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.
Reason (1) above, which is essentially changes in the Sun itself that affect its direct output, or irradiance (referred to as total solar irradiance, or TSI), is the second-most talked about explanatory reason attributed to climate changes. This one is controversial. While there are many scientists who are increasingly concluding that long-term changes in the Sun’s output (as recorded by sunspot variations) are responsible for centennial-scale warming and cooling periods, including the modern warming (see here for references to 18 such papers published in 2016 alone), there are still many doubters who believe such seemingly small changes in the Sun’s irradiance cannot have a significant effect on the Earth’s climate.
So let’s focus on Reason (2) as an explanatory factor for changes in the Earth’s radiation budget. This one rarely ever gets much attention. Most casual observers don’t think of clouds as an important factor affecting changes in climate. But they are – far more influential than CO2 within the longwave greenhouse effect. The prominent influence of clouds encompasses both Reason (2) and Reason (3), both albedo/shortwave reflectance and longwave (greenhouse) forcing.