No, global warming is not a myth, despite previous comments to the contrary.
The story of the mainstream science on this issue begins in 1811, when an William Charles Wells began an investigation of how dew forms, providing an early insight into radiation exchanges between atmosphere and the earth's surface. Other milestones came in 1826, (Joseph Fourier); 1860, (John Tyndall); 1896, (Svante Arrhenius); and 1938 (Guy Callendar.)
In 1958 measurements of atmospheric CO2 began at Mauna Loa. From 1959 to 2011, concentrations have risen from 316 parts per million to 392 ppm. In all, human contributions have raised CO2 about 40% from pre-Industrial levels.
By the end of the 1980s it was clear that the climate was warming. International efforts to investigate this question began, leading to several treaties, including the Kyoto Protocol of 1997, and the International Panel on Climate Change was established. To date, the IPCC has issued 4 Assessment Reports summarizing the science in the professional literature. (The IPCC does not perform primary research, and has no scientists on staff--its authors--hundreds of them per AR--volunteer their time.)
Today, there is a robust 30+ year warming trend; the first decade of the millennium was the warmest on record, and 2010 was the warmest year in many of the records. Glaciers are in retreat around the world; the Arctic sea ice is in sharp retreat; animals are adjusting their ranges to warmer temps (as are gardeners); absolute humidity, drought and extreme precipitation are increasing as predicted by climate models.
There is also a documented PR effort to deny all of the above, funded primarily by Big Energy--tax documents show that organizations such as the coal company Massey Energy, Exxon and (above all) the Koch brothers have spent many tens of millions of dollars to attempt to discredit the mainstream science. It is an effort which precisely parallels the efforts of Big Tobacco to discredit the fact that smoking causes cancer. Sadly, this effort has had considerable success.
A great resource to find out more on the topic is this website:
There are also a number of Hubs on the question; I've written some and there is an excellent series by William R. Wilson: