"I'm sorry." That was a message I delivered on Sunday to the students of Lancaster University, speaking as as a representative of my generation (I’m 48). "We've made a right mess of things." In Britain, and around the globe, we've got three crises all coming together at the same time: our economic, social, environmental systems are all failing.

It’s clear that young people are increasingly understanding this – and that this timing isn’t coincidental, but the result of the failure of decades of free-market politics and economics, which saw greed as good and the natural world as a storehouse to be plundered. The coming together of these crises makes the need for genuine change in our economy and society apparent, and that understanding was evident in Lancaster — with more than 100 students giving up their weekend to talk politics.

We were talking about exciting ideas like the idea of an unconditional basic income for everyone, about cooperative business models, about the need to stop the proposed US-EU free trade deal (TTIP), and much more. This change, although it needs to start now, isn't the work of a year or two, but of decades. It is the responsibility of young people today — and a huge opportunity for them .

I think it would be fair to say that the Green Party message went down well in Lancaster: making sure rich individuals and multinational companies pay their fair share of taxes and giving their workers a living wage, ending and reversing the privatisation of public services (particularly our NHS and bringing the railways back into public hands), defending the free movement of people in the EU and stopping the divisive, dangerous and damaging race to the bottom on immigration rhetoric. The response was a demonstration in person of the way politics is moving quickly — and one result of that is what’s known on Twitter as the #greensurge.

Membership of the Green Party has been growing steadily for a while, but in the past couple of months it has leapt, growing by around 1,000 a week, with numbers now over 80 per cent up on 1 January this year, having just past 25,000. That’s just England and Wales; the (separate) Scottish Green Party has seen an explosion – with membership numbers up by more than 450 per cent since the Scottish referendum.

The referendum has certainly been one factor behind this sea change. The idea that politics should be something that you do, not something that is done to you, is catching on fast, helped by the Scottish example of near-total political engagement.

In pictures: Alex Salmond's campaign for Scottish independence Show all 9 1 /9 In pictures: Alex Salmond's campaign for Scottish independence In pictures: Alex Salmond's campaign for Scottish independence The campaign for independence Alex Salmond stepped down as Scotland's First Minister and the leader of the SNP after the country voted no to independence Getty Images In pictures: Alex Salmond's campaign for Scottish independence The campaign for independence Alex Salmond said he accepted 'the democratic verdict of the people' PA In pictures: Alex Salmond's campaign for Scottish independence The campaign for independence First Minister of Scotland Alex Salmond chats to school children at Strichen Primary School in Strichen PA In pictures: Alex Salmond's campaign for Scottish independence The campaign for independence Scotland's First Minister Alex Salmond delivers his final independence speech to supporters in Perth EPA In pictures: Alex Salmond's campaign for Scottish independence The campaign for independence On the last day of campaigning before the polling booths open, the SNP leader wrote to voters in a final attempt to convince them to vote for independence Getty In pictures: Alex Salmond's campaign for Scottish independence The campaign for independence It was decided to give Alex Salmond an extra year in government AFP In pictures: Alex Salmond's campaign for Scottish independence The campaign for independence Alex Salmond and Alistair Darling taking part in a live television debate in Glasgow on 25 August Getty Images In pictures: Alex Salmond's campaign for Scottish independence The campaign for independence Alex Salmond during the live television debate with Alistair Darling at the Royal Conservatoire of Scotland on 5 August in Glasgow Getty In pictures: Alex Salmond's campaign for Scottish independence The campaign for independence Scottish First Minister Alex Salmond presents the White Paper for Scottish independence Getty Images

And the idea that your vote can genuinely make a difference was also buoyed by the Scottish referendum, with just the threat of a "yes" vote drawing the "Devo-Max" promise from the political establishment. That feeling is what is also driving, I believe, the surge in the Green Party’s poll ratings, which have reached record levels, rivalling and sometimes exceeding the Lib Dem count.

The message I’m hearing with increasing frequency is: "I've been voting tactically for years, but no more." Voters see no point in voting for the party or person they dislike the second-most, in the hope of stopping the party or person they really hate getting in. One reason is that it is increasingly hard to distinguish between those two options – the rhetoric of Labour and Tory might sounds a little different, but the policies are Identikit. When Labour says it will be tougher on welfare than the Tories, and will follow the same failed policy of austerity, what’s the point? Our political class is just continuing business-as-usual politics.

Just look at the past days: the Government has trumpeted its plans for building new roads, despite all the evidence about this being a massive, counterproductive waste of money. They should be investing in walking and cycling, local buses and local trains – helping local economies and communities to thrive.

The Green Party were not invited to participate in the leaders' debates

We also know that new roads will inevitably put us on track to massively overshoot our legally-binding carbon emissions targets. No government can credibly claim to be tackling climate change whilst investing in exactly the kind of high carbon infrastructure that has driven us so close to the point of no return. The good news is that voters are increasingly recognising that it is within their hands to deliver change. Imagine a turnout of 85 per cent next May, with people voting for what they believe in: the Vote for Policies website gives an idea of what a peaceful revolution that could produce.

So it’s simple, really: we have to entirely redesign the system. We have to make the same kind of leap that saw Thatcherism replace social democracy. We haven’t yet got a name for this system change – if I were being cheeky I might suggest “green political philosophy” – but rather than focusing on the label, I think it’s more useful to think about the basic criteria.

I’d suggest these are two: that the new system should provide everyone – in Britain to start with and eventually around the world – with a basic, decent, humane standard of living, and to provide that securely, removing fear. And the total system has to operate within the environmental limits of our one planet. We have to offer a new and inspiring vision of the future to young people. I’m convinced they’re up to, and up for, the challenge.