Critics of Climate Change (so-called "Climate Change Deniers") suggest that weather records have only been kept for the past century and a half. However, this is a bit misleading. First of all, it's a bit ethnocentric. In other regions in the world, some sorts of weather records have been kept for thousands of years, albeit not in the scientific and focused way it's done today. Furthermore, because there aren't such exact human artifacts of numbers left DOES NOT MEAN that we don't know what climate and weather patterns and atmospheric content was in the past.
For example cores taken of the earth's crust reveal considerable climate information. Ice cores taken in glaciers track climate, weather, and atmospheric conditions GOING BACK 100,000 OF YEARS!! We can even extrapolate the flux of CO2 in the atmosphere over this extremely long time period!