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- Herausgeber: Old Street Publishing
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
- Veröffentlichungsjahr: 2024
'Very well-informed, splendidly provocative – a must-read for the rich debates to come' JONATHON PORRITT Glaciers are melting, sea levels are rising and weather is becoming more extreme. Most of us know the solution: cut our carbon emissions. There's only one problem – we aren't doing it. So what else can we do? Over the past four years, Gwynne Dyer has spoken to dozens of the world's leading climate scientists, asking them this question and listening to what they have to say. Should we copy volcanoes and squirt sulphates into the stratosphere to reflect the sun's rays? Can we suck CO2 out of the air and bury it? Invent a clever way to prop up the 'Doomsday' glacier? Or dam the North Sea? The ideas might seem outlandish, but these are desperate times. From fission power to fake meat, from the deep seas to the jet stream, Intervention Earth is a magisterial survey of the most creative scientific thinking on how we might still solve the most frightening problem of our age.
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On Climate Wars
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edinburgh science festival
‘A truly important and timely book’
james lovelock
‘As an insight into what the military strategists imagine is going to happen as a result of climate change, this book is truly terrifying.’
New Scientist
On The Shortest History of War
‘Readable and sharp... does what it says on the tin’
Independent
‘From the first armies to clashes of drones and dirty bombs, this is eye-opening, big-picture stuff’
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Seattle Times
‘Dyer writes with eloquence and authority’
Irish Examiner ii
viiviii
INTERVENTION EARTH
LIFE-SAVING IDEAS FROM THE WORLD’S CLIMATE ENGINEERS
GWYNNE DYER
Contents
xi
Note on the Text
For commercial reasons, Tina Viljoen’s name does not appear at the front of this book, but from shooting every interview to debating all the available options for coping with the crisis, she has had an equal share in making it happen. Working with her on this and many other projects has been the delight of my life.
All direct quotations, unless otherwise noted, are taken from interviews with the more than 100 climate scientists, engineers and a few others who generously gave their time. There is a list of all the interviewees at the back of this book.xii
Introduction
In 2008, I wrote a book called Climate Wars about the science and the geopolitics of climate change, and for a couple of years afterwards I had a sort of intermittent double vision. In my mind’s eye, I would suddenly see the three-degree-hotter world overlaid on the world that existed at that time. Such afflictions, however, can usually be cured by a judicious combination of outdoor exercise and good wine – and now that you know the remedy you can be confident that reading this book will cause no similar derangement. But when I immersed myself in that world again more than a decade later, I did notice that some of the scientists I’d met first time around now seemed a bit more – what’s the word? – distracted. Not actual waking visions, you understand. Just moments lost in thought.
As the climate crisis deepens and the negative impacts multiply, public opinion and politics are finally responding, but there is no guarantee that our actions will be big and fast enough to avoid an outcome that is catastrophic for human beings and quite disruptive, at least, for the entire biosphere. We are not even sure yet how big and how fast those actions need to be, because the discipline of climate science is only about forty years old. But the answer is almost certainly: very big and very fast.
How extreme it could get depends on two main things: how intent we are on burning all the fossil fuels we already know about, and how sensitive the Earth’s climate is to that carbon injection. We could get to 8°c of warming fairly readily, but we probably wouldn’t, because it would be so catastrophic well before we reached that point that it 2would terminate our activities. We are still able to trigger warming of the order of 5°c globally, burning only a fraction of the fossil fuels, if we consider the feedbacks and tipping points. Very sobering for those of us who work on this day in, day out.
Tim Lenton, Professor of Climate Change and Earth System Science, University of Exeter
Scientists can now predict with some confidence how much extra carbon dioxide in the atmosphere will cause how much warming: there is still a range of possibilities, but the range has narrowed and all the possibilities past 450 parts per million (ppm) of carbon dioxide (co2) in the atmosphere are bad.*
We are coming up on 425ppm, and adding 2.4ppm a year. Scientists can even foresee how fast that warming will happen – if they can assume that this will always be a steady, ‘linear’ process. But we now know that warming is often ‘non-linear’: that is to say, the average global temperature crosses an invisible threshold, a sort of tripwire, and makes a sudden unscheduled leap upwards. ‘Tipping points’ are the specific points at which the upward leaps occur, but climate scientists have only a vague and uncertain knowledge of where they are. Two major questions addressed by this book, therefore, are the speed and accuracy with which scientists can map the ‘tipping points’, and how we might find ways to avoid crossing them even now.
Since my last foray into the field, there has been a significant loss of faith in the notion that emission cuts alone can stop us short of reaching the tipping points. There is an emerging 3debate – emerging into public view, at least; it has been raging inside the climate science community for some time – about which methods of direct human intervention into the workings of the climate system would be valid and safe, and which would not – in other words, about ‘geoengineering’ or ‘climate engineering’. (The former term is generally used in North America; the latter is preferred in Europe and elsewhere.)
This debate has become so fraught that quite a few climate scientists who favour only ‘Carbon Dioxide Removal’ (cdr) techniques such as Direct Air Capture (dac) or Bio-Energy with Carbon Capture and Storage (beccs) now want to remove these technologies entirely from the category of ‘geoengineering’ techniques with which they have usually been grouped. This would be done to distinguish the cdr technologies more sharply from the allegedly more dangerous but generally cheaper and quicker Solar Radiation Management (srm) techniques that involve direct human intervention to reduce the amount of solar energy reaching the planet’s surface. There are more and less desirable techniques within each category, but this is increasingly where the battle lines are being drawn: between cdr on the one hand, and more direct interventions like srm on the other.
Almost nobody in the climate science community really believes any more that we can stop the warming at a place that is relatively safe without direct human intervention of some sort in the climate system. Doing so merely by cutting emissions and planting lots of trees would have been possible (with a huge crash programme) in the year 2000, and it was still imaginable (just) in 2010, but it now hardly seems credible.
This book will assess the belief that some form of intervention in the climate system (cdr, srm or a combination of the two) will be required to stop the warming short of a catastrophe. 4A ‘never-exceed’ target that most governments now agree on, high though it may be, is an average global temperature ‘less than two degrees Celsius higher than it was in pre-industrial times’. (The shorthand for this is <+2°c.) An ‘aspirational’ target of only <+1.5°c was adopted by the Intergovernmental Panel on Climate Change in 2018, but that is already teetering on the edge of impossibility. If we overshoot +2°c, we are likely to enter a chaotic world in which sudden upward lurches in temperature are added to the relentless current rise (officially estimated at +0.18°c per decade), and the hurricanes, forest fires, killer heatwaves and the rest will grow correspondingly more severe and more frequent.
The 1.5°c target is one that science increasingly demonstrates is associated with substantial risk of triggering irreversible large change and that crossing tipping points cannot be excluded even at lower temperature increases.
Katherine Richardson, Director, Sustainability Science Centre, University of Copenhagen
In other words, we are already in the danger zone.
In recent years, the various methods for reducing emissions have improved greatly both in affordability and in variety. To take just two examples: solar power has become dramatically cheaper, while meat substitutes and ‘cultivated’ (lab-grown) meat are being developed which would, in theory, enable us to rewild huge amounts of pastureland now devoted to feeding beef cattle, to the great benefit of both biodiversity and emissions cuts. But in every case, the question that must be answered is: ‘When will this solution be available at scale?’ Because the ‘never-exceed’ deadline is drawing near.
This book is not a consciousness-raising exercise, and there 5will be no exhortations to pull up our collective socks and get on with ‘saving the planet’. Nor will I waste people’s time with a lengthy trudge across the now familiar territory of the climate emergency; others have already done that. But just as an aide-mémoire, here are the inconvenient facts that we must always bear in mind.
FACT NO. 1: We Are Running Out of Time
Actually, we probably have run out of time. Like soon-to-be-bankrupts, we can go on fiddling the books for a while longer, but we cannot stay below the 1.5°c higher average global temperature that was our recommended maximum increase according to the Paris Climate Agreement of 2015. As Johan Rockström, director of the Potsdam Institute for Climate Impact Research, told me in 2020:
We have been lulling ourselves into a comfort zone, believing we have a lot of time, but 2020 is the year when we need to bend the curve down on global emissions, because when you look at the more than 100 scenarios in the Intergovernmental Panel on Climate Change (ipcc) report – the 100 different scenarios that could [stop the warming at] +1.5°c – they all bend in 2020. You cannot succeed if you bend later. Everything is determined by the carbon budget… If you bend later, the speed by which we have to reduce emissions is no longer possible to achieve in any democratic way. You would simply have to bulldoze every coal-fired plant overnight.
Well, the emissions curve did not bend down in 2020, despite the Covid-19 pandemic, and they haven’t started bulldozing coal-fired power plants either. Global carbon dioxide emissions did drop briefly – by 17 per cent – at the peak of the 6first wave of Covid-19, but over the whole year the needle barely flickered. The planes stopped flying for a while, but the cows kept burping, the lights stayed on, and the houses of the developed world remained warm in winter and cool in summer.
For all the talk of cuts, the amount of carbon dioxide in the air has increased almost every year since the start of the industrial revolution. In 1800, it was only 280 parts per million (ppm). By 1988, when global warming first became a public concern, it was 350ppm. In 2020 it was 415ppm, and it’s still going up. There is little chance that the curve will turn down before 2025 at the earliest – whereas achieving the ‘aspirational’ target of not exceeding +1.5°c would have required an already implausible cut of 7.9 per cent in greenhouse gas emissions in each year of this decade, starting in 2021.
FACT NO. 2: Cutting Emissions Is Not Enough
There’s a dirty secret about the Paris deal and the <+1.5°c ‘aspirational’ limit: the target could never have been achieved by cutting emissions alone. It is abundantly clear from many sources that the negotiators in Paris were counting on ‘negative 7emissions’ technologies – that is, taking greenhouse gases out of the air – to avoid some of the warming.1
This is a problem because almost all of these ‘Carbon Dioxide Removal’ (cdr) technologies are either slower-acting or much more expensive (or both) than simply cutting co2 emissions, and most are not yet ready for deployment at a global scale. Half of them also have major implications for land use or the health of the oceans. Some of these cdr technologies do have longer-term possibilities as part of an attempt to stabilise the global climate, and I will discuss them in detail later, but they cannot be deployed fast enough to help us stay below the <+1.5°c target into the mid-2030s.
FACT NO. 3: Carbon Accumulates
So much for the aspirational goal. How about the real, ‘never-exceed’ goal of stopping the warming at or before +2°c?
The carbon dioxide that we put in the air stays there for a very long time: 200 years for the average co2 molecule. Plants absorb some co2 each spring and summer as they grow, but they put it back into the air again when they die and burn or rot. Even the rocks absorb some co2, very slowly – but these natural ‘carbon sinks’ are largely occupied with playing their role in the natural ‘carbon cycle’. Much of the co2 that human beings put into the air each year stays in the atmosphere and accumulates: even some of the co2 emitted by the coal-burning boilers on Thomas Newcomen’s eighteenth-century steam pumps is still there.
Now, 450ppm of co2 in the atmosphere is the point at which we are effectively committed to +2°c. Beyond that, very bad things begin to happen. With the amount of co2 in the air already at 425ppm, we only have 25ppm left before +2°c average global temperature becomes inevitable. The extra amount of co2 emissions caused by human activities that accumulated in 8the atmosphere in 2022 was 2.4ppm. If we continue at that rate, we will reach 450ppm around 2032. Even if we cut our emissions by half in the next decade – a heroic but unlikely achievement – we would still reach at least 435ppm by the middle of the decade (2035).
Nobody in their right mind would willingly go to 435ppm, because there is not always a predictable, direct relationship between parts-per-million of co2 and global average temperature. At various points as the planet warms – unfortunately we don’t know precisely which – ‘tipping points’ will be triggered and the global average temperature will lurch rapidly upwards. Most climate scientists – and the ipcc’s official ‘best guess’ – assume that these thresholds are almost all higher than +2°c/450ppm, and it might be that the climate really spins out only at +2.2°c. On the other hand, the true ‘never-exceed point’ could also easily be +1.8°c, in which case 435ppm would be more than enough to cook our goose.
Yet these figures feel so small that it’s hard to take them seriously. What’s the difference between 1.8 and 2.2? Or between 435ppm and 450ppm? Well, it’s similar in effect to the difference between a human body temperature of 36.5°c (normal), 38.5°c (fever), 40.5°c (brain damage), and 43°c (death). So yes, take it seriously. The better informed people are, the more frightened they are.
FACT NO. 4: Predicting the Climate is Hard
As meteorologist Edward Lorenz realised in 1960, if a butterfly flaps its wings in a certain way in Beijing in March, then by August hurricane patterns in the Atlantic could be completely different. The climate system is so complex and so interconnected that we cannot predict the weather for even one week, so how can we possibly predict the climate? Alan Robock, 9Distinguished Professor in the Department of Environmental Sciences at Rutgers University, New Jersey, explains:
We don’t have any data on the future, and there’s a lot of chaos in the climate system. We can predict the ‘envelope’ of possible weather, but not the specific weather. Then there’s natural variability: some years are warmer than average; some are colder. Some years you get El Niño, some you get La Niña, and you can’t predict those very far in advance. This is a problem that climate scientists have always had. We don’t have a laboratory with test tubes and accelerators to do our experiments; the laboratory is the real world, and the best we can do is the climate models we create.
We write down the equations that describe everything we understand and do multiple runs with slightly different initial conditions, putting in the flapping of butterfly wings and so on, and we get a swarm of potential climates. The real world will only go through one of those potential climates, but probably it will be somewhere within that swarm. Then we test those models on the past. If they do a good job simulating the effects of known volcanic eruptions, or if they do a good job simulating the global warming of the past century, then we have more faith in them for the future.
Alan Robock
That’s all we have, so it will have to be good enough.
FACT NO. 5: Averages Lie
The ‘Average Global Temperature’ is an indispensable concept when discussing the broad topic of ‘global warming’, but it is very unreliable as a guide to what the temperature will be in any 10specific location. Moreover, there is a big difference between temperatures at sea and on land. Temperatures are generally more extreme on land, because it heats up more quickly in sunshine and loses heat more quickly at night and in winter. The further away from the sea, the truer this is, which is why it’s deep in the interiors of the continents that most of the record temperatures, both high and low, have been observed.
But since two-thirds of the planet’s surface is covered by oceans, the Average Global Temperature is always closer to the average temperature over the oceans than it is to the average land temperature. These values are not usually calculated, but a rise in average global temperature of 2.0°c really means a rise of roughly +1.0°c in average maritime temperature and a rise in average land temperature of between +3.0°c and +4.0°c (depending mainly on how far inland).
FACT NO. 6: The Atmosphere Does Not ‘Bounce Back’
Even if we do manage to achieve Net Zero by 2050, that doesn’t mean everything goes back to ‘normal’. We would have stopped adding more greenhouse gases to the atmosphere each year, but all the carbon dioxide that drove the temperature up to +2.0°c or more would still be there, and it won’t leave of its own accord. If we want our old climate back and we are not willing to wait thousands of years for the rocks to do the job, we’ll have to take the excess co2 out of the air ourselves – a massive, centuries-long task.
Since the alternative is living indefinitely with the brutal climate of a +2.0°c world, we will probably try to do that. Indeed, that is likely to be the long-term role of the various ‘Carbon Dioxide Removal’ (cdr) techniques now being researched or, in a couple of cases, developed. 11
FACT NO. 7: ‘Runaway’ Is Possible
Terms like ‘runaway’ and ‘hothouse Earth’ do not mean Venus-like conditions inhospitable to all life. Our planet is considerably further from the sun than Venus is. It will not experience the extreme conditions of that planet until the sun has heated up another 6 per cent, around a billion years from now. But if tipping points cascade, a rise in average global temperature of +4°c or more is possible by the end of this century. Low-probability but high-impact events are precisely what you buy insurance for, but unfortunately they tend to be omitted from most official climate documents.
Human beings are a tough species. A few breeding pairs are bound to survive.
James Lovelock
Temperature rises of up to +6°c would still not mean the extinction of the human race throughout its range (pretty much the entire land surface of this planet), but it would drastically shrink the climate niches where humans could survive, implying a die-back in global population of perhaps 90 per cent. Hundreds of thousands or even millions of other species would become extinct, but such temperatures and mass extinctions have happened before and this would not be The End. Our current civilisation would be unlikely to survive, and it might become impossible to build another one, but actual human extinction is quite unlikely.
This future is not yet inevitable. A hyper-aggressive worldwide programme of emissions cuts combined with the super-charged development and deployment of cdr techniques capable of extracting vast amounts of co2 from the air and getting rid of it somehow, might make it possible to stay below +2°c even into 12the 2040s – and by then, like stepping stones to the future, better means for reducing emissions and removing carbon dioxide from the atmosphere might have become available.
If that doesn’t happen, the same goal of staying below +2°c might be achieved quite quickly, even at the next-to-last moment, by reflecting back enough incoming sunlight (aka Solar Radiation Management or srm) to cool the planet’s surface by a degree or two. This could not be a permanent solution, but it might win us a few extra decades to work on reducing emissions and deploying cdr techniques without crossing the tipping points and without suffering extreme warming that would topple global civilisation into famine, mass migration and war.
***
We probably came very close to +1.5°c in 2023 and may exceed it in 2024, but this is largely because an El Niño event came along and piled additional warming, unrelated to climate change, on top of the existing climate trend. (El Niños occur every three to seven years and the previous one was in 2016. That, too, was at the time the hottest year ever.) Many people will be hurt by the extreme weather of this period, and almost everybody will experience some discomfort – floods, droughts, forest fires, heat-waves, massive storms, etc. – but this is not climate Armageddon; it is a transient event. The average global temperature ought to drop back down to +1.3°–1.4°c by 2025, and the current assumption is that it will not reach and remain above 1.5°c until 2029.
***
The first part of the book, The Planetary Trajectory, explores the recent research into non-linear effects that has forced scientists to conclude that ‘runaway’ warming is indeed possible, has happened in the past, and could be triggered by far less human-caused heating than we had previously thought. The early chapters 13will therefore focus on the science that imposes this new urgency, on the fossil fuel lobby’s ongoing attempts to discredit or distract from the science, and on the scientists who have expanded our understanding of the climate system and the entire Earth System.
Next, in All About Emissions, I will examine our efforts to end greenhouse gas emissions (with a focus on carbon dioxide and methane), and also the new technologies for ‘negative emissions’ (taking those gases back out of the air). The aim is to arrive at a realistic estimate of how much emissions can be cut and how quickly – and this requires a case-by-case examination of how much each technique, from Artificial Ocean Alkalinisation to growing the algae Asparagopsis armata, can contribute to the effort. Will our present emissions reduction strategy work, or will more radical measures will be required?
The third part, Climate Geopolitics, is about how the political environment will evolve as the climate worsens. Politics is the way mass societies must make their decisions, and that will not change even in the most extreme emergency. There are plenty of good ideas around – to stop glaciers sliding into the ocean, to expand the food supply, to suck carbon dioxide out of the air and bury it – but they will have to be prioritised, negotiated and paid for by groups of frightened, desperate people. No matter what we do now, the climate will go on getting worse for at least another twenty years, and human beings don’t typically respond well to this sort of challenge. We will have to learn to act like grown-ups all the time.
By the final part, Desperate Scientists and Last-Ditch Ideas, I hope to have shown that we are likely to run out of time faster than many of these promising ideas can be deployed at scale to save us. At some point very soon, I will argue, we must have the great srm debate. Indeed, we should start right now, because our whole future may depend on it. 14
* References to the amount of carbon dioxide in the atmosphere in this book will, unless otherwise stated, mean ‘carbon dioxide equivalent’. That is, the sum of all the ‘greenhouse gases’ in the air that cause warming, including methane, nitrous oxide, etc., expressed in terms of the warming that would be caused by an equivalent amount of carbon dioxide alone.
Part One
The Planetary Trajectory
… in which it is revealed that the Earth System is less stable than was formerly supposed, and that the risk of ‘runaway’ warming is real 16
1
‘Hothouse Earth’
Once upon a time all we had to worry about was the greenhouse gases relentlessly building up in the atmosphere, slowly but surely raising the average global temperature. Those were the Good Old Days, before the alarm was sounded by a very influential study published in 2018. You can access it directly on the web, but as it was written by scientists for scientists, the language can be a bit challenging.2
We explore the risk that self-reinforcing feedbacks could push the Earth System towards a planetary threshold that, if crossed, could prevent stabilisation of the climate at intermediate temperature rises and cause continued warming on a ‘Hothouse Earth’ pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene. We examine the evidence that such a threshold might exist and where it might be. If the threshold is crossed, the resulting trajectory would likely cause serious disruptions to ecosystems, society and economies. Collective human action is required to steer the Earth system away from a potential threshold and stabilise it in a habitable interglacial-like state. Such action entails stewardship of the entire Earth System…
From ‘Trajectories of the Earth System in the Anthropocene’, published in the Proceedings of the National Academy of Sciences of the United States318
That abstract set off a cascade of alarm bells, not only in my head but in the heads of thousands of other people around the world. Let me translate some of the passages that set the bells ringing into non-scientific English.
‘self-reinforcing feedbacks’
Natural and unstoppable sources of further warming triggered by human-caused heating.
‘prevent stabilisation… at intermediate temperature rises’
We’re heading towards the +2°c limit, and the brakes have failed.
‘“Hothouse Earth” pathway’
Runaway warming.
‘continued warming… even as emissions are reduced…’
We have lost control.
‘much higher global average temperature’
+4°c, +5°c, +6°c, and on up.
‘sea levels significantly higher’
Up to two metres sea-level rise in this century (goodbye Miami, Lagos, Calcutta, Bangkok); up to 5 metres by 2200 (goodbye New York, London, Mumbai, Tokyo).
‘serious disruptions to ecosystems, society and economies’
Famine, war, mass refugee flows, mass die-backs of human and animal populations.
‘stabilise it in a habitable, interglacial-like state’
We can’t go back to the old climate, but we might be able to stop in one that is still habitable.
‘stewardship of the entire Earth System’
We must become Planetary Maintenance Engineers.
The phrase ‘planetary maintenance engineers’ popped into my head as soon as I read the abstract. It comes from James 19Lovelock’s first book about the ‘Gaia hypothesis’ – now known in universities as ‘Earth System Science’ – published in 1979. At that time, Lovelock wasn’t talking specifically about climate, because global warming wasn’t yet high on the scientific agenda. He was discussing how Gaia (the self-regulating ‘Earth System’ that he identified and named) was being slowly crippled by the ever-increasing pressure of human activities:
If man had encroached upon Gaia’s functional powers to such an extent that he disabled her… he would then wake up one day to find that he had the permanent lifetime job of planetary maintenance engineer… The ceaseless, intricate task of keeping all the global cycles in balance would be ours… and whatever tamed and domesticated biosphere remained would indeed be our ‘life support system’.
This was the first time any serious scientist publicly suggested that human beings might have to step in and manage the planet’s many autonomous and hitherto self-regulating biogeochemical systems. Lovelock wasn’t suggesting it with any pleasure, because forty years ago we lacked the knowledge, tools and sheer processing capacity to do that job (as we still do today). But we are nearing the point when we may have to take on at least some of that ghastly, thankless job, ready or not, for we are overloading and disabling the systems that kept the climate and much else stable. It is much less uncommon today than it was in 1979 to hear scientists talk, in some desperation, about our having to take on ‘the stewardship of the entire Earth System’. So who wrote this remarkable ‘Trajectories’ paper, and why?
There were sixteen authors, all leading climate scientists working in eight countries.4 The paper contained no original 20research, no new facts. All it did was summarise and analyse the recent research in the field (admittedly a monumental task) and draw the obvious conclusions. Yet it had a huge impact, with a sky-high 6,061 score on Altmetric, which tracks the impact of academic articles. This kind of massive, instantaneous response only occurs when you say in public for the first time what a great many people are already uneasy about in private.5
‘I knew the paper would just explode in the faces of people interested in climate science,’ said Hans Joachim Schellnhuber, Director Emeritus at the Potsdam Institute for Climate Impact Studies. ‘Persuading somebody of a completely different worldview is impossible, but to bring out some of the things they already know deep inside – or not so deep inside – that’s possible.’ This phenomenon is common in fields like climate change, where the conclusions of new research are often provisional and conditional. As the evidence accumulates, experts may begin to suspect or even fear that it adds up to a really big deal – but nobody wants to be the first to say so aloud. The media and the public are liable to grab the wrong end of the stick, and your own colleagues may conclude that you are just seeking attention. Scientific research is a highly competitive field, and scientists can be torn to shreds by their peers if they go one millimetre further than the evidence supports. Think piranhas.
By 2018, the evidence was piling up that the projections for future warming that the big climate conferences and treaties had been working with were much too optimistic, but in professional terms it would still have been unwise for an individual scientist to take all that evidence, drawn from many different domains of research, and offer sweeping conclusions about it. It was information that people didn’t want to hear, and there would inevitably be a desire to shoot the messenger. What was needed was a major review paper by multiple authors, each an 21expert in some aspect of the accumulating research, and at least a few of them really big names in the discipline.
One such name was Tim Lenton, Professor of Climate Change and Earth System Science at the University of Exeter.
A fairly sizable group of us had been researching the possibility of different tipping points in the Earth’s climate system, so it was natural that we would ask: ‘Is there a global tipping point? Is there an instability of the whole climate?’
We burn a certain amount of fossil fuel, warm up the planet a certain amount, but then the feedbacks within the climate system start to amplify that until the climate change becomes almost self-propelling, because carbon is being released from degraded ecosystems and the permafrost and so on.
We weren’t alone in being concerned, but we came together to raise a flag: this is a risk that can’t be ruled out. And if it’s a risk that can’t be ruled out, let’s have a go at trying to work out how big a risk it is.
Tim Lenton
Many of the ‘Trajectories’ authors had met in the 1990s as members of the scientific steering committee of the UN’s International Geosphere Biosphere Programme (igbp). Among them were Katherine Richardson, an American oceanographer now leading the Sustainability Science Centre at the University of Copenhagen, Hans Joachim Schellnhuber (German, founder of the Potstdam Institute), Paul Crutzen (Dutch, Max Planck Institute for Chemistry), who was awarded a Nobel Prize for discovering how the ozone layer was affected by human activities, Johan Rockström (Swedish, at the time studying systems ecology at Stockholm University), as well as the then director of the igbp, Will Steffen (American, Australian National University). 22
In 2009, they had co-authored the paper ‘Planetary Boundaries: Exploring the Safe Operating Space for Humanity’, which broached the notion of specific thresholds in various parts of the Earth System that, if crossed, could trigger abrupt and radical change. In the 2015 update, produced to coincide with the Paris climate summit, they reported that four of the nine planetary boundaries they had defined – climate change, biosphere integrity, biogeochemical flows and land system change – might already have been breached.6
Soon after the Paris summit, the group began working towards the key ‘Earth Trajectories’ paper of 2018. Only in retrospect did it become clear what had changed. Will Steffen explained:
During the years leading up to [the ‘Trajectories’ paper], I was spending a fair bit of time in Stockholm working with colleagues at the Stockholm Resilience Centre, especially Johan Rockström [then the Director] and Katherine Richardson from Copenhagen. We all had a common concern that despite a lot of research on feedbacks, on tipping points, in the biosphere, in the oceans and in the cryosphere, every time a big report came out from the physical climate guys it had very even trajectories completely dominated by human emissions, [as if] that was going to decide where your system went.
At the Resilience Centre, they also do a lot of complex system thinking, so they’re looking at states in transition, and it’s not smooth changes, it’s abrupt changes between alternate stable states.
Will Steffen
Katherine Richardson gives a lot of the credit to Lovelock and his idea of Gaia: to systems thinking on the biggest scale, 23slowly but irresistibly reshaping the perspective of climate scientists (and of every other educated human being, whether they realise it or not):
We have a geology department, a physics department, an economics department, a humanities department. The object is to find out as many details as you possibly can in your box. It’s as if we believe that if you could put all of your details in a big pot together with everybody else’s details from their boxes, then we’d understand how the Earth works…
It’s like asking doctors who have different specialties – heart and brains and lungs and reproduction and skin and feet – to put all their knowledge in a pot and stir it up. We still wouldn’t know what a person is, because it’s the interactions between them that are really important. And that’s what James Lovelock did. He focussed on these interactions. He said that if you wanted to find life on another planet, you should just analyse the atmosphere around that planet, because there would be certain elements if there was life that wouldn’t be there if it wasn’t. He was one of the very early pioneer thinkers about systems. It’s no accident that Earth System science didn’t develop at a university.
Katherine Richardson
When I spoke to him in 2021, James Lovelock agreed with Katherine Richardson:
The source of the trouble here is, I’m afraid, the university system. It somehow made the ghastly mistake a long time ago that you cannot teach all the subjects together, you have to separate them, and you teach chemistry in one building, physics in another, engineering in yet another. The students 24in each of these buildings never meet, and this produces an incomplete knowledge in the community at large. This is disastrous.
The final piece of the puzzle came in a book written by Marten Scheffer, a Dutch mathematician and expert on complex system theory. Scheffer had pointed out that during the glacial-interglacial cycle of the current Ice Age – 50–100,000-year periods of deep glaciation, followed by 10,000–15,000-year ‘interglacial’ periods like our own that were thought to be around 5°c warmer – the actual ‘forcing’ (that is, the changes in the amount of solar heating reaching Earth) is very small. The minor shifts in the Earth’s orbit known as the Milankovitch cycles, which slightly alter the amount of energy being received from the sun, account for only about one degree C of the regularly repeated five-degree C change up or down in average global temperature. What had caught Will Steffen’s eye in Scheffer’s work was that the rest – ‘80–90 per cent of the heavy lifting’ – was down to feedbacks within the Earth system, and they were non-linear. ‘So why,’ he asked, ‘would we expect under pretty horrific forcing (like now) for the Earth to be well-behaved in a nice linear fashion?’
Steffen, Richardson and Rockström decided it was time to share their insights with the wider scientific community.
We’d been mucking around with this for a while… publishing papers on tipping points. We didn’t have a model yet, but we needed to put this out on the table, to say ‘we think the Earth System could operate this way, not the way most people think.’
They flew a group of scientists to Stockholm for a workshop, including John Schnellnhuber and Marten Scheffer. And it 25was on the plane that Scheffer came up with one of the critical figures in the Trajectories paper.
What he had in mind was a diagram showing ‘limit cycles’ of temperature, one loop describing the range of temperatures we live in now, with a transition, already underway, to a different limit cycle that is much hotter: ‘Hothouse Earth’. Our planet might eventually return to its earlier, cooler limit cycle, but not for a couple of hundred thousand years – the time it would take for the carbonate and silicate cycles to remove the co2 from the atmosphere. Crucially, the diagram also showed a much smaller, tighter circle labelled ‘Stabilised Earth’ in which the planet’s average temperature never exceeds +2°c. Not ‘Stable’, mind you; just ‘Stabilised’.
Switching to the ‘Stabilized Earth’ pathway would require 26‘deep cuts in greenhouse gas emissions, protection and enhancement of biosphere carbon sinks, efforts to remove co2 from the atmosphere, possible solar radiation management [my italics], and adaptation to unavoidable impacts of the warming already occurring’.
The article, published in August of 2018, really did get the donkey’s attention, both within and beyond the scientific world.
The Anthropocene
For years, climate scientists had been quietly panicking. Now the ‘Trajectories’ paper gave form and substance to their worst fears. It said two things that had not been said with such authority before. One was that we have already left behind us not only the ‘Holocene’ epoch, a period of stable, moderate climate, suitable for large-scale agriculture, that has prevailed since the last major glaciation ended 11,600 years ago, and in which we have built all of human civilisation. We have also left the whole Quaternary period behind: the alternation of long periods of glaciation with briefer ‘interglacial’ warm periods that has obtained for the past 2.6 million years. We are now on a new trajectory, heading up into the recently named ‘Anthropocene’ epoch, a hotter period when the pressure of human activity on the Earth System is the dominating factor in determining outcomes.7
The other even more dramatic conclusion is that human decisions that must be taken in the next couple of decades will determine how hot the Anthropocene is. Intervene intelligently and ‘take charge’ of the climate, and we could probably stabilise it at around 2°c higher average global temperature: a good deal hotter than now, and very hard on at least some people, but not utterly catastrophic. ‘Stabilised Earth’, as the authors call it.
But don’t do enough, or do it too late, or do the wrong 27things, and the average global temperature will end up as much as 5°c or 6°c higher (‘Hothouse Earth’), with no way back on a time-scale relevant to human beings. As Johan Rockström explains:
Thirty years of climate science has given us so much understanding, and what I now see very clearly as a red thread during that entire journey is that the more we learn about the Earth system, the more reason for concern we have… In 2001, at the Third Assessment [of the ipcc], the best assessment of the risk of crossing catastrophic tipping points, of destabilising the biosphere, was estimated to be somewhere between +5°c and +6°c of warming. Even now, we believe we could avoid that. So you could argue that in 2001, twenty years ago, we assessed that the risk of catastrophic change and causing a tipping point was basically zero.
Then for every new assessment the level [of average global temperature at which the risk of crossing tipping points gets serious] just goes down, down, down – until 2018, where the assessment is somewhere between +2°c and +3°c. People think we’re raising the alarm because human pressures are increasing, but that’s not the case at all. It’s just that we are learning how the planet works, and the more we learn, the more we realise how vulnerable she is.
When humans start this massive global experiment of putting pressure on the planet, with greenhouse gases and cutting forests and loading nutrients into the oceans, what does the Earth system do? It responds by buffering and dampening the impacts, just shoving our planetary debt under the carpet, because we are so far away from the tipping point that the systems have huge redundant capacity – what we call resilience. We’ve followed a 150-year journey since we 28kicked off the industrial revolution and we’ve gradually been losing resilience, but up until recently the models have been right. Things change linearly when it’s resilient, but when you lose resilience… Bang! Things can crack, and you tip over into new states.
That’s what the ‘Trajectories’ paper is telling us: that promises of ‘Net Zero by 2050’ and other pie-in-the-sky-when-you-die schemes for eventually reducing emissions aren’t going to cut it, because the crisis is coming a great deal sooner than that. It would be nice if all we had to worry about was the slow, steady, linear growth of emissions, but our bigger, much more urgent problem is figuring out how to avoid triggering the feedbacks in the next ten to twenty years.
Or to put it in the paper’s scientific language:
In most analyses, [the Earth System’s] trajectories are largely driven by the amount of greenhouse gases that human activities have already emitted and will continue to emit into the atmosphere over the rest of this century and beyond – with a presumed quasi-linear relationship [author’s italics] between cumulative carbon dioxide emissions and global temperature rise…
But the ‘quasi-linear relationship’ is precisely what the ‘Trajectory’ authors say we should not presume:
Our analysis implies that even if the Paris Accord target of a 1.5°c to 2.0°c rise is met, we cannot exclude the risk that a cascade of feedbacks could push the Earth System irreversibly on to a ‘Hothouse Earth’ pathway.8 [Author’s italics] 29
Positive Feedbacks
A ‘positive feedback’ is what you get once you have crossed a tipping point: it’s a self-reinforcing, self-amplifying process that human beings cannot stop. For example, as the Arctic region warms, some of the permafrost thaws. The permafrost is a permanently frozen layer of ground that lies beneath the ‘active’ surface layer that thaws and refreezes each year, and it contains large amounts of dead vegetable and animal matter that has been buried within it for centuries or millennia. If the permafrost layer starts to thaw at its southern edges (as it is doing now), it will release some co2. So long as the thawing is only happening on a small scale, however, those emissions would stop again if, in the near future, we stop dumping large amounts of greenhouse gases into the atmosphere.
But suppose that we go on heating the world with our emissions, the climate warms, and the permafrost thaws over a large area. The dormant bacteria in the soil revive and start consuming the dead organic matter, and as that rots it presumably releases the ‘gases of decomposition’ (carbon dioxide, methane, nitrogen and hydrogen sulfide) into the air in large quantities. Two of those, carbon dioxide and methane, are greenhouse gases, and they, too, begin to contribute to the warming. This is the feedback process in action: we didn’t produce those gases directly, but it was the warming we caused by our own emissions that started the process rolling – and permafrost emissions could become truly massive.
There is a permafrost layer ranging from a metre to a kilometre thick beneath about a quarter of the land surface of the Northern Hemisphere, and it contains around twice as much co2 as there is now in the entire atmosphere. If we ever get that feedback moving fast, our own emissions will become almost irrelevant: we’re heading for 1,500ppm, and we can’t stop it. 30
Fortunately, this particular feedback is one of the slowest to activate on a grand scale, and it would take many centuries to thaw all the permafrost. But even a relatively small area of the permafrost thawing releases a very large amount of co2 and methane, and there are other feedbacks that act a lot more quickly. Especially since they may act in cascades.
Tipping Points
The tipping points are not a single row of dominoes (topple the first and all the rest go down), but they do come in clusters, and these clusters can cascade in a domino-like manner. The ‘Trajectories’ paper lists fifteen tipping points to worry about, such as die-back in the Amazon forest and in the boreal forests of the far North, permafrost thawing, increased bacterial respiration in the ocean, and the weakening of land and ocean physiological carbon sinks. Of course, there may be ‘unknown unknowns’ out there too.
By the time human emissions have caused 2°c of warming, that warming will itself have triggered feedbacks that add another 0.47°c to the average global temperature, for a total of (rounding up) +2.5°c – and maybe, by the year 2100, +3.8°c. There’s some room to quibble on all these numbers, but mostly on the upside, and they clearly put us on the ‘Hothouse Earth’ trajectory.
This does not mean (nor did the authors intend it to mean) that the world will experience conditions lethal to all human life in this century, or even ever. They are saying that if the human race continues on its present trajectory, with modest and slow reductions in greenhouse gas emissions and no rapid deployment of other relevant measures like carbon dioxide removal and perhaps albedo modification techniques (reflecting sunlight), then within a decade or so we will be almost 31irrevocably committed to a pathway that will ultimately make the planet quite inhospitable to human civilisation. That is to say that we will be past the tipping points and committed within decades, even if we do not suffer extreme consequences for a much longer period. It is definitely an alarm call, but it is not a shriek of despair.
A Cascade of Feedbacks
What would a cascade of feedbacks look like? The earliest we are likely to see would start with the loss of the summer ice cover on the Arctic Ocean. This process is well underway, as the warming in the Arctic is three to four times faster than in the rest of the world. Since satellite records began in 1978, the area of ice left on the Arctic Ocean at the end of the summer melt season (early September) has fallen by an average of 40 per cent. And since the 1990s, the volume of the ice is down by more than half: when old multi-year ice (up to three metres thick) melts in a particularly warm summer season, it is now generally replaced by single-year ice, often less than one metre thick, that will melt quickly in subsequent summers. Shipping using the ‘Northern Sea Route’ across northern Russia and Scandinavia as a shortcut between the Atlantic and Pacific Oceans is already in the high hundreds of commercial vessels annually.
As every year passes, we realise that the climate models, which do have some uncertainty, have all under-predicted the speed of the melting. Things are worsening at a much greater rate than anyone had thought, particularly in the Arctic. Three years ago, if you had asked me ‘Should we hold the warming down artificially?’, I would have said ‘Look, I hope we don’t have to do any of that geoengineering crap’, but now I just can’t see this predicament going in any other 32direction. And I really hope we do proper, government-funded research on how these geoengineering techniques work, and how we can do them safely because, sure as eggs is eggs, we’re going to have to do it.
Hugh Hunt, Centre for Climate Repair, Cambridge University
A summer will come, perhaps within the next decade, when there is no ice left in early September except among the Canadian Arctic islands.9 The Arctic Ocean will continue to refreeze in the winter for many more years, but the melt will come earlier each spring and the freeze-up later each autumn because the vast area of open water each summer will absorb and retain more heat. The loss of the sea ice and of snow cover on land (both of which bounce most incoming sunlight back into space) will speed the warming throughout the region – and that, plus the warmer water, will start the final melting of the Greenland ice cap. Warmer air will melt the glaciers from above, and as the altitude of the top of the ice cap drops, the air temperature at the surface of the glaciers will rise further: air temperature goes up one degree Celsius for every 350-metre drop in altitude. At the same time, the warm water lubricates the base of the glaciers, accelerating their slide into the sea. And unlike the Arctic Ocean ice, which is already floating on the ocean, the glacial melt-water raises the world’s sea level.
The sea level would rise seven metres if all the Greenland ice cap melted, but that would take centuries. The more urgent question is whether the volume of fresh water flowing into the North Atlantic might once again destabilise the warming sea currents of the Atlantic Meridional Overturning Circulation (amoc), better known as the Gulf Stream. 33
On one spectacular previous occasion, during the planet’s emergence from the last major glaciation, the flood of glacial melt-water came from Lake Agassiz, centred on what today is the Canadian province of Manitoba, which covered an area as great as the Black Sea. The volume of water was large enough to raise global sea levels by at least one or two metres. It scoured out the valley of the Mackenzie River all the way north to the Arctic coast, surged east across the Arctic Ocean, and reached 34the amoc’s ‘overturning point’ between Greenland, Iceland and the United Kingdom. As a result the amoc shut down, causing a 1,400-year return to Ice Age conditions worldwide between 12,900 and 11,550 years ago.
The moral of this story is that seemingly minor events (an ice dam collapses on a big inland lake) can flip the entire global climate into huge and long-lasting changes. It’s not just sensitive; it’s hyper-sensitive.
How Tipping Points Cascade
David Thornalley, an oceanographer at University College London, studies the past behaviour of the amoc in the hope of understanding its future behaviour. I asked him what the consequences would be now if the Overturning Circulation stopped or slowed down drastically.
They would be global in range. If there is an abrupt shutdown you would get widespread cooling over the Northern Hemisphere, focussed on the North Atlantic region: anywhere from 8 to 10 degrees Celsius, and it could happen very rapidly. You can imagine just going into one winter and it being very, very severe. The sea ice could suddenly encroach down past Iceland, all the way down to off the uk, and that could happen over the course of a year or so. That has happened in the past. [Author's italics]
Don’t get the wrong idea. That wouldn’t mean that global warming had been cancelled. The amoc would stop delivering a huge amount of heat to north-western Europe, whose temperatures would swiftly drop to what you might expect in those latitudes (Ireland is directly east of Labrador; most of Norway is level with Greenland). But the planetary warming would 35continue, and would probably overpower the local cooling after a generation or so. A roller-coaster ride in regional average temperatures is entirely possible.
The general view among climate scientists is that the amoc (the Gulf Stream) is unlikely to stop completely in this century, although since 1950 it has already slowed down by 15 per cent and is likely to slow further (see Chapter 5 for an alternative view). In any case, the heating of the Arctic will continue, leading to further changes in the Jet Stream, intensifying boreal forest die-back, and, as we have seen, thawing permafrost. Any rearrangement of the Atlantic currents, even a minor one, might also have an undesirable impact on two other tipping points: the Amazonian rainforest and the West African monsoon. The knee-bone is indeed connected to the thigh-bone.
None of these events, if and when they occur, would be directly caused by human action. Once human greenhouse gases and other human activities have caused the initial warming, the rest of the changes are independent of human action – and unstoppable. That’s how tipping points can cascade. The positive feedbacks that are the fine structure of these events self-amplify, and so great changes can come from relatively small inputs.
One last thing about tipping points: hysteresis. This is a term in physics which means that it’s easier to go through a change in one direction than to come back the opposite way. If raising the temperature by two degrees melts the Greenland ice cap, it will take considerably more than two degrees of cooling to start the ice growing again. The cost of avoiding a flip in the state of some element of the climate system is almost always much less than the cost in time and energy of trying to bring it back once it has flipped – if that is possible at all. 36
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