[esquire (OP)]

decrease the salinity in brine and the electromagnetic wave attenuation also decreases.
will the continued desalinization of the oceans via the fresh water input of melting polar caps,
further weaken the Earth's magnetic shield?

Does increasing the oceans temperature, in conjuction
with reduced salinity, increase the moisture content in the atmosphere? Does this increase moisture
content result in more chaotic climatic swings? 

[evan_au]

will the continued desalinization of the oceans via the fresh water input of melting polar caps,
further weaken the Earth's magnetic shield?

The change in conductivity of Earth's oceans will not be dramatic:
- I see the average depth of the oceans is about 4km
- A severe polar ice melt scenario might see 100m rise in sea level, which is a 2.5% increase (dramatic for coastal cities, but small for the ocean as a whole)
- The same amount of dissolved salt (ie number of conductive ions) exists in this slightly increased ocean depth
- So it won't affect how much magnetic field passes through the ocean

More importantly, Earth's magnetic field is an (almost) steady field
- Conductivity of seawater attenuates varying magnetic fields of high frequency (which is why radio communication to submarines use frequencies like 40-80Hz)
- But seawater does not attenuate steady magnetic fields.

The Earth's magnetic field is generated by movement of liquid iron in the core.
- The convection patterns in the core are continually changing, at a slow rate
- This changes the surface magnetic field, which has been in steady decline since around the year 1700 (ie before the industrial revolution really got underway)
- It has been suggested that if the decline continues at the current rate, Earth could be facing a magnetic field reversal in about 2,000 years
- But the decline is not equal everywhere, and it is likely that even if the "dipole" (North/South) field disappears for a few decades, there could be a residual "quadrupole" field that is at least partially protective for some regions of the Earth's surface.
See the diagram of magnetic field strength at: https://en.wikipedia.org/wiki/Earth%27s_magnetic_field#Magnetic_field_reversals