Right now, the world is closer to superpower conflict than at any time since the Cuban Missile Crisis of 1962. Putin's nuclear weapons are on high alert, and he regularly threatens first-use in order to shield his aggression in Ukraine under a nuclear umbrella. American inspections under the New START treaty have been stopped by Russia, and China's furious reaction to Nancy Pelosi's visit to Taiwan has put America on a collision course with this emerging superpower, too. Russia may well use a tactical nuclear weapon imminently on the battlefield in Ukraine if it continues to lose the conventional war, breaking the nuclear taboo which has lasted since 1945. Numerous countries are in flagrant breach of non-proliferation treaties. The geopolitical arms control system that was established at the end of the Cold War has begun to break down.

There are around 12,000 deployable nuclear warheads currently in the arsenals of more than half a dozen countries. A full-scale nuclear exchange would lead to the extinction of much of the biosphere, and the end of human civilisation. Recent research suggests a famine-related death toll of two billion following a nuclear war between Pakistan and India, and five billion following a nuclear war between the United States and Russia. Either would destroy in a week a civilisation that has taken millennia to come into existence.

Despite receiving very little attention - there are no marches, no COPs, and no nuclear Greta - nuclear war is a greater immediate threat to humanity's survival prospects than climate change. Global warming will play out over decades, and humanity has multiple off-ramps as temperatures rise; there are increasing reasons for optimism. The nuclear outlook is far more worrying, not least because virtually the entire world seems to be in denial. Those who grew up during the Cold War have become accustomed to thinking of nuclear armageddon as yesterday's threat. Younger generations simply have no idea what it means, and everyone tries not to think about it. Strikingly, no-one has written a significant book on this issue for decades. The most important issue of our time and humanity's greatest existential risk is simply being ignored.

There is a general sense that the most dreadful impact of a thermonuclear conflict - the nuclear winter - has been somehow debunked, or might be survivable, and that global warming is a greater worry. This is not true. The same climate models that give us future global warming scenarios have been run to simulate nuclear fallout, and their results are unambiguous. In the post-nuclear global darkness, which could be near-total for many months, there will be no photosynthesis. Temperatures will fall below freezing for the entire year over most of the globe. Food system models show that virtually everyone will starve, and there would be no reliable refugia.

What makes the current moment so dangerous is a unique combination of geopolitical circumstance and technological advancement. While the dual superpower confrontation of the Cold War was a relatively comprehensible problem, managing mutually assured destruction between multiple players on a chessboard where the rules of the post-1945 settlement have now broken down is going to be uniquely difficult. Who can doubt that Vladimir Putin, if reduced to cowering in a bunker like Hitler in April 1945, would fire Russia's entire inventory of intercontinental ballistic missiles if that option still remained open to him as a last resort? North Korea and Iran, as pariah states, know that deployable nuclear weapons are their only absolute guarantees of regime survival. Ukraine's experience with the worthless 1991 Budapest Memorandum - which saw it attacked by one of the supposed guarantors of its security after it gave up its legacy Soviet nukes - is a salutary lesson to any would-be disarmers.

There is no way that nuclear weapons can be recalled or banned, or the technology uninvented, despite the strong moral case for complete global disarmament. The methods of creating nuclear arms are too well understood and too easily replicated, and this knowledge will not go away. This does not mean, however, that nuclear war is an inevitability.

Mark Lynas decided to write this book on 25 Feburary 2022, the day after Russia invaded Ukraine. Before that moment, he felt, like most people, that humanity should be able to muddle through the nuclear minefield, and that climate change must remain our top priority. But nuclear conflict is now a much greater existential risk to the human species and planetary survival than any other. Trying to forget about it is no more a solution to the nuclear threat than it was to the climate one. We must and can do better, and Seven Minutes to Winter will outline both the risks of nuclear conflict and the ways these risks can be reduced in credible and pragmatic ways that make conflict less likely, and make a nuclear exchange more limited and survivable if one does take place.

In the same way that Six Degrees was a crucial starting-gun in climate change's battle for hearts and minds, Seven Minutes to Winter aims to start a movement. It may feel impossible now that nuclear weapons risk could be addressed, but very few people took climate seriously 20 years ago, yet today virtually every nation on Earth is engaged in an effort to get off fossil fuels; today we are talking about two degrees, not six. Humanity is cooperating to address climate change, just as it must cooperate to address the risk of nuclear war. It may take a decade or two to build political and grassroots momentum sufficient to put this issue at the top of the world's agenda, but it is an achievable goal - and the effort starts here, with this book.