No, it's because we can't get new data from the past. There has to be a statistical analysis on what the average temperature looks like given the fact that a 150 years ago people did average temperatures by throwing wooden buckets off sailing vessels.
That information was recorded and it means something. We can also look to other measurements, atmospheric measurements, and geologic measurements and get a better idea of what the average temperature would have looked like given the information we have. Each data point has to be analysed, accounted for, and given an error.
No, it's because we can't get new data from the past. There has to be a statistical analysis on what the average temperature looks like given the fact that a 150 years ago people did average temperatures by throwing wooden buckets off sailing vessels.
That information was recorded and it means something. We can also look to other measurements, atmospheric measurements, and geologic measurements and get a better idea of what the average temperature would have looked like given the information we have. Each data point has to be analysed, accounted for, and given an error.