Firstly, the precautionary principle is not just about risk assessment of human projects, since they relate to the environment - ie everything on the planet. It is not about whether we need to proceed with caution - it is about whether we should proceed with our projects at all.
Second, on the evidence for and against human-induced climate change, we tend to be self-deceivers: that we believe what we want to believe is alas dreadfully true. All science is ideology (I say this reluctantly since I am sympathetic to Descartes).
Arguments about climate change are used by the various social cliques that struggle for political power. Of course, actual truth might well reside in those scientific arguments, if we could but recognise it. I would argue that just as human activity has damaged the planet in other ways - the increase in cancer caused by industrial processes and materials, the reduction in biodiversity, damage caused by acid rain and so on - the idea that human activity either does not affect the earth's climate or only affects it in a way that is not damaging to life is rather laughable.
Those who say climate change may be a good thing, that we should be more positive about humanity are dangerous (if they have social power) self-deceivers looking for arguments to defend their own projects. Furthermore, these arguments are often an unconscious obfuscation. These self-deceivers are often really saying 'I am going to do what I like' because their mothers (or Mother Nature) 'will always come along and clean up after me'.
As for architecture, most architectural projects (indeed, most human projects) subjected to the precautionary principle probably should not go ahead in the form envisaged - we may need more homes, but not those kind of homes.
David Humphreys BArch, MA