Last Saturday, as Brexit continued to dominate the headlines, Momentum activists sought to draw the nation’s attention to a slightly more pressing issue. The group staged protests outside bank branches across the UK to put pressure on financial institutions such as Barclays to stop “financing climate chaos” after a report revealed that the bank is the largest single lender to fossil fuel companies.

And chaos is exactly what we are facing. On current trends, the planet is set to warm by at least 1.5 degrees by 2030. At such temperatures the environmental systems that sustain human life would start to collapse. Harvests would fail, water cycles would be disrupted, and extreme weather events would become the norm. Huge swathes of the planet would become uninhabitable, killing millions of people and displacing many more.

Historically, climate activists have often been fond of telling us that “we’re all in this together” and that only by working across political and ideological boundaries can we hope to save the planet. They have focused on grand, international summits where politicians, business leaders and bureaucrats meet to decide the fate of humanity and return to their nation states with plans for carbon taxes, emissions trading schemes and nuclear power stations.

But climate change is, and always has been, a class issue. It has been caused by the wealthy, and its effects will fall on the poor. Just 100 companies are responsible for 70 per cent of all carbon emissions. Globally, the wealthiest 10 per cent are responsible for 50 per cent of all lifestyle consumption emissions. In the UK, the top 10 per cent is responsible for nearly 25 per cent of lifestyle consumption emissions, with the bottom 50 per cent responsible for just five.

And it is the poorest parts of the world that will be most ravaged by the effects of climate change. Low-lying states like Bangladesh are incredibly vulnerable to flooding as sea levels rise. Africa’s Sahel region – home to nearly 200 million people – will become desertified as temperatures increase. As brutally demonstrated by Cyclone Idai, the poorest states are also those least able to protect their citizens from extreme weather events.

And if climate change is a class issue, then decarbonisation should be a class project. The only way to halt climate change is to challenge the logic of capitalism itself: that the value of everything – land, knowledge, and even human life – is determined by how much money it can make for the wealthy.

Some argue that such a mission is impossible. Averting the apocalyptic scenario laid out in the recent IPCC report requires reducing global emissions by 45 per cent by 2030 – an incredibly ambitious target. Others are slightly more optimistic, pointing out that the world is currently experiencing something of a “solar revolution”. Costs have fallen by a factor of ten over the last decade and solar power generation is increasing at a remarkable rate of 30-40 per cent per year.

Both the optimists and the pessimists have a point. We do have the technology to prevent our planet from decaying into an uninhabitable hothouse earth. But as long as corporations, banks, and even governments, continue to profit from fossil fuels, decarbonisation will remain a distant dream.

Saturday’s protesters understand this point all too well. In targeting major financial institutions like Barclays, they were attempting to strike at the heart of modern, financialised capitalism. Without access to financial markets, most fossil fuel firms would struggle to survive.

But given the scale of the challenge, we must be far more ambitious. Dealing with the existential threat humanity is facing requires the kind of radical state intervention that no liberal government would consider and no international institution would allow: it requires a global green new deal.

Citizens must pressure their political leaders into implementing a just transition towards a zero-carbon economy. This would mean a huge increase in state spending – in the area of 30 per cent of GDP per year – to decarbonise energy and transport infrastructure and boost investment in green technologies. The costs of such a project should be imposed on the wealthy. This will require tax reform, constraints on capital mobility, and the replacement of private financial institutions with green, democratic, publicly-owned alternatives.

Providing for green growth over the long term would also require increasing public and collective ownership over the most important economic assets. Pension funds must be reformed and democratised so their members can put pressure on private corporations to take climate change seriously. The state should also start to act as an activist investor, using the funds from quantitative easing to buy up corporate bonds and pressuring companies to reduce their emissions. And some industries will need to be nationalised outright to deliver the levels of investment required to make the green new deal a success.

The green new deal must be global – states must work together to achieve these goals. But they will have to do so outside of existing international institutions. The kind of state intervention required to tackle climate change – democratic public ownership over most of the economy, dramatic increases in state spending, and the controls on capital mobility required to achieve this – are not merely frowned upon by the World Bank and the IMF, they are actively prohibited.

Such a mass mobilisation of society’s resources would be unprecedented during peacetime. And it has to be undertaken alongside a fundamental reorganisation of the international system. Were the planet not facing an existential catastrophe, this vision would seem utterly utopian. But the reality is that there is no alternative.