Those 29 years are just a baseline that's used by convention. You can pick whatever period you want to use as a baseline and it doesn't change the result (i.e.

Those 29 years are just a baseline that's used by convention. You can pick whatever period you want to use as a baseline and it doesn't change the result (i.e. the absolute magnitude of the anomalies will change but not the relative change which is what we are interested in).