To get a deeper understanding of the energy transfer mechanism for the whole global warming process it is necessary to start with studies of the SUN RADIATION, its nature, size and effects. Even if the Sun radiation has its major effect in the long run other occurrences like large volcanic eruptions might have some influence in the short run over limited or larger global areas. The strength of the radiation emitted from the Sun has over time some variations giving detectable problems on Earth – like disturbances of the functioning of electronic equipment - but they can often be regarded as of lower importance in the context of long term global warming.
The processes going on in the Sun plasma continuously produce an immensely large amount of radiation. The radiation consists of quanta, called photons, which are the smallest amount of energy and each one of them can be characterized either as a wave of discrete wave length or as a vibrating particle with corresponding frequency and emitted at a very high speed. The photons are the Sun light and cover a large spectrum of wave lengths/frequencies in the visible part of the sun light area as well as outside thereof. A good part of the latter part is in the infrared area where thermal radiation (heat) is dominating. Figure 2 (published by WIKIPEDIA) gives a view of the spectrum of the Sun radiation entering the atmosphere and at the Earth´s surface in the interesting wavelengths for the global warming (part of the ultraviolet area, the visible light area and part of the infrared area).
Figure 2 by Nick84 - http://commons.wikimedia.org/wiki/File:Solar_spectrum_ita.svg, CC BY-SA 3.0, Link
In an overview of the Sun radiation energy arriving at the Earth globe it is appropriate to emphasize that the varying energy flux entering its atmosphere is about 340 W/m2 (Watts per square meter) and that about 30% of this amount or about 100 W/m2 has been found to be reflected back to space while the rest, or about 240 W/m2, is absorbed by land, water and the atmosphere (including clouds).The increasing amounts of greenhouse gases emitted by human activities do, however, change this energy flux - although only slightly – by increasing the part being absorbed. This increase of the absorption during the last 250 years has been estimated to be about 2.4 W/m2 which corresponds to about only 1% increase of the total absorption. Although comparatively small this amount is significant in terms of global warming and climate change.
When the Sun radiation hits the greenhouse gas molecules energy is transferred from the light quanta (photons) to the greenhouse gas molecules if the photon´s spectrums of wave lengths/vibration frequencies are compatible with enough of the absorption spectra of the gas molecules at discrete values (harmonic vibrations). This process of energy transfer causing “warming”/temperature increase of the greenhouse gases is particularly effective within the infrared area of the light spectrum.
The emissions of the greenhouse gases produced by the burning of oil, natural gas and coal are now generally acknowledged to be the main cause of the global warming (mostly water vapor and carbon dioxide are co-emitted). All the energy of fossil fuels did originally derive from sun radiation energy that has been absorbed and stored in green matter (e.g. in plants) during the photo synthesis process a very long time ago and saved in the Earth´s crust until man did dig/pump them up and burned them. The great role played by the consumption (burning) of the fossil fuels is demonstrated by the following Figure 3 (published by WIKIPEDIA) over the consumption of the world energy from the different sources documented and available. It clearly illustrates the dominating and increasing role which the fossil fuels (coal, oil and natural gas) have for the global warming in comparison with e.g. nuclear and renewables.