The last time I met Caroline Lucas, she was about to stand trial. The Green party co-leader was prosecuted in 2014 following an anti-fracking protest in Balcombe, West Sussex, and was due in court the week after we met, yet her spirits that day were remarkably high. If optimism has been the defining quality of Lucas’s political career, one might say it has served her well: in 2010, she became the first – and remains the only – Green politician elected to parliament and in 2014 she was cleared of all charges.

Her subdued demeanour when we meet this week, therefore, is unfamiliar. The 57-year-old was more upbeat when facing a court case, it strikes me, than she is facing today’s political landscape.

Lucas’s mood may be due in part to the visit she had made earlier in the day to Yarl’s Wood detention centre in Bedfordshire. When we meet, her expression has a sober, preoccupied air, as though she would rather be processing what she had seen in quiet solitude than giving a press interview. But the MP has always shown a talent for reconciling light and shade, having spent her career warning us of impending armageddon, while simultaneously assuring us we still have time to avert it. Her capacity to sustain hope has always impressed me, so I begin by apologising if my questions sound defeatist to her. I put their gloom down to my state of general pessimism. In a barely audible murmur, Lucas admits: “Well, I’m not in such a different place, to be honest.”

The member for Brighton Pavilion had until recently thought the campaign against a third Heathrow runway had been won. “We’d been lulled into a bit of a false sense of security, thinking the government would do – or in this case, not do – what it said it wouldn’t do.” In 2009, the then prime minister, David Cameron, had promised: “The third runway at Heathrow is not going ahead – no ifs, no buts” – but in the Commons last week the transport secretary, Chris Grayling, announced it would be built after all and could be open by 2026. Tory MPs representing constituencies that will be ravaged by the new runway are up in arms; Boris Johnson and Justine Greening and others may yet thwart Grayling’s plan. But, for Lucas, the fight goes way beyond defending villages from bulldozers.

Lucas was found not guilty of obstructing a public highway and a public order offence in relation to an anti-fracking protest in West Sussex in 2013. Photograph: Gareth Fuller/PA

Grayling’s Commons speech did not even mention climate change, yet this omission attracted negligible attention until Lucas tweeted her incredulous dismay – which, I suggest, tells us that most people now think one more runway will make no difference to climate change, but a massive difference to the UK economy. Might they be right? Lucas addresses her reply to the carpet between our chairs, like a pop star performing an old hit she can’t believe anyone could still need to hear again.

“If you measured impact on climate change by each individual action then you’d never be able to talk about the cumulative impact of a set of actions on the climate. We know aviation is one of the fastest growing sources of emissions; we know emissions at altitude are a lot more damaging to the climate than they are at ground level; we know that if Heathrow expands then it’s almost like an arms race between the different airports across Europe, because they’re all in a fight for passengers.”

But we keep being told we must not concede a competitive advantage to rival European airports. She counters wearily: “If you were talking to campaigners in Charles de Gaulle [airport in Paris], they’d tell you they’re told exactly the same thing: don’t concede defeat to London! We’re all being pitted against one another in this incredibly dangerous race to the bottom. If we were to follow the logic of those people who think every time we build a runway our economy miraculously benefits, then why would you not just cover the whole country in concrete? That’s the logic of that argument. The bottom lines is that aviation is a very good example of why you can’t say: ‘We’ll have a demand-led approach’ – because the demand will go on. I think there needs to be a mature conversation about limits to growth. I think we need to ask: growth for what?”

Growth for jobs? Growth for our kids to leave home and afford a mortgage and enjoy the living standards our parents took for granted? “Growth that is not tackling inequality,” she rejoins. “Growth that’s destroying the planet we depend on. Growth that we know, by simply measuring prosperity in terms of GDP growth, is an incredibly blunt instrument. GDP simply measures the circulation of money in the economy, not whether or not the outcome of using that money is positive or negative. A major pile up on the M5 is wonderful for growth, because it means people go out and buy more cars. But by any other measure of what’s useful or helpful, a pile up on the M5 is bad news.”

We’re in danger of losing something incredibly precious. If you don’t have an environment, you don’t have anything else

She does not blame MPs such as Johnson for objecting to the runway on local, self-interested grounds. But if, as is widely predicted, the foreign secretary absents himself from the parliamentary vote by contriving an excuse to be abroad, “I think it would be despicable. He’s promised to stand up for something; he’s gone to the polls and said: ‘This is what I stand for.’ And no one is going to believe that absence from the country was seriously unavoidable. I just think the cowardice of that is grotesque.”

For all Johnson’s ostentatiously theatrical opposition (he promised to “lie down in front of the bulldozers”), it is Jeremy Corbyn whom Lucas believes has the power to determine the third runway’s fate. With the support of Tory rebels, the Labour leader could defeat Grayling’s bill by imposing a three-line whip – and Lucas thinks he will. “There’s a good chance. What I’m hearing is that there’s a good chance Labour might come out against it.” As a long-time Corbyn enthusiast, could Lucas forgive him if he did not? “No. I think it would be unforgivable.”

Corbyn would make a convenient culprit, but should the blame not lie ultimately with her party? The Greens are forever predicting environmental catastrophe – if it is not Heathrow, it is plastic or diesel or eating meat – but when the world continues to turn, despite their apocalyptic warnings, does the environmental movement become a casualty of its own hyperbole?

“I don’t think so. I recognise the danger of crying wolf. But when you look at the data – or, indeed, just look around green spaces – the apocalypse is happening. You don’t hear the same birdsong any more. Not that long ago, if you drove at night through the countryside, your windscreen would become full of moths and now there are no moths any more.”

I wonder how she explains why this does not translate into votes for her party. Which half of her message does the public not buy – that we are heading for armageddon or that we still have time to save the planet? “I think for the past 10 years the public has been struggling just to get by. They’ve been hit by a wall of austerity: struggling to get their kids into school, to get a doctor’s appointment, to keep their job. So, the environment feels like a luxury to be discussed on another day. And I understand that. But, at the same time, we’re in danger of losing something incredibly precious and that ultimately all this other stuff is built on. If you don’t have an environment then you don’t have anything else.”

‘If Heathrow expands then it’s almost like an arms race between the different airports across Europe’ ... Lucas protesting against Heathrow expansion in 2008. Photograph: Mike Webster/Rex/Shutterstock

No one could doubt the sincerity of her commitment. The daughter of Tory-voting, small-business-owning parents, she was converted to environmental politics in adolescence and has devoted herself to the Green party for most of her adult life. And yet last month Lucas announced that she will not be standing for re-election this summer.

“Well, it’s not ‘And yet’,” she objects. Having dedicated her leadership energy to overdue structural reform, she insists she is standing aside in order to allow fresh talent to flourish. This will give her more time to “campaign around nature”, but she adds: “The overarching context right now is Brexit.”

Lucas is a passionate campaigner for what she calls a “people’s vote” – a referendum on the terms the government agrees with the EU for Brexit. Earlier this year, she put the odds of such a vote at 50/50 – and, to my surprise, she believes they have since shortened.

“I do. I think it’s very significant that Labour have refused to rule it out. They have been careful to leave the door open. I think they might well find it would be an elegant way for them to resolve the very uncomfortable position they’re in with so many of their constituencies, particularly in the north, being pro-leave while having a very strong remain support as well.” For the PM, too, “it’s one way of getting herself off a very awkward hook. And the number of people supporting a people’s poll is absolutely growing. It is.”

Her worry is that remain MPs are so focused on the parliamentary mechanics of securing such a vote that “they’re forgetting that, were we to get it, we’d still need to win it. Which is not a given.” Liberated from leadership, she plans to use her time to visit leave areas and “start listening to them, instead of telling them they’re wrong”.

The question this plan does not answer is why the electorate would decide a former leader of the Green party was right about anything. Ever since my childhood, the environmental movement has been celebrating alleged breakthrough victories, from the election of German Greens post-Chernobyl in the 80s, right up to Lucas’s electoral triumph in Brighton. As each has proved premature, is it time to conclude that the cause, no matter how right, is hopeless?

The public has been struggling just to get by. So, the environment feels like a luxury to be discussed on another day

She shakes her head. “It is a long slog. But not hopeless. If we could change the electoral system …” Perhaps, I suggest, the problem is not our electoral system, but human nature. Turkeys don’t want to vote for Christmas.

“No. It’s not human nature. There are Greens in many other countries, in government.” But the US elected Donald Trump! “They’ve got a crap electoral system, too. And Trump didn’t win by the popular vote. So I never think the game is up. But I do think it’s a bloody long slog. And I certainly didn’t think, when I joined the party back in 1986, that by 2018 we’d have one MP. I didn’t think that. But I still passionately believe there’s a massively important role for the Green party. Even within our hopeless system.”

Lucas thinks a Corbyn-led Labour government might introduce a proportional representation system that would translate their 1.1m votes in 2015 into 20 MPs. And yet, I say again, she has chosen not to be their leader.

“I’m not going to be very far away. I’m still going to be sounding off, I hope.”

Does she think she can defeat the Heathrow runway? “I think there’s a perfectly good chance we’ll defeat it.” As I am not sure if this is a prediction or ambition, I ask which way she would bet if she had to gamble £1,000 of her own money.

“Bet £1K on it?” Lucas laughs nervously, buying time, but in the end her old optimism wins.

“Because I’m a bit of a risk-taker, I’ll put my thousand pounds on it not happening.”