The earth has been subjected to many warming and cooling periods over millions of years, none of which were of human origin. Data from many independent sources have mutually corroborated these effects. They include data from coring both the Antarctic ice cap and sediments from the Sargasso Sea, from stalagmites, from tree rings, from up-wellings in the oceans, and from crustaceans trapped in pre-historic rock formations.
The onset of each 100,000-year abrupt warming period has been coincident with emissions into the atmosphere of large amounts of both carbon dioxide and methane greenhouse gases, which absorb additional heat from the sun, a secondary warming effect. Solar radiation would appear to be the initial forcing event in which warming oceans waters release dissolved carbon dioxide, and melt methane hydrates, both of which are present in the oceans in vast quantities. Subsequent declines in radiation are associated with long cooling periods in which the green house gases then gradually disappear (are re-absorbed) into terrestrial and ocean sinks, as reflected in the data from coring the Antarctic Ice Cap and Sargasso Sea.
The current 100 year solar radiation cycle may now have reached its peak, and irradiation intensity has been observed to be declining. This might account for the very recent net cessation of emission of green house gases into the atmosphere starting about 1988, in spite of increasing generation of anthropomorphically-sourced industrial-based green house gases.
While it seems likely that solar radiation, rather than human activity, is the "forcing agent" for global warming, the subject surely needs more study.
In the American Thinker today a nice overview of the different sun and climate cycles known in climate science and how they effect the temperatures on Earth. It's a very interesting read, so I encourage you to go there and read it. But for those in a hurry, here's the summary: