> If this happens, wouldn't we also see dramatically increased levels of solar radiation reaching the earth? It probably depends on how long a reversal requires.
>

Possibly. The magnetosphere, or the geomagnetic sheath of ionizing layers which extends out into space and protects us from the bombardment of radiation from solar wind, is generated somehow by the earth itself. Complete reversal is gradual, taking 3000 to 5000 years. The last one was 780,000 years ago according to the paleomagnetic record. We're overdue for another one -- but the good thing about this is that life in all the major taxa (by class, not necessarily by species) managed to survive the last magnetic reversal.

On the other hand, both Mars and Venus no longer have functional magnetic fields, which may explain their lack of atmosphere -- with no magnetospheres, their atmospheres may have been stripped long ago by the ablation of the solar wind. The earth's magnetosphere field continually regenerates itself, even after it approaches zero.


> And then, to needlessly complicate matters, the draining of lakes has been known to produce alternating magnetic bands almost identical to those found around rifts in the ocean floor, the single best evidence for pole reversals. So maybe we aren't due for a reversal.
>
> gab"Better stock up on our sunblock..."by

The "alternating magnetic strips" which record magnetic reversal are never surface phenomena. The paleomagnetic record is locked into rocks of the ocean floor and in some deep lava flows: it is created when molten paramagnetic magma cools slowly, and the directional "dipole moments" of the iron-rich mineral crystals -- within the cooling lava -- align themselves along the existing field lines of magnetic force. Alternate (reverse) dipole alignments in subsequent lava strata indicate exposure to reversed magnetic fields, at some point in planetary history.

The example of lake drainage only means (1) the lake had eroded through an already existing paleorecord of reversed magnetic strata. However, (2) Why would anyone want to use data from lakebeds as a reliable billenial geological record, since lakebeds are, as I said, a surface phenomena which can be easily changed and destroyed by ordinary weather precipitation and glaciation? There very well might be "alternating magnetic bands" in lakebeds that 'look' like rifts in the ocean floor. But then again, someone is confusing the fact that the shoreline's plunge-and-debris zone can build up -- along the lakesides -- alternating layers of different types of debris (magnetic and nonmagnetic) as the water occasionally cuts through iron-bearing mineral strata. At a later date, the debris layers are compressed into sedimentary rock by the pressure of more water or glaciation. So sure, these lakeside layers have "magnetic banding" but the dipoles they contain are disorganized and RANDOM: they *don't* have the unique dipole-alignment markers of having being created, unidirectionally, under uniquely different timeframes when the intensity was *different* from what it is now. The earth's magnetic field strength is currently around 0.6 Gauss. At various times in the past it has been stronger, and sometimes it has approached zero.

Anyway, it appears humans survived the last reversal 780,000 years ago. Species like birds and water-dependent mammals (which have special sense organs in their noses or brains for detecting changes in the force of magnetic flux) may not be so lucky if magnetic reversal starts happening again.

Wolf "Maybe the Martians are transmitting neural images of red rocks directly into our brains, but we don't have the sense to tell" spirit