Prince Charles is right to speak up on climate change, but some of his ideas are completely off the wall

"Did you know," someone once asked me, "that in medieval Spain, Muslim scholars knew the secrets of the atomic bomb?" Apparently they were too wise to make use of their knowledge, but instead encoded it in the decoration of the Alhambra. In the 20th century, I was told, physicists such as Einstein and Niels Bohr made discreet trips to Granada to unravel the palace's code.

My source was Professor Keith Critchlow, architectural guru to the Prince of Wales, and cited in the latter's book Harmony. The tale of the nuclear Alhambra is not repeated, but the Prince does draw on Critchlow to show that, with only a little fiddling, you can inscribe an equilateral triangle in the cross-section of Chartres Cathedral. Within its plan you can draw a vesica, a shape symbolising – quaint term – the "female organ of birth".

This "sacred geometry" is used to support the book's argument that there is an innate harmony and interconnectedness of all things, known to almost all cultures except for western civilisation from the 17th century on. Our big mistake was to be lured by rationalist theories into forgetting God and putting all our faith in material things. The consequences of this "great divorce" were the industrial revolution, global capitalism and environmental peril for the planet.

At times this book gets very bizarre. Gnostic and alchemical texts such as the Gospel of Mary Magdalene and the Emerald Tablet of Hermes are quoted. The future head of the Church of England puts forward ideas, linking Osiris and Jesus for example, that would once have verged on heresy. Before we came over so stupidly rationalist three centuries ago, such writings could have had him burned at the stake.

At other times the problem is mere amateurishness. A Le Corbusier building said to be in the Indian city in Chandigarh is actually a quite different one in Ahmedabad, nearly 600 miles away. The Pritzker architecture prize becomes Pritzka. As Le Corbusier is cast as a villain, there is no mention of the fact that he shared the Prince's fascination with theories of the golden section and Platonic forms. Touchingly, we read about "vast, as yet unnumbered, creatures with which we share this miraculous planet". I think the Prince meant to say something like "vast, as yet unknown, numbers of creatures", but I like the idea of leviathans roaming the earth, which have bravely escaped attempts to stamp serial numbers on their hides.

When it is not being weird or wonky, Harmony says things with which only nutters, or Republican candidates for the US Senate, would disagree. That there is global warming, that it is manmade, and that it is dangerous, for example. That there might be downsides to the world's food production being run by a small number of enormous companies. That mass extinction of species is a bad thing. He and his assistant authors describe reasonable-sounding efforts at organic farming and seabird-friendly fishing techniques, albeit without convincing that these solutions are equal to the scale of the world's ecological problems. They, or he, go awry again when championing homeopathy and osteopathy. The Prince uses science when it suits him, to establish climate change, and drops it when it fails to support his views on alternative medicine.

The Prince's musings follow a pattern. He treats his views, not always original, as personal revelations. He regards opposing views as cynicism or blindness. He likes to overlook complexity. It is all very much about him: he keeps popping up in photos, like an impeccably tailored Forrest Gump, beside Buddhist temples in Indonesia, or a mothering albatross in New Zealand.

Harmony is far from the smartest book on the environment. It disdains, on sentimental grounds, big cities and genetically modified crops, which the environmentalist Stewart Brand argues are essential. It wouldn't get reviewed in national newspapers if it weren't for its author. You wonder what the point is, until you look at the bare-faced liars who get airtime on behalf of climate change denial, and then think that, maybe, there is value in someone famous stating the basics once again.