I don't know much of his perceptions on climate change, but Allegre was a venerable giant in petrology.

As for this:

Pierre:

the magnetic field has been stable now for one of the longest periods in the history of the earth and is weakening like it did before previous inversions

Yes, it is weakening like what is seemingly happening before other inversions, but stable? The last inversion was (top of my head) some 800.000 years ago (Wikipedia check: 780.000 years). That's not particularly long, considering the 37 million years of the Cretaceous Long Normal Superchron. And during the past 780.000 years it doesn't look that stable to me...