Who discovered global warming?
The answer depends on how far back in history you really want to go.
Humans have discussed for some time how human actions can affect climate and that the climate has not always been the way it is today. Greek philosophers noted that humans affect the climate. James Hutton, the father of geology, found evidence of ancient glaciers in areas that would have currently been far too warm to support them.
In 1896 Svante Arrhenius argued that burning of coal and other fossil fuels will add carbon dioxide to the atmosphere and raise the average temperature of the planet. Thus, if any one person discovered global warming, we'd likely give him the credit.
Scientist John Tyndall also played an important role in the history of global warming though. In 1862, he made the discovery that certain gases don't allow heat rays to pass through them, understanding that the earth's temperature is connected to the atmosphere. In 1824, Joseph Fourier noted that the planet would be much cooler if it lacked an atmosphere and is considered to be the first to explain the greenhouse effect.
The term "global warming" was first used in a scientific paper published by Wallace Broecker titled, "Climate Change: Are We on the Brink of a Pronounced Global Warming?." The term became widely known when James E. Hansen testified to Congress in 1988 (this is the same year the UN formed the Intergovernmental Panel on Climate Change).
For a more in-depth telling of the history of global warming, see this Scientific American article.
For a timeline of global warming alongside relevant events in history, click here.
If you really want to learn more, check out the book The Discovery of Global Warming by Spencer Weart .