since 1750 (through 2011) is just 1.8 W m-2. Again, that’s the total accumulated radiative effect attributed to CO2-forcing of climate changes over the last 260 years.
To put this into context, consider that the total amount of radiative forcing attributed to the +22 parts per million CO2 increase for the 2000-2010 period is claimed to be just 0.2 W m-2 by Feldman and co-authors (2015):
“Here we present observationally based evidence of clear-sky CO2 surface radiative forcing that is directly attributable to the increase, between 2000 and 2010, of 22 parts per million atmospheric CO2. … The time series both show statistically significant trends of 0.2 W m−2 per decade (with respective uncertainties of ±0.06 W m−2 per decade and ±0.07 W m−2 per decade)”
Remember that. CO2 climate-forcing amounts to merely 0.2 W m-2 per decade with a 22 parts per million increase in atmospheric concentration during the first decade of the 21st century, when there was a pause in global warming.