Green Party Spring Conference this week. We're in Swansea, soon to be the scene of the election of our first Welsh Assembly member. Rhodri Griffiths, our lead candidate for South Wales West region, opened proceedings with a thoughtful attack on the ruling Labour Party in Wales. First Minister Rhodri Morgan having handed us a gift recently by saying that he sees global warming as a bit of a business opportunity for Wales - perhaps a flourishing wine industry may emerge? Hmmmm.

Exposing ignorance of the global scale of the problem apart, our Green Rhodri was keen to point out that Welsh Labour's lack of care and ethics would have the early pioneers of the Labour Party turning in their graves. It was after all here in South Wales a century ago that Keir Hardie became the first Labour MP, after all.

Rhodri also reminded us that 250 years ago, it was here that the industrial revolution took hold and that, if any country in the world has a duty to make up for the consequences and take a lead in reducing carbon emissions, it should be Wales.

The first panel session of conference followed, and it was a contentious one, also with huge relevance to the area. How do we make use of the tidal energy of the Severn Estuary? It's clear there is enormous potential for generating clean, renewable and, crucially, predictable energy from the flow of the tide in and out of the Severn but it's also one of the most rare and unique habitats in the world, so we have to be careful.

How exactly to harness the energy while doing the least damage to this environment is the big question, and something the government's Sustainable Development Commission is looking at now. It will assess the different options (based on existing evidence) and publish its conclusions later this year.

The question is the subject of disagreement between environmental groups and is also a hot debate within the Green Party, so we have had a special Working Group on the case for the past six months. They are now putting a pair of motions to this conference on Friday to allow us to decide on our position.

The controversy and debate surrounds whether or not to completely enclose the Severn - along with nearly 200 square miles of estuary - with a hydroelectric barrage. The potential for power generation through this route is massive - equivalent to around three new nuclear power stations. However, the plan is guaranteed to permanently disrupt the wetlands and mud flats behind the barrage, reducing the range between high and low tides by half.

Today's panel discussion heard from both sides of the argument.

Insisting he didn't represent 'a plan for a barrage' but calling strongly for a more detailed government-funded review of the Severn, we heard from Jim Redman of the Severn Tidal Power Group, which includes several large engineering companies such as McAlpine and Balfour Beatty.

He did have a lot to say in favour of a barrage though, pointing out the predictability of the energy source, the lack of carbon emissions, the fact no fuel from abroad is needed, and the fact that, with carbon credits at the level currently provided to wind power (via the current Renewables Obligation, or feed-in tariffs as we would prefer - see previous blogs) private-sector energy companies would be willing to fund it. His call for a closer look at the options is supported by Greenpeace.

Peter Jones, from the RSPB for Wales, spoke in a personal capacity about the dangers to the environment of a barrage. His view is shared by Friends of the Earth the RSPB and WWF. The uniqueness of the Severn Estuary, with the second highest tidal range on earth, and home to tens of thousands of birds, means that, legally, for any barrage plan to get approval it would have to show there was no alternative and that there was an over-riding public interest at stake.

A possible alternative was outlined by Peter Ullman of Tidal Electric, a company specialising in enclosed tidal lagoons. He was keen to stress that there is nothing experimental or revolutionary about what his company does - everything used was a 'mature' technology back in the 1920s. The principle is simple - a barrier encloses a small area of shallow water, filling up at high tide, and then the water is let out of the lagoon via turbines when power is needed. A more complex arrangement of interlinked pools is able to make power available for longer than a barrage - around 80% of the time - and the environmental effects are negligible outside the pool itself.

There are well-developed plans for a lagoon in Swansea Bay that would enclose two square miles and generate around 30MW of power. Brig Oubridge, the Green Party member most famous for organising the annual Big Green Gathering, is proposing that the Swansea project is run as a 'public-public partnership' through a company backed by regional and local government with local people as significant investors. He estimates a down payment of £24 million needs to be raised, and that bank loans can complete the £81 million budget. With profits of £13 million a year expected after the loans are repaid, this could generate useful long-term cash for the local community.

To give a sense of the reduced environmental impact of lagoon projects, Peter Ullman superimposed little blue blobs on a photo from Google Earth to show that only fifty square miles within the estuary would have to be enclosed to generate the same amount of power as a barrage enclosing nearly 200 square miles.

Interestingly, the two technologies are not mutually exclusive. Although the area upstream of a barrage would be less viable for lagoon power, the dynamics of the tides mean that, downriver of a barrage, the amount of energy provided by each enclosed area would be increased. The debate is very complex and, even though I grew up around the Severn and am very attached to it - not least to the exciting tidal bore that rushes up river when tides are highest - I'm still not 100% sure how I will vote when the policy motions come up tomorrow.