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 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 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 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
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 the 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 435 ppm and 450 ppm? Well, it’s similar in effect to the differ- ence 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, Distinguished 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."
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 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).

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.

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 the 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.