We are on the 3.30pm train from Birmingham to London and it feels like a race against time. Outside, the fields are flooded and the rain is lashing down. More bad weather is heading in fast from the west. Two days before, on Wednesday, Virgin Trains called a temporary, early-evening halt to services in and out of Euston and passengers are worried it may do so again.
Aside from a nagging anxiety about getting home, Ed Miliband is having a decent day. Earlier, he dropped in on south Manchester, where Labour had won the Wythenshawe and Sale East byelection with ease, before heading to Birmingham to explain his union reforms to a group of mostly enthusiastic party activists.
Now, however, he wants to discuss something far more important: climate change, its consequences, and his plans to combat its worst effects. Between 2008 and 2010, Miliband was secretary of state for energy and climate change at a time when David Cameron was positioning himself as a believer in everything green. It seems like an age ago that they trod such similar ground.
"I genuinely believed he believed it," he says of the prime minister's ultra-green phase as opposition leader. "He talked a lot about it. It seemed to be close to his heart."
Since then, though, much water has passed under the Tory bridge and Miliband is not at all sure what Cameron believes. The economic crash and recession helped put green politics out of fashion in Conservative circles and austerity made the whole agenda seem, at least to some, like an expensive luxury the country could not afford.
In 2012, Cameron sacked a green energy minister, Charles Hendry, and appointed a climate change sceptic, Owen Paterson, as environment secretary. Where being green had once been a defining mission, for Cameron and others it had become a financial burden and a source of party division. Last year, Cameron was said to have been wandering around Downing Street talking of his wish to be rid of all this "green crap" .
As the wind buffets our carrriage, Miliband describes Cameron's claims to be leading the "greenest government ever" as nothing more than a joke these days. Last week – after more than a fortnight of storms and wall-to-wall media coverage of a country under water – he tuned into the prime minister's press conference and heard Cameron equivocate when asked about the link between climate change and storms and floods.
Cameron said: "I think the point I would make is, whatever your view, clearly we have had and are having some pretty extreme weather. So whatever your view about climate change, it makes sense to mitigate it and act to deal with that weather." Everyone had a right to their own view, but the prime minister would not stick his neck out. Had he lost faith in climate change as the cause of the extreme weather and was he no longer prepared to lead the debate? Miliband was mystified and dismayed.
"It is pretty extraordinary that [in Cameron's case] it has gone from a core conviction, a part of his irreducible core, to a matter of conscience as to whether you believe it or not," he says.
For the Labour leader there is no doubt. "In 2012 we had the second wettest winter on record and this winter is a one in 250-year event. If you keep throwing the dice and you keep getting sixes then the dice are loaded. Something is going on," he says.
Miliband has plenty of experience on climate change. In 2008, he navigated the Climate Change Act, which committed the UK to cut its emissions by at least 80% by 2050, through parliament. It passed with only five votes against.
At that time, all three political parties appeared united about the scale of the problem and the measures that needed to be taken. The act was regarded around the world as a model. But today, squabbling and inconsistent messages from ministers, and pressure from Ukip has split the Tories from Labour and the Liberal Democrats on green issues.
Miliband is keen not to be seen to be laying too much blame at Cameron's door, when so many people are suffering the terrible effects of flooding. It is not a time for political point-scoring but for leadership, he says.
But he struggles to avoid personal criticism, because he believes the Tories' divisions on the issue are having an effect on policy. "The reality is that the action we take as a country depends on whether you believe in climate change. If you believe that the climate has been changing for centuries – and that this is no different – then why would you believe that it is necessary to take all the measures that are required?
"What we have seen for the last couple of weeks is that that [attitude] has impacts.
"So when the government downgrades flood protection, cuts the floods budgets, cuts the adaptation budget – all of those things – that has an impact."
He says it is urgent that a national consensus is rebuilt behind the scientists' view that climate change is to blame for extreme weather – because otherwise the planning about how to respond will continue to be inadequate.
More money, he admits, will have to be spent on flood defence – though he is adamant that a Labour government would find it by reordering priorities rather than increasing overall spending.
He wants the floods to serve as a wake-up call and suggests that the need for national unity on this issue is just as urgent as it is in wartime. "We have always warned that climate change threatens national security because of the consequences for destabilisation of entire regions of the world, mass migration of millions of people and conflict over water or food supplies," he says.
"But the events of the last few weeks have shown this is a national security issue in our own country, too, with people's homes, businesses and livelihoods coming under attack from extreme weather. And we know this will happen more in the future."
Miliband's declared mission is to rebuild a national consensus on the subject. That task is, however, hugely complicated by the divisions in the Tory party between climate change deniers, agnostics and believers.
"The problem is that either denial or dither on climate change will damage the country. Denial is damaging because it means you won't take the steps necessary, but dither is damaging, too, because it means you are half-hearted about taking the necessary measures.
"The science is clear. The public know there is a problem. But, because of political division in Westminster, we are sleepwalking into a national security crisis on climate change."
He calls for "decent people" in the Tory party and the Liberal Democrats to join the cause, "to come forward and say, we can't have this ambivalence any more because it will be disastrous for this country".
