In 1988 James Hansen gave his famous testimony to the US Senate. For the short story, go to Tamino at Open Mind. For the longer story, Gavin Schmidt at RealClimate, plus the commentary thread is best.
Hansen told the politicians that our production of greenhouse gases, principally CO2, N2O, CH4 and CFC, were warming the climate. He said temperatures would go up in the coming years: Continue reading Hansen got it right