Just about anything is possible to imagine. If you want an answer that is
consistent with natural law, then that answer is an unequivocal *no*. If you
don't require that natural law be obeyed, then you can say anything you want
about the matter because then science has nothing to then say. So in order
to be able to apply science to the issue I will assume below that you want
natural law to be obeyed.
First of all, the very existence of a vapor canopy with any significant water
content would violate natural law if the surface conditions (temperature and
pressure) were fit for human habitation. Second of all, if there was such a
significant (i.e. Noahically useful) vapor canopy there is no mechanism for
its sudden collapse as the enormous latent heat locked up in the vapor would
have to be removed for it to collapse. The earth is incapable of radiating
that amount of heat into space in only 40 days--even if somehow the sun could
be temporarily turned off to keep any more energy from going into the system.
Third, according Einstein's well-verified theory of general relativity
Newton's constant of gravity G is a universal constant of nature which is
unaffected by any physical process (short possibly of the extreme conditions
immediately after the big bang, i.e. temperatures of the order of 10^32 K
within a Planck time of 10^(-43) sec of the big bang). Even if general
relativity turns out to be wrong and something like the Brans-Dicke theory
of gravity turns out to be correct (not too likely)--even then--the constant
G is determined by the entire distribution of matter throughout the whole
universe, and not by any local process which could take place at the earth.
If by "constant of gravity" and "gravity" in your question you do not mean G,
but rather the local strength g of the earth's gravitational field at the
earth's surface, then there would be a tiny effect if such a vapor canopy
collapsed. For every meter of extra water condensed onto the earth's surface
there would be a net *decrease* in the value of g of 1 part in 2.3 X 10^(-7)
at the earth's surface. This is because the increase in g due to the
increase in mass (1 part in 8.55 X 10^(-8)) is overcompensated by the effect
of the decrease in g (1 part in 3.14 X 10^(-7)) due to the increase in the
earth's radius provided by the extra water. Besides the direct gravitational
effects of mass on the surface value of g there would also be another *tiny*
effect which would tend to decrease the effective value of g, because the
collapse of the canopy would lower the moment of inertia of the Earth/
atmosphere system, and this would result in a slight increase in the earth's
spin rate (from the conservation of angular momentum), and this would
increase slightly the (latitude dependent) centrifugal effect felt at the
surface which tends to cancel part of the full strength of g -- resulting in
a lower effective value for g. Since the canopy collapse would not create
any additional matter (unless you wish to postulate the violation of certain
physical conservation laws) the strength of the earth's gravitational field in
space away from the atmosphere would be completely unaffected by the canopy
collapse. (Thus, for instance, the moon's orbit would be completely
unaffected by the collapse.)
In any case, these direct effects on g at the surface would be completely
swamped by the decrease in bouyant lift that a land-based "megafauna" would
experience caused by the decrease in the surface density of the air. The
reduction in the surface air pressure produced by the condensation of each
meter of liquid water is 0.097 atm. For this decrease in pressure there is a
corresponding decrease in the surface density of the air which would make a
proportionate decrease in the bouyant lift. Currently with 1 atm of pressure
the air provides a bouyant lift of 0.129% of a land-based fauna's weight
(assuming the fauna has a density close to that of liquid water). For a
water-based fauna there would be a smaller decrease in bouyant lift caused by
the dilution of the (high density) seawater with the (low density) fresh
water from the canopy. If, for some reason, after it condensed the fresh
canopy water could remain floating on the original seawater without mixing
and the original sea creatures remained at their original depth (measured
from the top of the salty part), then they would experience no change in
bouyancy lift whether the canopy water was in the air or on the ocean as a
liquid. Of course eventually the collateral effects of the canopy collapse
could disrupt the heating and cooling cycles of the oceans which would change
their temperature and thus make a slight density change in the seawater with
a slight corresponding change in its bouyancy effect.
As far as the contribution of the earth's magnetic field to the strength of
its gravitational field goes, this is a *completely negligible effect*. The
energy of contained in the earth's magnetic field contributes something like
1 part in 10^(-22) to the earth's total gravitational field strength.
Clearly, it makes no significant difference whatsoever (regarding the
gravitational field strength that is) whether or not the earth's magnetic
field is increasing, decreasing, or whatever.
David Bowman
dbowman@gtc.georgetown.ky.us