The Berkley Study is missing a crucial piece. While the magnetic field is getting weaker in the western and southern hemispheres, it is gaining strength in the eastern hemisphere. You may refer to US National Geophysical Data Center calculator . The north magnetic pole in Antarctica measures a drop of 4% between the years 1900 and 2014 (from 69,175 nT at the magnetic pole location on 71 degree South and 148 degree East in the year 1900 to 66,749 nT at the magnetic pole location on 64 degree South and 137 degree East in the year 2014. the south magnetic pole in the Arctic has dropped by 6% (from 61,120 nT at magnetic pole location on 70 degree North 96 degree West in the year 1900 to 57,177 nT at the magnetic pole location on 85 degree North 149 degree West in the year 2014).
The fact that both poles are moving eastward means that the field will have to cover a larger surface area of Earth in the west and a smaller area in the east and therefore an explanation to why the field getting weaker in western and southern hemisphere and vice versa in the eastern and northern hemisphere.
However, the reason behind the drop of 4% and 6% when measured at poles’ locations has another explanation that you may find it at https://planet-earth-2017.com/2014/06/23/current-magnetic-field-map-confirms-3-magnets-configuration/