When discussing climate change with a flat earther, inevitably the argument is used that the climate is always changing. Scientists understand this phenomenon about Earth fairly well, it describes how Earths natural changes in movement affect the climate over thousands of years. These natural processes are called Milankovitch after Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he theorized that variations in eccentricity, axial tilt, and precession of the Earth's orbit resulted in cyclical variation in the solar radiation (insolation) reaching the Earth, and that this orbital forcing strongly influenced climatic patterns on Earth.
Well the online environmental publication Grist.org published a highly informative graphic by XKCD.com. Take a look below, gain some perspective and use it whole educating others about climate change.