Naomi Klein, social activist, author and now environmental campaigner, is not universally popular, even among the big conservation groups – she argues they have come to reflect the values of the corporations whose money they have come to depend on – not to mention those big companies who try to ‘greenwash’ their dirty businesses. But when she spoke in Oxford recently as part of a whistle-stop tour of Europe, the audience, packed high into the galleries, loved her. The staff at the Sheldonian Theatre, the university’s 17th-century ceremonial hall, said they had never seen a speaker received with such rapture by a normally cynical, hard-bitten Oxford crowd.

And, I have to say, it’s hard not to like her. You might expect this world-famous anti-capitalist campaigner to be intimidating – impressive, but remote, unapproachable, overwhelming, and a little scary. How wrong you would be. Her face forms effortlessly into smiles. And along with her undoubted determination, she exudes personal warmth.

She is also a class act. Her new book, This Changes Everything, is the impressive product of five years’ hard work, and it shows. It’s brimming with well-told stories, compelling arguments, inspiring insights, irrefutable evidence – and it will provide a rallying call for climate action for years to come. Like Ed Miliband, Klein speaks without a script. But with her command of her subject, she’s not about to accidentally forget to mention the economy, or anything else. Nor does she pull her punches. On climate change, she began: “There are no non-radical options left… If we stay on the road we are on, then our leading scientists as well as some of our most conservative institutions like The World Bank, the International Energy Agency, PriceWaterhouseCooper tell us that we are headed directly towards four to six degrees Celsius of warming from pre-industrial levels – and at that level, all bets are off…” We are, she said, looking at massive crop failures, famine, whole countries beneath the water, big cities including New York and London in jeopardy.

As to how we got into this mess, she is absolutely clear – it’s the corporations, and politicians’ subservience to corporate interests and the neoliberal, free trade agenda that has ruled the world since round about 1988, when the Intergovernmental Panel on Climate Change was formed.

“The reason we have failed to tackle the climate crisis is that it landed on us at the worst possible time, in the late 1980s, at the very peak of capitalist triumphalism,” she argued, as I began my interview with her in a private room at Blackwell’s bookshop, looking out over Broad Street to the carved stone heads of Roman emperors. “Time magazine had just declared planet Earth ‘man of the year’ and there was a real willingness to take these issues seriously and act to solve global problems.”

But then along came the Canada–United States Free Trade Agreement, which later morphed into the NAFTA, and the whole ideological baggage that went with it. “The idea was there was only one way to run the world – free markets, free trade, privatisation, deregulation, low taxes, investor rights, the cult of consumerism, the cheapest possible everything. But tackling climate change demands the reverse – collective solutions, more regulation, restrained consumption, carbon taxes, and so on.”

And therein, Klein says, lies the tragedy of a quarter-century of climate inaction. The end of the Cold War created the opportunity for a new international order, guided by cooperation, justice and equity, in which the “peace dividend” could be harnessed to tackle real, serious global problems – such as climate change. But that was not the way that the US and its corporate powers saw things. For them it was a chance to exploit their Cold War ‘victory’, extend their imperial reach, and impose their financial domination on the world.

As for putting all that on one side to tackle climate change, well, that had to be a communist plot, because the demands it made were the outright antithesis of freewheeling, devil-may-care capitalism. “As the Heartland Institute realised very early on, if the science of climate change was right it would give the Left the right to do whatever they wanted. So they set out deliberately to undermine the science, or rather people’s belief in the science, using the same techniques used to make people doubt that smoking tobacco caused lung cancer. Climate change denial is not scientific – it is ideological, and funded by corporations.”

Or as Klein puts it in her book, “Climate change detonates the ideological scaffolding on which contemporary conservatism rests. A belief system that vilifies collective action and declares war on all corporate regulation simply cannot be reconciled with a problem that demands collective action on an unprecedented scale and a dramatic reining in of the market forces that are largely responsible for creating and deepening the crisis.”

Just think: all the companies growing rich on oil, coal and gas would have to face fast, deep cuts in their production and write off trillions of dollars in what we now know as ‘stranded assets’ or ‘unburnable carbon’. But that’s not all. Entire economies would need to be restructured to support vast public spending programmes.

“To tackle climate change we need to cut carbon emissions by eight to ten per cent a year and to do that we would need to invest massively in public infrastructure on a scale we haven’t seen since Roosevelt’s New Deal,” Klein says. “We could be 100% renewable by 2030, and then there’s all the public works to become climate change resilient. What we need here is an enormous collective response.”

And that means a complete transformation of national and global economies to focus on the climate imperative. “Eight to ten per cent emissions reduction per year is not compatible with the current system. We don’t want a great depression, but a great transformation. Parts of the economy must grow, parts must shrink, and we must make those decisions.”

And within that transformed economy, with an ethic of ‘reduce, conserve, share and play fair’, people can actually be better off. “Telling the truth about climate change doesn’t make it a heaver lift, but a lighter one. The economy is already letting down the majority of people on this planet and that’s one of the things we will also change,” Klein insists.

But can we really imagine the global powers that be taking on changes of such magnitude? “Is it realistic to insist on transformation rather than incremental tweaks? It’s the only way we can do it! The same brutal logic that foreclosed on so many homes is now foreclosing on our collective home. The fossil fuel industry has five times more carbon in proven reserves than we can burn. We have to confront that, and show people the way to a better life – less pollution, better health, safer jobs. We are fighting for a future that’s better than anything else on offer...

“A crisis this big, this encompassing, changes everything! The stuff we have been told is impossible must start happening right away. And all that stuff we have been told is inevitable has to stop – right away.”

But there is a problem. As emissions have soared, as the signals of climate change become ever more discernible – for example in extreme weather events and melting Arctic sea ice – so the free-market ideology has also advanced and become ever more powerful, its grip on global governance ever more complete, Klein argues. One result: “The same ideology has informed every aspect of our response to climate change, and our ‘solutions’ have been shoehorned into acceptable forms – like individual consumer choice, not collective action, and carbon trading, not carbon taxes – and it has all failed.”

And governments are still pushing for more “free trade” agreements, such as the TTIP (Transatlantic Trade and Investment Partnership) between the European Union and the United States, and the TTP (Trans-Pacific Partnership), the 12-nation agreement currently being negotiated between the governments of the US and 11 other Pacific-facing countries.

“These will make it impossible to effect sensible climate policies,” Klein suggests. “We have tried to solve the climate problem without engaging in the ideological battle, as it offered green groups the path of least resistance. But it has failed.”

The only answer, Klein insists (and this is at the very core of her message) is a “grand coalition of movements – not just climate campaigners, but people fighting so-called free trade, opposing austerity programmes, Occupy Wall Street, people of colour, communities affected by fossil fuel exploitation, fracking, oil refineries… Over 400,000 people came together to march for climate in New York under a banner of ‘Change everything – we need everyone’. Only a broad-based movement can take on the fossil fuel lobby and win. Our problem is that we have been treating this as a carbon problem when the truth is, it’s a capitalism problem.”

There is, I can’t help thinking, a smidgeon of a problem in all this. Most campaigners I know (and thanks to my job that’s quite a few) already have their hands full with whatever it is they are already campaigning about, whether it’s fair trade, over-fishing, land grabs, getting decent pedestrian and cycling provision, the planning system, transgender rights, housing justice, nuclear weapons, racial discrimination, genetically modified organisms, militarism, the occupation of Palestine, keeping the National Health Service public, fracking, dangerous drivers, social services cuts, air pollution... You get the picture?

Trouble is, it’s hard to imagine them suddenly finding the time and energy to take on the cause of climate change as well as everything else. And, frankly, it’s unreasonable to expect them to. And then, even if they do come together to campaign on climate, they have to agree on what it is they want to do about it. We may broadly agree that we want to make a rapid transition to a fairer and more equal world, powered by clean energy, sustainably farmed, richly forested, and so on, but exactly how are we to get there?

This is where, sadly, agreement is likely to break down. Climate campaigners are already riven between those who believe we must rely on a massive expansion of nuclear power and those who think it is a dangerous distraction. (Klein and I are both in the latter camp.) Then should we keep the failed Kyoto Protocol, or ditch it and develop a new system that actually works? Are market-based solutions intrinsically flawed, or have all the ones we have tried so far just been badly designed? Should we regulate carbon where it’s burnt, or where it comes out of the ground?

I never got the chance to throw this barrage of questions at Klein, which is perhaps just as well, but I did get to ask how high she rated her – or rather our – chances of ultimate success. “I wrote my book on the premise that what we are doing is failing – and a broad, justice-based agenda represents our best way of winning. A good chance? I don’t know. But we have a chance. What matters to me is that there is any kind of chance, however slight.”

This Changes Everything by Naomi Klein is published by Allen Lane.

Oliver Tickell is the editor of the Ecologist website.