"Global warming" has been around as long as there's been an atmosphere.
If you're referring to warming caused by *humans* then it's been around at least since the Industrial Revolution kicked off in the late 18th century. This was counterbalanced somewhat by what climatologists call the "Little Ice Age" (or the latest thereof), which ended around 1850.
But naturally-occurring global warming has always been with us thanks to cyclical changes in the Earth. The Jurassic period was, in all likelihood, warmer than we are now, for example. There is also evidence to suggest that the ancient Mediterranean world, from Mycenean Greece through the Roman Empire, was also a warmer clime than the current residents enjoy.
As for politicizing, there's a lot of that going on at both ends of the scientific theory spectrum. One of the worst offenders is the UN's IPCC, the Intergovernmental Panel on Climate Change, which first caused a major flap in their 1995 report by rewriting the scientists' findings (by one Dr. Benjamin Santer) to exaggerate the dangers, but leaving those scientists' names on the report. In Chapter 8 of said report, those scientists specifically said that their projections were models, and that we didn't know enough about global climate to make any certain predictions; but Dr. Santer altered or eliminated any passages that cast doubt on the conclusions the IPCC was forwarding.
More recently, their 2000 report not only altered its own findings to portray Antarctic ice cores they sampled as being 95 years younger than they were (thus "increasing" their carbon dioxide content) and ignored the "Urban Heat Island Effect"--the established fact that the air around cities generally is warmer--but they also based their projections based on a uniform planetwide model of warming--even though there is little evidence to suggest a warming trend in the Southern Hemisphere.