By Gary O'Donoghue Political correspondent, BBC News, in Liverpool

Sian Berry and Derek Wall are both principal speakers for the Greens

For years, the environment was a fringe issue, mocked by the mainstream media as the preserve of the sandal-wearing brigade.

But over the past 10 years, that's all changed.

Global warming has moved to the centre of the political agenda and all the main parties have embraced green issues, putting them at the heart of the debate.

The problem for the Green Party is that this public awakening has done little to benefit their electoral position.

True, they have seen a steady increase in the number of councillors across the country - about 120 seats in Green hands after last May's local elections.

They can also boast two members of the European Parliament and two members of the London Assembly.

However, the Greens lost ground in the elections for the Scottish Parliament, where they went from seven seats to two.

And there is still no sign of a breakthrough at Westminster.

Like many small parties, they blame the electoral system for this failure to have an impact.

"Proportional representation," they cry - though with just over 1% of the total vote at the 2005 General Election, it's far from clear that PR would deliver them any Westminster seats with that kind of showing.

Appointing a leader

Some within the party blame their troubles on the lack of a proper leadership.

For 20 years, the party has shunned traditional hierarchies, appointing "principal speakers" rather than leaders and deputy leaders.

Robin Harper of the Scottish Greens (r) met the SNP after May's election

That could all change now, as the party is holding an internal referendum to institute a more traditional form of political leadership.

Those in favour say it would allow the party to communicate more effectively; those against fear that the party will end up more concerned about "ego" than "eco".

The result of that vote will not be known until November, though many of the party's most high-profile figures - such as MEP Caroline Lucas and London Assembly member Darren Johnson - are supporting the change.

That debate will undoubtedly be one of the main talking points over the next three days as the party convenes for its autumn conference in Liverpool.