In 2008, I wrote a book called Climate Wars, about the science and the geopolitics of climate change, and for a couple of years afterward, 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. 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 the 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,” says Tim Lenton, professor of climate change and earth system science at the University of Exeter, “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 would 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.”
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 in the atmosphere are bad. (The reference here is not to CO2 alone but means 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 CO2.)
We are coming up on 425 ppm and adding 2.4 ppm 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 upward. 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.
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 debate—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 geo-engineering or climate engineering.
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 geo-engineering 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 anymore 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.
A “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, and the hurricanes, forest fires, killer heat waves, and the rest will grow correspondingly severe and more frequent.
“The 1.5°C target,” says Katherine Richardson, director of the Sustainability Science Centre at the University of Copenhagen, “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.”
In recent years, the various methods for reducing emissions have improved greatly, both in affordability and 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.
Just as an aide-mémoire, these are the inconvenient facts that we must always bear in mind.
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. . . . You cannot succeed if you bend later. . . . 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 percent—at the peak of the first 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 CO2 in the air has increased almost every year since the start of the industrial revolution. In 1800, it was only 280 ppm. By 1988, when global warming first became a public concern, it was 350 ppm. In 2020, it was 415 ppm, 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 in greenhouse gas emissions of 7.9 percent each year of this decade, starting in 2021.
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 emissions” technologies—that is, taking greenhouse gases out of the air—to avoid some of the warming. This is a problem because almost all of these 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 stabilize the global climate, but they cannot be deployed fast enough to help us stay below the <+1.5°C target into the mid-2030s.
Carbon Accumulates
The CO2 that we put in the air stays there for a very long time: 200 years for the average CO2 molecule. Plants absorb some of it 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, 450 ppm 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 425 ppm, we only have 25 ppm left before +2°C average global temperature becomes inevitable. The extra amount of CO2 emissions caused by human activities that accumulated in the atmosphere in 2022 was 2.4 ppm. If we continue at that rate, we will reach 450 ppm 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 435 ppm by the middle of the decade (2035).
Nobody in their right mind would willingly go to 435 ppm, 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 upward. Most climate scientists—and the IPCC’s official best guess—assume that these thresholds are almost all higher than +2°C / 450 ppm, 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 435 ppm 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°C and 2.2°C? Or between 435 ppm and 450 ppm? 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.
Predicting the Climate Is Hard
As meteorologist Edward Lorenz realized 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, distinguished professor in the department of environmental sciences at Rutgers University, 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.”
That’s all we have, so it will have to be good enough.
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 specific 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).
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 CO2 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 CDR techniques now being researched or, in a couple of cases, developed.
“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 percent, 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.
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 percent. 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 civilization 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 the 2040s—and by then, like stepping stones to the future, better means for reducing emissions and removing CO2 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 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 civilization into famine, mass migration, and war.
Excerpted from Intervention Earth: Life-Saving Ideas from the World’s Climate Engineers by Gwynne Dyer. Copyright © 2024 Gwynne Dyer. Published by Random House Canada, a division of Penguin Random House Canada Limited. Reproduced by arrangement with the publisher. All rights reserved.
