We’ve had a lot of interest in the Clean Energy Shift, and lots of questions to field via email and social media. In order to address them better for a wider audience, below you’ll find an FAQ on the whole concept addressing some of the issues and challenges that have been raised. I hope you find it useful.

To start with, here’s a few of the key paragraphs from the initial paper:

In 2024, Earth’s global mean surface temperature averaged 1.55 °C above pre-industrial levels2, and the average for 2023–25 is 1.48 °C, perilously close to the limit. Keeping to the Paris target now looks impossible by any realistic measure. Yet this moment should not invite despair. Instead, it demands an urgent reframing of how climate progress is measured and mobilized.

The world today looks very different from that in 2015 when the Paris goal was framed. Although emissions are still rising and global actions on climate change are slow, a lot of progress has been made. Clean energy is expanding rapidly and decarbonization, not fossil fuels, is the new ‘business as usual’. In the first three quarters of 2025, growth in clean electricity generation outpaced that in energy demand for the first time, implying that fossil fuels are being displaced.

We argue that the main focus of climate action in 2026 and beyond should be on accelerating the clean-energy revolution. And the rate at which clean energy displaces fossil fuels in the global economy should become the key measure of climate progress. Here we describe how such progress can be tracked and incentivized using a metric we call the clean-energy shift. Unlike chasing intangible temperature targets, cleaning up the energy sector is a more-focused battle that the world can win.

FAQ on the Clean Energy Shift metric

• What is the exact definition of the CES metric?

  CES is defined as the difference between the percentage annual growth rate of clean energy supply and the percentage annual growth rate of total energy demand over the same time interval. A positive CES indicates that clean energy is expanding faster than overall energy use. If sustained for long enough and at a high enough value, the economy will decarbonise with mathematical certainty.

• Are you suggesting CES replaces existing climate policy, including the Paris targets like 1.5C?

  CES is a different kind of metric that we hope policymakers will find useful as the 1.5C Paris limit is passed in realtime. Temperature targets remain useful normative guideposts, but their achievement can only be assessed probabilistically and over long time horizons, making them poorly suited as near-term performance indicators.CES provides a near-term, operational metric of transition speed. By focusing on annual structural change in the energy system, it offers policymakers a real-time indicator of whether the clean energy transition is accelerating sufficiently to improve long-term temperature outcomes.

• Why do we need a new metric like CES when COP28 already agreed actionable targets like tripling renewables and doubling energy efficiency by 2030?

  CES is complementary to these measures, and helps understand and guide their implementation. But tripling renewables in and of itself does not guarantee any reduction in fossil fuel use if demand rises faster than increased renewable energy supply. This is why CES matters: keeping the metric positive requires that the rate at which renewable and other clean energy increase their supply outpaces increases in overall demand. It is this differential – not increases in renewable capacity and efficiency per se – that guarantee the reduction and eventual elimination of fossil fuels and any delivery of the Paris Agreement’s goals.

• What are the future CES values needed for a net-zero/2 degrees/1.5 degrees pathway to guide policymakers?

In our paper we provide historical CES values and indicate the direction and approximate magnitude needed: CES would need to rise substantially from its recent level of around 4% and keep climbing through the 2030s and 2040s to eliminate fossil fuels by 2050. However, in our paper we chose not to present future pathway numbers – for example CES values guiding towards a 2050 net zero date – because of problems in the way that energy datasets are currently presented. The most widely available datasets use primary energy, which has decreasing relevance for an economy that is moving away from fossil fuels and towards electricity. A more appropriate dataset would be for useful energy. See question below on primary energy vs useful energy.

• What about primary energy vs useful energy?

Most global energy statistics are presented in primary energy terms. Primary energy embeds large thermodynamic losses — for example, roughly two-thirds of the energy content of coal is lost as waste heat in a conventional power station, whereas solar photovoltaics generate electricity directly. As economies electrify, many of these losses disappear.

Primary energy accounting can therefore overstate the total energy input needed to deliver equivalent energy services as the system electrifies. A more economically meaningful metric would be “useful energy” — such as lumens of light or kilometres travelled — but economy-wide useful-energy datasets are not yet available in open, harmonised form. In our paper we therefore call for systematic research to develop accessible, useful-energy accounts to inform future transition modelling.

• If CES is already positive why aren’t we already past peak emissions?

A positive CES does not automatically imply falling emissions because the effect depends on the relative size of the clean and fossil portions of the energy system. CES measures relative growth rates; emissions depend on absolute quantities.

Consider a system with 1000 EJ total energy and 100 EJ clean energy. If total energy grows at 3% and clean energy grows at 6%, CES equals 3%. Total energy rises to 1030 EJ and clean energy to 106 EJ. Fossil energy therefore still increases in absolute terms.

If instead clean energy represents 500 EJ of a 1000 EJ system, the same 3% CES would stabilise fossil use in absolute terms. Once the clean share is sufficiently large relative to the growth differential, a sustained positive CES drives fossil energy downward in absolute terms. The timing of peak emissions therefore depends on both the growth differential and the starting shares.

• What about hard to abate sector (aviation, chemicals, steel, agriculture)?

CES measures the balance between clean and fossil energy growth. It does not by itself solve non-energy emissions such as those arising from agriculture or land-use change, which require sector-specific strategies. Hard-to-abate sectors such as aviation, cement, steel and chemicals currently constrain the upper bound of achievable decarbonisation. Whether CES slows once electricity is largely decarbonised depends on the rate of technological substitution in these sectors. Historically, sectors once considered intractable — such as road transport — have shifted rapidly once cost and policy conditions aligned. The frontier of achievable CES therefore shifts as innovation and policy expand the set of sectors that can be decarbonised.

• What about other gases (methane, NO2 etc)?

CES is an energy transition metric. It does not directly measure emissions or differentiate between greenhouse gases. Some gases — such as nitrous oxide from agriculture — require independent mitigation strategies. Others, such as fossil methane, decline as a co-benefit of reducing fossil fuel production and use. Biogenic methane and land-use emissions remain outside the scope of the metric and must be addressed separately.

• What does CES tell us about emissions? Does it differentiate between different types of fossil fuel?

CES does not differentiate between coal, oil or gas within the fossil portion of the energy system. A system dominated by coal has a very different emissions profile from one dominated by gas, even at identical CES values. Emissions accounting and fuel-specific policies therefore remain essential complements to the metric.

• What about the carbon costs of manufacturing clean energy sources like renewables?

Energy used to manufacture clean technologies forms part of total energy demand. If that manufacturing relies on fossil fuels, it will temporarily increase fossil energy use. A positive CES therefore requires clean generation growth to exceed both baseline demand growth and energy used to build the transition itself.

• What about data centres and other potentially large future sources of energy demand? Are they factored in?

CES is agnostic regarding the source of demand growth. Any increase in energy use — including from data centres — enters the total energy term. Rapid demand growth therefore requires proportionally faster clean supply growth to maintain a positive CES. In this sense, CES embeds demand-side discipline within the metric itself.

• Does CES assume that markets are driving a clean energy transition that is already inevitable?

CES does not assume any particular driver of change. It is a descriptive metric of growth differentials within the energy system. That said, sustaining a high and positive CES over time, however, is unlikely to occur without deliberate policy support, infrastructure build-out, capital investment and regulatory signals that enable clean energy to outpace total demand growth.

• What do you do with biomass: do you consider it clean or not?

In our analysis we classify traditional biomass with fossil fuels and modern bioenergy with clean energy. This avoids creating a perverse incentive to count subsistence biomass use — particularly wood and charcoal for cooking — as evidence of transition progress.

We recognise ongoing debate regarding land use, food competition and ecological impacts of bioenergy. Our classification is designed to avoid overstating transition progress by excluding traditional biomass from the clean category, while acknowledging that some modern bioenergy may play a transitional role despite unresolved sustainability concerns.

• Who came up with the CES concept?

The conceptual foundation was articulated by Michael Liebreich in his essay “The Pragmatic Climate Reset, Part 1.” Our Nature paper formalises and operationalises the concept into a tractable policy metric. We are grateful for his constructive input during the drafting process.

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Thank goodness those days are over, and the Antarctic is now protected from all exploitation. Except that it isn’t. Each year massive Norwegian and Chinese supertrawlers visit the feeding grounds of recovering whale populations and vaccum up as much krill – the tiny crustacean that is food for whales, penguins and seals like – as they can take. Last year was the highest take for two decades, 620,000 tonnes of krill removed by Norway’s Aker QRILL and other trawling companies. I’ve calculated that this is enough krill to feed 850 blue whales for a whole season!

And almost the worst thing is that the Marine Stewardship Council – them of the blue tick – is about to re-certify Aker QRILL’s activities as ‘sustainable’ despite fears from scientists and conservationists that the competition for food may be hampering whale recovery. Witnesses have seen krill supertrawlers sailing right into pods of feeding whales in order to get to the densest krill swarms, literally hoovering up the whales’ food from right in front of them.

Please make sure you boycott all krill products – don’t EVER buy omega-3 krill oil supplements, for example, as vegan algal alternatives are easily available. And take care because krill also goes into pet food and fishmeal used in salmon farming. Most importantly, please also tell the MSC not to re-certify the damaging krill fishery. Time is running out; they’re due to announce the decision any day, and all the objections from conservation NGOs have so far been rejected.

Here’s the full shocking story on my Substack, and how you can help before it’s too late…

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Want to solve climate change? Then stop with the negativity. Yes, global warming is undeniably real and demands urgent reductions in the use of fossil fuels globally. But to achieve this we need to move the conversation away from negative framings like direct emissions cuts and reducing energy use and start talking about a more actionable approach: saving the world by accelerating the production of enormous amounts of clean power for a growing global economy.

For the full post, see my Substack.

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[I’m a co-author on this Nature piece, which is open access and can be read here.]

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For the whole thing, see Geographical magazine.

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So far, we’re matching exactly this trajectory. We’ve sent probes far beyond the solar system – and vast quantities of electronic data fizzing in all directions at the speed of light – but we’ve also got thousands of thermonuclear weapons on hair-trigger alert that can in just a few hours reduce our entire world to a dark, freezing wasteland.

Whether or not we can confound the apparent inevitability of this doomsday outcome is perhaps the ultimate test of our species-level intelligence. But the omens aren’t good. Russia and the United States currently have 1,500 fully deployed nuclear warheads and over 5,000 each in their total arsenals. China is racing to reach nuclear parity by 2030. The current geopolitical instability raises the risk of superpower world war to perhaps the greatest level since the hottest periods of the Cold War.

America’s ‘launch-on-warning’ posture means that ICBMs (inter-continental ballistic missiles) must be out of their silos and in the air while the incoming missile attack is still blips on a radar screen. Once launched ICBMs cannot be recalled, nor can their targets be altered. When warning is received of imminent attack, the US president has as little as six minutes to decide whether to launch an all-out retaliation that would destroy most life on Earth.

A major thermonuclear exchange would likely kill about 770 million people in blasts and city firestorms across the major combatant powers. Technical language can obscure the reality here: this means several hundred million people – women, children, men, the elderly – being burned alive. Many more would quickly die from radioactive poisoning, but the biggest killer would come after: a decade-long worldwide nuclear winter that would starve billions more to death and wreck our civilisation beyond repair.

And all for what? Nothing matters this much; not any current-day flashpoint or contested piece of territory. The probability of nuclear war in any single year is small, probably around 1%, but this compounds to a two-thirds risk over a century. We have been extremely lucky with past near-misses, from Black Saturday of the Cuban Missile Crisis in 1962 to the notorious ‘3am phone call’ incident in 1980 when President Carter was nearly woken in the night when 2,200 incoming Soviet missiles were erroneously displayed on warning screens.

We would be unwise to trust to luck forever. It should be obvious that nuclear weapons and human civilisation cannot co-exist together long-term. Either we abolish them or they abolish us. To do so we will need to build a citizens’ movement singularly focused on the goal of total abolition. This will need to be very different from the anti-nuclear movements of the past: we can learn a huge amount from both the successes and failures of CND and other previous campaigns. Nor is it about abolishing nuclear power, which is a vital technology to tackle the climate crisis and can even help remove warheads from our world by burning them up as fuel.

Nor can we be unilateralist, because the process can only work via simultaneous trust-building disarmament by all the nuclear nations. This new movement must involve millions of people of all political persuasions in every country of the world whose only reason for participation is that they want to survive. We have a good head start in the 2017 UN Treaty on the Prohibition of Nuclear Weapons, already signed by 93 countries. In other words, almost half the world’s nations are now on board with the drive towards abolition.

This should be an easy decision. Collective suicide is in nobody’s interest. We don’t need to burn alive millions of men, women and children, and usher in a nuclear winter that destroys the biosphere. But the first step is to break out of the fatalistic denial that views nuclear weapons as inevitable and the aim of abolishing them as impossible. That way the Fermi paradox can remain unanswered, and humanity can continue to flourish on our beautiful, living planet.

For the full published version see the NewScientist magazine.

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First published in the Wall Street Journal, 15 May 2025

The world is on the brink of a climate apocalypse—not one caused by gradual greenhouse emissions but by a sudden exchange of nuclear weapons, a possibility made more salient by the current conflict between India and Pakistan. While the long-term effects of emissions are uncertain, we know that a nuclear war would result in an immediate nuclear winter.

When we think about nuclear apocalypse, we tend to think of the immediate effects: thermonuclear explosions that incinerate cities and vaporize populations. But the worst consequences unfold long after the weapons have detonated. A major thermonuclear exchange would shroud the atmosphere in soot, plunging the world into darkness and ushering in a decadelong winter. While hundreds of millions of people would likely be killed in the initial conflagrations, most of the human population—including those in the combatant nations—would likely die in the subsequent winter famine.

It’s comforting to think that an exchange of nuclear warheads in a regional conflict such as that between India and Pakistan might be more limited. The death toll from the detonation of a few dozen weapons might only number in the low millions, and there would be little effect on planetary temperatures.

But if India bombed Islamabad and Pakistan bombed Mumbai in retaliation, it would be hard to prevent further escalation. Moreover, once intercontinental ballistic missiles are in the air, it’s virtually impossible for other nuclear-armed nations to determine where they’re headed. Leaders in Washington, Moscow and Beijing would need to make decisions in a matter of minutes about whether to launch their own weapons.

Midrange scenarios involving a few hundred weapons would cool the climate enough to decimate global food production and trade and would likely kill hundreds of millions.

Under worst-case scenarios, droughts and crop failures would quickly spread across the globe. Hundreds of millions of refugees would cross continents in search of food, safety and shelter. Some would die of disease and illness, most of starvation. Human civilization would be over.

In comparison, there’s no conceivable global-warming scenario that would kill off most of the world’s population in only a few years. Climate change damages natural systems such as coral reefs and the Arctic and will increasingly stress human societies, but it’s not an existential risk akin to nuclear war.

Unless we reduce and ultimately eliminate greenhouse gas emissions, global temperatures will continue to rise. Climate change could also lead to abrupt changes in earth’s ecosystems, such as irreversible melting of the ice sheets in Greenland and Antarctica. While these changes will be fast on geological time scales, they’ll be slow on human time scales, unfolding over decades and centuries. Humanity will have time to adapt food production to climate change and become more resilient to extreme weather and sea-level rise. We also have many available off-ramps, from nuclear energy to solar geoengineering, that can limit future warming.

Nuclear winter, by contrast, would destroy civilization beyond repair within months or years. Yet unlike climate change, which has preoccupied activists for decades, it is largely ignored. Politicians, journalists and activists don’t travel by the tens of thousands every year to attend conferences on the threat of nuclear annihilation. Philanthropists such as Bill Gates and Jeff Bezos don’t spend billions on efforts to eradicate the threat. There’s no nuclear equivalent to Greta Thunberg lecturing the United Nations General Assembly about its failure to assure our survival. The antinuclear movement has bizarrely focused on eliminating clean power-generating nuclear reactors instead of city-incinerating nuclear weapons.

Arguably, President Trump is the most prominent figure warning of nuclear war, with his frequent invocations of World War III. Mr. Trump was also an advocate for arms control in the 1980s.

The arms control regime that world leaders painstakingly built during the latter stages of the Cold War is in tatters. The New Strategic Arms Reduction Treaty (New START), an accord signed in 2010 that limits Russia and the U.S. to 1,550 deployed warheads each—still plenty to destroy civilization—expires next February. Russia and the U.S. each hold more than 5,000 additional warheads in reserve. China, meanwhile, is rapidly building its nuclear inventory.

Against this backdrop of rising economic and geopolitical instability, the contrast is stark between the genuinely existential—but largely ignored—threat of nuclear warfare and the immense amount of attention and political effort lavished on the climate issue. Climate change is real, and there’s much that we can and should do about it. But nuclear war is the far more imminent threat.

Whatever else one thinks about the current administration’s novel approach to longstanding geopolitical alliances, President Trump deserves some credit for pushing Russia and Ukraine to agree to a cease-fire and appears to have played a significant role in brokering a cease-fire between India and Pakistan over the weekend.

Conflicts between nuclear-armed adversaries remind us that no other risk to human societies remotely rivals nuclear warfare. Zero nuclear weapons may be as much a pipe dream as net zero, but there should be no higher priority for politicians, philanthropists and civil society leaders, whatever their political stripe, than to de-escalate that threat.

Mr. Nordhaus is director of the Breakthrough Institute. Mr. Lynas is author of “Six Minutes to Winter: Nuclear War and How to Avoid It.”

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Looking up at the sky, one of the girls sees a brief metallic flash high up near the sun, far above the chrome spire of a Midtown skyscraper. She does not know it, but she has seen a re-entry vehicle from a 5 megaton (Mt) intercontinental ballistic missile (ICBM) and has only a few seconds left to live. Thinking about an upcoming maths test she has been working hard for, she looks down when the road-crossing signal turns to green. The girls are not even halfway across when the bomb explodes.

For the full extract, see LitHub

To buy the book, visit Bloomsbury Sigma

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And not just by a small margin. Trump won handily, in almost all the so-called ‘swing’ states. If the Republicans hold the House, this puts the political Right – arguably the far Right – in charge of all parts of US government: presidency, Congress and Supreme Court. There are no checks and balances with the same party, now controlled by one man, in charge of the executive, legislative and judiciary branches at once.  

There will be plenty of time for post mortems. How could American citizens vote for a sex-abuser felon con-man with no political programme to speak of other than his own narcicissm and petty grievances? Not once, but twice? The answer is the one that those of us who believe in science, democracy, truth and rule of law are most reluctant to face: that in much of the world ordinary people have lost trust in the very institutions that we most value. 

To make smart choices we need an evidence-based worldview. What could be further from the Trump mentality? He lies constantly, and the authenticity he radiates is wrapped up with his determined and proud ignorance: he is a perennial anti-elitist because he knows nothing and cares less. For voters alienated by modernity, this toxic and reactionary worldview has enduring appeal. But it’s the opposite of everything we are working for.   

Whether Trump is a capital ‘F’ fascist is arguable; a debate that will likely be resolved one way or another by events over the next four years. But his schtick is very much ‘triumph of the will’, belief over evidence, feeling over reason, and distrust in any notion of objective, observable truth. Trump says you can’t trust the media, the scientists, the government – anyone but him. And when he changes his mind from one day to the next, and spouts random nonsense, you must do likewise if you are not to become the enemy within. You must follow the Leader. 

There have been lots of pronouncements about what Trump might mean for clean energy or the climate. I feel these are missing the point. Trump is only pro-fossil fuels because the elites are green, and renewables are woke. His acolyte Elon Musk made his name with Tesla after all, and certainly did more to advance the EV revolution than any environmental NGO, whatever he now seems to have become. The issue with Trump is something deeper and darker than merely his stance on the environment, and we need to understand what that is.  

Yes Trump is a danger to the climate, but more importantly in the short term he is a danger to freedom. He does not value democracy, except where it can provide him with a path to power. He is a true authoritarian, and has never made any effort to disguise this: he sided openly with Putin from the get-go, and has managed to get the entire Republican party to fall in line. If American institutions allow it, he would doubtless prefer not hold another free and fair election in four years.  

This should remind us that freedom is never a given. It always has to be fought for. Once taken for granted for long enough, it ebbs away like water through sand. Trump is a disruptor, but only against institutions that constrain his power and act to protect the rule of law. Authoritarian populists can be defeated at the ballot box, as we have seen in Poland and Brazil. They can be defeated on the streets too, as pro-democracy revolutions have shown. People will fight and die to defend their freedom, as the Ukrainians do every day. Freedom isn’t granted from above, it’s seized from below and once won must be perenially renewed and defended.  

But freedom depends on truth. How can you hold free elections if there is no truth and you don’t know who to believe? Trump has vowed to put anti-vaccine quack Robert Kennedy Jnr in charge of America’s health service. There could be no better emblem of what he represents than that. RFKJr opposes all evidence-based medicine. But remember where he came from – the environmental movement. With post-modernism and later wokery it was the Left that began the attack on institutional trust that the Right is now taking to a destructive extreme. We must look in the mirror too when we seek to analyse what Trump represents and where he came from.  

There have been periods of darkness before in human history, sometimes very long ones. Absent a nuclear war – and Trump will, from 20 January next year, have sole launch authority over America’s nuclear missiles – there will be an end to this nightmare. Our task is not to despair, and not to give in to hopelessness or nihilist extremism of our own. We must keep the light burning. Objective reality cannot simply be wished out of existence, and sooner or later science will win out and truth will return.   

When that day dawns is not up to Trump, it is up to us. 

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