'There is," as Shakespeare put it, "a tide in the affairs of men, which taken at the flood leads on to fortune." The exhortation by Brutus to his fellow assassins in Julius Caesar is echoing around Whitehall in the wake of the fall of another all-powerful leader.
This may be the moment, senior ministers say, to capitalise on one of Britain's greatest assets, the 45ft tide that races through the Severn estuary, making it the second best place in the world - after Canada's Bay of Fundy - to harness tidal energy.
By building a barrage, they hope to be able to meet a large chunk of Britain's electricity needs from a single renewable, reliable source. It is just one of a number of clean energy technologies they want to employ to keep the lights on, while cutting back the pollution that causes global warming.
On Wednesday, the Government will publish the next stage of its long-awaited energy White Paper. Until now, debate has focused on whether this will lead to the construction of more nuclear reactors, following Tony Blair's repeated insistence that these hold the key to combating climate change. But the nuclear debate has masked the beginnings of what some experts are calling a renewable energy revolution - the potential scale of which has shocked even Whitehall mandarins.
The revolution is being driven largely by legally binding targets for renewable energy agreed by European leaders in March, to help bring global warming under control. At the time, Tony Blair - who had helped to get them agreed - boasted that they were "groundbreaking, bold and ambitious".
He was not wrong. The targets are much more ambitious than the UK renewable electricity plans set out in Wednesday's White Paper and will mean that they have to be scaled up. Few people - least of all the politicians who agreed the European targets - have fully thought through their implications. And the reality is only now beginning to dawn.
Under the EU agreement they have promised, like other European governments, to meet 20 per cent of national energy consumption from renewable sources by 2020. That seems stretching enough. But because of the difficulty of running cars and heating homes on renewables, most of this target is going to have to be met from the energy used to generate electricity. Experts say that will mean boosting the renewable share of the nation's power from the present 4 per cent to 35 per cent in less than 13 years.
At the same time it is becoming clear that building enough nuclear power stations to replace those that will be taken out of service through old age in the next decade is going to be much harder than the outgoing Prime Minister fondly believed. At present they generate 19 per cent of our electricity: by 2020 this could be as low as 5 per cent unless new ones are built.
But constructing atomic power stations is expensive and slow, and has a history of mammoth time and cost overruns. Investors are not rushing to take up the opportunity, and Gordon Brown has made it clear that there will be no subsidies.
With the changeover at No 10 both the plans and the rhetoric are changing. Ministers are determined to keep the nuclear option open, knowing it may be needed, but are placing their hopes in energy saving and re-newables, both of which feature heavily in the White Paper.
They are concentrating on three main options - the Severn barrage, another form of tidal energy being already tried out off the Devon coast, and wind-power. Energy experts reckon that by 2020 about a fifth of the country's electricity will need to come from wind farms with about another tenth coming from hydroelectric power and tidal power. Wavepower - though potentially a huge resource - is lagging behind.
The advantage of the tides is they rise and fall like clockwork. …