//What a ridiculous suggestion that if we try to harvest some of the absolutely immense energy in tides, it will slow the spin of the earth.//
It is not ridiculous, it's a scientific fact. Some of the angular momentum in the Earth's rotation is lost every day as the tides rise and fall. You can see this principle clearly when an ice skater performs a spin and moves her arms in and out. Move out she spins slower, move them in she spins faster. The bulges which are the high tides are equivalent to the skater's arms. It is transferred to the moon (because total angular momentum in a system cannot be gained or lost) which responds by moving away from the Earth. I learnt about this when I was about twelve years old. It isn't a great influence - it's about a second in 50,000 years. But over aeons it makes a difference and it is estimated that when the Earth and moon were first formed the day was about 5 hour long. But I didn't say it was significant. I was simply commenting that you get nothing for free.:
http://burro.astr.cwru.edu/Academics/Astr221/SolarSys/earthmoon.html
A tidal barrage forms artificially high water levels and so adds to this loss of angular momentum. But again, I never said it was significant. But instead of simply trashing a suggestion out of hand it would be better to do some research first.
//So what was NJs question?//
The "bloke in the pub's" question is quite simple: accepting that climate change is real and man made for the sake of this argument, tell me what effect all the radical and expensive measures our current Prime Minister is currently advocating for the UK will have on the global climate. If the issue is so clear cut as suggested, it can't be that hard.