Shortly after the Fukushima nuclear catastrophe, the celebrated Guardian journalist George Monbiot wrote one of his most widely read articles. It was entitled 'Why Fukushima made me stop worrying and love nuclear power'. Whatever you might think of his judgement, you cannot fault his sense of the dramatic, or the paradoxical. "As a result of the disaster at Fukushima, I am no longer nuclear-neutral," he announced. "I now support the technology."

Monbiot is not the first environmentalist to back nuclear power as the solution to climate change. But this article gave him an unequalled public profile among his pro-nuclear fellow-travellers, like Mark Lynas and former Greenpeace Director Stephen Tindale. And while his intervention has yet to split a single atom, it sent deep fissures running through the British green movement. Until then, greens of various hues were generally agreed that climate change was the world's greatest environmental threat, or something close to it. But on the question of how to respond, there was no universal accord – rather an agreement to differ, and to let sleeping dogs lie. By coming out so forcefully for nuclear power, Monbiot brought that accommodation to an end. Suddenly Greens had to come down on one side of the nuclear debate or the other, or hover uncomfortably between the two, wondering whose judgement to trust in an increasingly complex and at times vicious debate. The green instinct has always been to oppose nuclear power. But if it really is the only viable way for the world to curtail carbon emissions, then we must surely accept it. To borrow a thought from Sellar and Yeatman's 1066 and All That, it is better to be right but repulsive than romantic but wrong. So this is the question: does the world need nuclear power for us to solve the climate crisis, as Monbiot claims? To borrow a second thought, this time from Margaret Thatcher, must we accept that there is no alternative? Let's look at the figures. In 2010 the world demand for primary energy was equivalent to 12,000 million tonnes of oil (Mtoe), 87% of which was provided by oil, gas and coal. Nuclear power contributed a gross 626 Mtoe, about 5% of the total, while renewables accounted for 935 Mtoe, almost 8%. To solve the climate problem, the world must not only reverse the trend of increasing carbon emissions over the next few decades, but bring them down to less than they are now. So can nuclear power do it? Assume a 2% growth in primary energy demand per year over the next 35 years, and that demand will double to some 24,000 Mtoe. Rely on nuclear power to accommodate all the growth, and knock out 4,000 Mtoe-worth of coal, and it will have to produce 16,000 Mtoe of energy per year – a 25-fold increase on its current level. Today the world has 440 operational nuclear reactors, so 25 times more means 11,000 reactors. To have these in 35 years means building, on average, about one a day. Or in an exponential growth scenario, the world would need to sustain an annual increase of 8% per year in the number of operational nuclear reactors for 35 years. Given that nuclear power generation has flatlined over the last decade, and has sharply declined in the last few years, that looks like a tall order. There are currently plans for about 200 new nuclear reactors around the world, mainly in China, the Middle East and the USA. But few observers expect all of these to be built, since the economics of nuclear power are unattractive to private investors, owing to high construction cost, long lead time, electricity price uncertainty, political hazard and long-term liabilities. Realistically the world might build 100 or so new reactors over the coming decade or so – perhaps one every 35–50 days. Over this same period a similar number of existing reactors will reach the end of their lives and close, leading to a net growth rate close to zero. That does not mean it's impossible to build 11,000 reactors in 35 years if the world dedicates sufficient resources to the task. At a construction cost of about US$10 billion per reactor, we would need to dedicate US$110 trillion, or about two years' gross world product, while also providing for long-term liabilities. But before we seriously consider doing so, we should ask what an 11,000-reactor world would be like. For a start, it would be much more radioactive than it is now. Routine radioactive discharges, for example of gaseous fission products like xenon-133, would be 25 times greater. Serious accidents, such as those at Windscale, Three Mile Island, Chernobyl and Fukushima – the last of which came very close to making Tokyo uninhabitable for decades to come – would become commonplace events. To date the nuclear industry has produced one major radiation release for every 3,000 years of reactor operation. On that basis our 11,000 reactors would give us four such events a year. Safer reactor design would reduce the danger, but as nuclear power reaches into countries where safety standards are not so exacting as in the UK, the US, Russia and Japan, and where suitably trained personnel may be hard to recruit, the risk would surely rise. And what about the nuclear fuel? The only naturally occurring fissile substance, uranium-235, is in short supply, so to power all those reactors we will have to 'breed' new fissile material. This may be done in two ways: by irradiating abundant uranium-238 with neutrons to make fissile plutonium-239, or by irradiating abundant thorium-232 with neutrons to make fissile uranium-233. And to use the newly bred fissile material, it has to be reprocessed – a complex, expensive, hazardous and polluting process that inevitably discharges significant amounts of radiation into the environment. A further hazard is that both plutonium-239 and uranium-233 can be used to make nuclear bombs, so the wholesale expansion of nuclear power and the widespread use of breeder reactors would create an uncontrollable proliferation hazard. The world already has some 2,000 tonnes of weapons-grade plutonium and uranium, and is producing a further 75 tonnes of plutonium per year from its 440 reactors. Just 8kg of plutonium is enough to make a small nuclear bomb, so it is inconceivable that proliferation could be contained securely in a 11,000-reactor world producing enough plutonium for hundreds of thousands of bombs every year. So it seems that this 11,000-reactor world is not only an improbable one, but also decidedly unpleasant. But what's the alternative? Other than nuclear, what other low-carbon energy sources could possibly rise up to the challenge? Renewables? Surely not! Most renewable energy production is from large hydroelectric dams, and there are very limited opportunities for expansion. And in 2010 renewables other than hydro contributed just 160 Mtoe, a mere 1.5% of primary energy. However, non-hydro renewables are growing very fast – up 15% in 2010. And within this figure just three power sources are responsible for most of the growth: wind power, solar PV and solar hot water. From 2005 to 2010, global solar hot water and wind power capacity both grew at 25% per year, while solar PV capacity grew at over 50% per year. If these growth rates were to be sustained for 35 years, wind capacity would rise 6,300-fold from 200 gigawatts (GW) in 2010 to about 1.25 million GW, solar hot water 6,300-fold from 185 GW to 1.15 million GW, and solar PV 40 million-fold from 40 GW to 1.6 billion GW. These figures are not predictions. Exponential growth will not continue for so long, as prime sites for wind turbines and solar panels get used up. Other technologies, such as concentrated solar power, will also become important. And there will be demand-side constraints: the projected 1.6 billion GW of solar PV capacity alone would produce over 3 billion billion kilowatt hours per year, equivalent to a primary energy burn of some 30 million Mtoe – over 1,000 times our projected world primary energy demand in 35 years. We would not even know what to do with so much energy. But while not predictive, the figures are highly indicative of the low-carbon energy choices the world should make. The one, nuclear power, is expensive and becoming more so. It will be a practical impossibility to increase its capacity to a scale big enough to make a real difference to global climate within a realistic time frame. Worse, if we were somehow to build our 11,000 nuclear reactors, we would face the certainty of repeated catastrophic accidents and the spread of nuclear weapons, not to mention unimaginable liabilities for decommissioning and long-term nuclear-waste management. We can fairly say that nuclear power is both repulsive and utterly wrong. The other choice, renewable power, already costs less than fossil fuels for many applications, thanks in large part to generous subsidies in Germany, Japan and other countries, which have had the effect of greatly reducing prices. Solar electricity is now cheaper than power from diesel generators in the tropics and subtropics – and so the rapid spread of solar power across China, India, Africa and Latin America is being driven not by subsidy but by the market. And it is getting cheaper all the time as increased demand, caused by its lower price, stimulates greater competition among manufacturers, technological advance, and even greater price falls, in a delightful virtuous circle. Moreover, renewable energy is free of catastrophic dangers and long- term liabilities. It is both romantic and right. That does not mean that the transition to a renewable energy world will be easy or straightforward. We will need to reconfigure power grids so they operate as networks accepting high volumes of 'embedded generation', not just as distribution systems; to build new long-distance electricity links to smooth out fluctuations in supply and demand; to develop the technologies to convert electrical power into liquid fuels for road vehicles and aviation; to create 'smart grids' in which the demand for power responds to the available supply; to find ways to store surplus power for those days or weeks when the wind isn't blowing and the sun isn't shining; and to waste less of the energy that we produce. All of this will require considerable investment in research, development, manufacture and installation – and will incidentally create many millions of jobs. All the more reason then not to throw our finite national capital into the bottomless pit of nuclear subsidies. Currently 86% of the entire budget of the UK's Department of Energy and Climate Change (DECC) is dedicated to decommissioning old power stations – power stations that have already cost the country dear to build and operate. Any more money we throw at nuclear power now will only create additional liabilities for us and for generations to come. As for renewables, wind, solar PV and solar hot water technologies have already reached a point of no return. The question is not whether they will come to dominate world energy supply, but when. By investing wisely in the critical enabling technologies, Britain can make a huge contribution to bringing that time forward, not just here in the UK but across the world. To really make a difference to global climate, and to achieve energy security and abundance for ourselves and for the wider world, we must wholeheartedly back the renewable revolution – and bring a decisive end to the nuclear nightmare. • Oliver Tickell is the founder of Nuclear Pledge