Climate 201: What on Earth is a Carbon Budget?

Author’s note: In 2020 I started publishing a series of podcast episodes under the title of “Climate 201”. The idea was to introduce topics in climate change, climate science, and climate policy — which I’ve studied for some years as a student- and explore them — and their implications — in much more detail than the simplistic framings you often see on the news. Ideally in the process I wanted to answer any questions that my audience had about climate change. By the end, the interested layperson should be up on some of the jargon and nuances of the discussion surrounding climate change.

You can find all of the episodes in the Climate 201 series here.

What on earth is a carbon budget? (TCRE)

To some extent, any “goal” that we set in terms of climate change is arbitrary. I think this is important to emphasise. We will talk often about 2 degrees C of warming as a “target” (when once it was considered a dangerous upper limit, that would take us beyond the range of temperatures that human historical civilization has lived in throughout the past).

But this doesn’t mean that the world at 1.9C, where we have technically succeeded at the Paris Agreement, is fine — and the world at 2.1C is some kind of apocalyptic hellscape. There are some systems in nature that can quite abruptly transition from flourishing to dying — such as the coral reefs, which in recent years have slipped past the point of no return and regular, mass bleaching events are now common. But there are other climate impacts, such as the disruption to patterns of rainfall that people depend on for food, the impact on extreme weather events and unusual extreme heatwaves, and the melting of ice sheets that results in sea-level rise — which we expect to get worse with each fraction of a degree of warming that we experience.

Scientists might disagree on how well we can predict things like carbon-cycle feedbacks that could result in the whole climate system spiralling out of human control. But it’s clear that the risks of something like this happening only increase as we push the climate system further and further from the stable, Holocene state that it has happily existed in for so many years.

We are already living in a climate-changed world. Depending on your baseline temperature, we’ve already warmed by between 0.9 and 1.1C from the preindustrial state of the climate. The impacts are only going to become more extreme over time.
I say all of these disclaimers at the start of the episode simply to point out that — while we often frame the discussion of climate change in terms of particular temperature “targets” and what we would need to do to avoid getting to that level of climate change — it’s not the only way of thinking about it. The reality is, if we exceed 1.5C of warming, or 2C of warming, or 2.5C of warming, etc. — it will still be worth doing all we can to decarbonise as rapidly as possible and avoid further warming in the future. But this is not a problem that switches on and off with some arbitrary target; it accumulates. The temperature “targets” are important in that they give us some nominal, notional barrier to prevent too much climate damage from accruing and accumulating. It’s convenient to talk about them to give us something concrete to model and to avoid, but they are not the be-all and end-all of climate change.

With all that said, we’re going to talk about the concept of carbon budgets and the associated temperature targets today. Broadly speaking a “carbon budget” is the cumulative amount of CO2 that can be emitted by humanity while staying within a certain temperature target, with a certain level of probability.

The idea here is that we can calculate the amount of CO2 that we could still emit and stay below 2C, as a guideline for humanity on how quickly we need to cut emissions.

So let’s unpack this a little bit. You might hear someone say, for example, that we can emit 400 billion tonnes of CO2, cumulatively, from today, to have a 66% chance of staying below the 1.5C temperature target. How is this probability determined, and what’s being included in this argument of a carbon budget?

Firstly, the reason we can only express this as a probability of staying below the temperature target arises because of our uncertainties about the climate sensitivity — how much warming we expect when CO2 concentrations change. Since we don’t know this number exactly, the best we can do is express it as a probability distribution. So part of the uncertainty, or error bar, that comes with a carbon budget will be due to uncertainty around the climate sensitivity. If climate sensitivity turns out to be on the higher end of estimates, then you have a smaller carbon budget leading to the same amount of warming.

So, for example, if we assume climate sensitivity to be at its median value from some certain distribution, and then calculate a carbon budget based on that… there’s a 50% chance that the climate sensitivity is higher, and a 50% chance that it’s lower. Therefore, we can only say that there’s a 50% probability that, if we use up the whole carbon budget, we’ll actually stay below 2C.

Already, then, this can create some interesting issues with how people express carbon budgets — because the budget itself is different depending on with what certainty you want to keep below a certain target.

For example, the IPCC’s special report on 1.5C gave a few different “most likely” carbon budgets for different levels of warming. They expressed the carbon budget with a 66% chance of avoiding 1.5C as 420GtCO2, and the carbon budget with a 50% chance of avoiding 1.5C as 580GtCO2.

This said, you can call into question what’s meant by setting a target. If you’re trying to avoid 1.5C of warming, is it enough to take actions that only result in a 66% chance of avoiding 1.5C? Or 50%, so that you’re more likely to avoid that warming than not? In many walks of life, promising a 50/50 chance or a 66% chance that you will meet a particular goal or target is not especially convincing. But if you want to be more stringent, and insist that your budget would result in, say, a 95% chance — almost certainty — that you won’t exceed 1.5C of warming — then you’re going to have a much smaller carbon budget, to account for the chance that the climate is more sensitive than we estimate it to be on average.
Incidentally, the carbon budgets in the 1.5C report are likely the main source of the — frankly kind of misleading — “12 years to save the planet” claim that was widely reported as the headline conclusion of that report. This claim, as far as I can tell, seems to have been reached by people just dividing the 66% budget of 1.5C by the annual emissions at the time, which were about 36 billion tonnes of CO2 a year. You do that, and you get pretty much exactly 12 years.

But obviously this is a really overly simplistic message to come out from the report, and one that is frankly too simple to be useful and far more likely to confuse. For a start, 12 years assumes that emissions don’t change. If they increase, then obviously, you have less time. If they decrease rapidly due to climate policies designed to reduce them, then obviously, you have longer than 12 years.

And then, of course, 12 years to do what? Say emissions are basically constant for 12 years. Then we know that 1.5C is likely to be exceeded. That’s bad, but it’s not the end of the world — it’s not like there is suddenly no point to climate action after 12 years, because we still want to limit the damage from climate change as much as possible. However, if emissions really are the same in 2030 as they were when the 1.5C report was written, obviously we’re finding it hard to cut emissions. So the final temperature is much more likely to be a lot higher than 1.5C — even if we started cutting emissions straight after that, the time it would take to get to net zero would guarantee further warming coming down the pipeline in the future.

So the “12 years to save the climate” slogan, which I’ve seen in some truly ridiculous forms by people who really don’t care about getting the science right — including “12 years of habitable climate”, “12 years to save humanity”, “12 years until the point of no return when feedback loops will kick in and take the climate out of our hands” (BigThink), “12 years to stop climate change” (Metro) — it’s all bullshit and not even close to what the report actually said. It’s misleading because, again, it gives the picture of the world dropping off some horrendous cliff in 12 years time unless we do something, which is simply not what is going to happen. I understand people’s desire to create a sense of urgency around some certain time-frame, but I don’t know how effective it is at getting people to take action.

The reality is that, if we want to keep to below 1.5C, we really don’t have any time to lose at all — emissions have to start falling pretty much straight away for any kind of reasonable pathway to net zero that won’t blow through that carbon budget, and the more climate action we can take now, the safer and less damaging the future will be for everyone.

So away from the specifics of these carbon budgets, let’s talk about carbon budgets more generally, because there are several important bits of science and debate that go into how they can be constructed and then how they are historically and politically used.

It’s not scientifically obvious — at all — that you can construct a simple carbon budget. Instead, it relies on an observation in modelling — that, regardless of the path you take to get there, the global mean temperature increase seems to depend approximately linearly on the cumulative emissions of CO2. In other words, within certain limits, the amount of temperature increase you get is approximately proportional to the total amount of CO2 you’ve emitted.

This allows you to define a new kind of climate sensitivity, the Transient Climate Response to Cumulative Emissions or TCRE — which says, for example, that every trillion tonnes of CO2 you emit, the climate likely warms somewhere between 0.8 and 2.5C (with a best estimate of around 1.35C per trillion tonnes of carbon.) Then, with this nice linear relationship between cumulative emissions and warming, you can say that every tonne of CO2 that’s emitted gives you a certain amount of additional warming, and come up with basic estimates for how much CO2 you can emit and still likely remain below a given threshold of temperature increases.

Now this is only an approximate relation. And at first glance it might be a surprising one. In the episode on greenhouse gases, we pointed out that the relationship between CO2 concentrations and radiative forcing is logarithmic. In other words, as additional molecules of CO2 fill in an increasingly opaque atmosphere to the infrared radiation that’s escaping, each molecule is less effective at producing warming — more of the light that it would absorb has already been absorbed by other CO2 molecules as the concentration increases. In terms of CO2 concentrations, the warming that we expect is logarithmic in nature — in other words, it’s the same every time we double. So, ignoring feedbacks for a second, you’d get the same effect in doubling CO2 concentrations from 250ppm to 500ppm, and then the same warming again in doubling from 500ppm to 1000ppm, although clearly the second doubling requires twice as much CO2 to end up in the atmosphere.

So how can we reconcile this with an approximately linear dependence of warming on CO2 emissions? The answer comes from modelling, where these trends were first observed across a range of different models around 2010, leading to the development of the TCRE and carbon budgets. However, this specific question is well explored by a paper by MacDougall and Freidlingstein in 2015 — the Origin and Limits of the near proportionality between temperature change and cumulative carbon emissions.

Essentially, as you add more CO2 to the atmosphere, it’s true that each CO2 molecule is less effective at trapping heat in the atmosphere. But it’s also true that, as the planet warms and the atmosphere becomes more saturated with CO2, the ocean becomes less effective at taking up heat and CO2. The ocean is like a vast regulator when it comes to CO2 emissions and the heat that they trap. Approximately half of the CO2 that we emit ends up dissolved in the oceans, but between 90–95% of the heat that’s trapped by greenhouse gases ends up as a change in the ocean heat content. You can often view the role of the oceans in the Earth’s climate as acting like a big inertial thermal bath. This will be familiar to anyone who has lived near the ocean. In this phase, the oceans absorbing heat are slowing down the surface warming that we’d normally expect to happen. Relatively small changes in the ability of the ocean to absorb this excess heat can result in large effects on the temperature at the surface.

In the models, then, it has been observed that under a wide range of different conditions — under a range of different modelled scenarios — the temperature ends up being roughly proportional to the cumulative CO2 emitted, regardless of the pathway you took to get there. And this is likely due to these two effects compensating and cancelling each other out — extra CO2 has a diminishing effect on heating the planet as it’s emitted, but the oceans have a diminishing ability to absorb the heat and CO2 that we are producing as we emit more, resulting in a larger fraction of that heat changing the surface temperature that we experience, and the net result is that the total surface T change that we get is approximately proportional to the total CO2 we’ve emitted.

So now to bring in the caveats. This is only true within certain limits — if we emit more than 2 trillion tonnes of carbon dioxide, the relationship starts to break down. If climate-carbon feedbacks start to become more important — in other words, if the natural emissions of CO2 from feedbacks such as forests dying off in the heat or methane being emitted when the permafrost melts — then this relationship will also break down. And one major thing to point out is that this ignores all of the non-CO2 ways in which we influence the climate, both regionally and globally — changing the albedo of the surface by cutting down forests and melting ice-caps, emitting aerosols that interact with clouds and reflect additional sunlight to space, having a cooling effect, and also via all of the non-CO2 emissions of greenhouse gases like methane and nitrogen dioxide that also have an impact on the climate.

The general argument, when it’s presented, is that this is a CO2-only budget. We know that CO2 is the most important driver of anthropogenic climate change, so it makes sense to focus on that. And some further argue, although I would say this is still unclear, that the warming short-lived climate pollutants like methane approximately cancel out the cooling effects of aerosols at the moment. That would mean, then, that most of the warming we see is due to CO2, with the effects of other pollutants cancelling each other out.



That may be true, but of course, it does make the carbon budgets a little bit misleading. If we actually look at the trends that are taking place on Earth at the moment, and that we might expect for the future, there’s no reason to expect this cancellation to continue into the future. As Europe got wealthier, it instituted lots of air pollution laws and substantially reduced the amount of aerosols that are emitted into the atmosphere, due to their harmful health effects. [This was also of course helped by Europe offshoring a lot of its industry to other parts of the world.] We might expect something similar to happen in China and India, as air pollution restrictions become tighter to reduce the urgent health issues associated with smog. This will reduce aerosol pollution and therefore reduce the cooling effect associated with those aerosols. Don’t get me wrong: it’s urgent to do this; air pollution leads to one in six premature deaths globally, and it’s frankly an international disgrace that we allow companies to pollute the atmosphere with such impunity and exacerbate health conditions for so many people.

At the same time as aerosol emissions will likely reduce, though, it seems that natural gas is only going to become more popular as countries move away from expensive coal. And, in addition, as more and more people become wealthy enough to demand meat, the methane emissions from agriculture are only likely to increase. So, to me, at least, it seems most likely that methane emissions will increase in the near future, while aerosol emissions will decline — and that means that the net effect of these non-CO2 pollutants will be towards short-term warming in the future.

If this turns out to be true over the next few decades, then it means that we could easily exceed the 1.5C temperature target well before the 1.5C carbon budget is used up. Carbon budget purists would argue that this would only be temporary — and that if we could get a hold on short-lived climate pollutants, we could return back under 1.5C relatively quickly, compared to the case when we’ve exceeded 1.5C of warming due to CO2 and would need negative emissions to return to below 1.5C. This is true, but since “getting ahold of methane emissions” means massive reforms in agriculture and other sectors, it’s not clear that it will be all that easy.

This is the source of one of the debates surrounding carbon budgets, and what it means to actually “exceed” a temperature target. Is a temporary overshoot allowed, providing we get temperatures back below 1.5C before the end of the century, as specified in Paris? What is the definition of surface temperature, anyway — because depending on which dataset you use for surface temperature, it can be a different number based on how much you weight different regions and compensate for imperfect measurements. And, similarly to this, temperature increase relative to what? There are arguments over whether to define the preindustrial era as the few decades surrounding the 1800s — when we know that human influence on the climate was minimal, but our measurements are also pretty bad — or whether to use a more “near-term” baseline for warming. Naturally, because this changes the definition of 1.5C of warming — relative to what, you might say — it also changes the carbon budget that we have left. And, indeed, it was changes in what the definition of temperature increase meant which changed the carbon budget between the last big IPCC report and the latest one. The AR5 IPCC report, before the special report on 1.5C, had a much smaller carbon budget than we are currently using due to all of these different changes in definition.

Carbon Brief, in its article on carbon budgets, summarises the causes behind some of these disagreements in the following way:

The precise meaning of the 1.5C target.

• Disagreement about what “surface temperature” actually refers to.
• The definition of the “pre-industrial” period.
• What observational temperature datasets should be used.
• What happens to non-CO2 factors that influencing the climate.
• Whether Earth-system feedbacks like melting permafrost are taken into account.

So, again, like many debates in the climate space — some of this is sure to seem very arcane and pedantic. But, naturally, the implications of some of these seemingly minor differences can be pretty huge. Between AR5’s definition and the new definition, we’ve gone from 1.5C being just 3 years of current emissions to 12 years. In other words, 1.5C goes from a hopeless target to something that one can just about dream as being attainable with unprecedented, massive international action to cut emissions, just based on the definition of the budget.
The carbon budget and its definition is also important when it comes to trying to understand what level of carbon emissions humanity has left before we’ve conclusively failed at our Paris Agreement target of keeping warming below 2C. So disputes over its precise nature can have global importance. Personally, knowing a few of the people involved, I feel like there was some movement in the 1.5C report to try and come up with a way of defining the carbon budget that made the target still seem achievable, to spur policy-makers into action by saying that we could still reach the 1.5C target. After all, the whole report was about the differences between 2C and 1.5C, and what we lose by exceeding Paris’ first target of 1.5C. So to investigate all of that, and then say that 1.5C is impossible, would be too depressing a conclusion for the report to reach. Even though, to all intents and purposes, barring something truly astounding, 1.5C is unreachable. Similarly, people can choose much smaller carbon budgets and more pessimistic definitions of temperature if they want to make a different argument. It’s almost as if the idea of a “carbon budget” and “temperature targets” aren’t really doing what they are supposed to — getting everyone to agree on what we need to do — because of the leeway and flexibility that remains in interpreting and defining them. But hey, that’s just me talking.

And sadly, estimates for carbon budgets are still a little bit all over the place. For example — we quoted the carbon budget from the special report for 1.5C, which gives us a 66% chance of staying below 1.5C, as 420GtCO2. But there was a paper in 2017 by Richard Millar et al. Which argued that the actual budget was more like 540GtCO2 — mostly due to changes in the models used and a different definition of a temperature baseline. Meanwhile, a paper by Jason Lowe of the Met Office and others in 2018, which attempts to take into account some more carbon-cycle feedbacks, suggests that the definition of the budget for 1.5C might be as low as 67GtCO2. Zeke Hausfather’s 2018 article on Carbon Brief, “How much carbon is left to limit warming to 1.5C”, draws together estimates from a whole range of different papers of different carbon budgets, including some way more in-depth analysis about why they’re different than I will go into here. Suffice it to say that even using the same sensitivity and methodology as the IPCC report, but with different definitions of temperature, you can get a carbon budget of anywhere from 260GtCO2–570GtCO2.So now you can start to see why the 12 years meme is so unhelpful. Not only is the carbon budget for 1.5C not some kind of magical climate cliff-edge, but plenty of respectable climate scientists would disagree about when or whether we’ve actually exceeded it… or what it even is. There is certainly not a magical deadline here. Even just changing how you define “global mean temperature” can change this silly number quite reasonably from 7 years to 16 years.

Beyond the technical fripperies of a carbon budget, there are of course other objections. Again, this idea of a budget that runs out sort of implies that you can “safely” emit billions of tonnes of CO2 into the atmosphere up to a point, and then somehow it becomes “unsafe”. This is obviously not what’s happening at all. The CO2 that we’re emitting now is harming some people, on average; it’s making the world riskier in the future; it’s certainly not “safe” to do it until our budget runs out.

If you’re sharing a kitchen, you hardly agree on a deal that says it’s okay to let the washing pile up in the sink, as long as it doesn’t hit the ceiling. But there is a concern that there is a “permissive” aspect to carbon budgets.

One key point that’s worth noting here is how, over the years, the idea of “temperature targets” for climate change has been changed and corrupted. The idea of a “carbon budget for 2C” implicitly suggests that we can, and probably should, “spend” all of that budget to get there. And so 2C has gone from a dangerous, upper limit for climate change — one that takes us well outside of temperatures the human race has ever experienced — into suddenly some kind of “target” that we are aiming to hit by spending our carbon budget.

And of course, also, the presence of negative emissions technologies in so many IPCC scenarios — this idea that we’ll just suck loads of CO2 out of the atmosphere at the end of the century with technologies that are yet to be invented or deployed at scale — that obviously also completely blows up the idea of a carbon budget. If you allow yourself to imagine that you’ll suck 10 billion tonnes of CO2 out of the atmosphere at the end of the century, and the only thing you care about is the “net cumulative emissions” of CO2 by the end of the century, then you can argue that it’s still “within budget” to emit that CO2 now. You are, effectively, “expanding your budget” by borrowing loads of CO2 emissions from the future, which you trust they’ll be able to pay back with negative emissions later on. You can imagine how attractive this policy of prevarication is to economists, with their models with discounting which argue that money today is worth more than money tomorrow, because the economy will grow forever…. Or how attractive it is to the politicians, who can kick the job of cutting emissions down the road to someone who’ll be in office a few decades later. Yikes. So negative emissions, and the increasing dependence of the world on imagining them to meet the more stringent climate targets, also blow up the concept of a carbon budget.

Having spent nearly the whole episode explaining all of these disputes surrounding how valid it is to define a carbon budget, let’s talk about what the implications are for what we think it is at the moment — and for this, we’ll take the 66% budget for 1.5C and 2C that come from the IPCC report, even though not everyone will agree with them. You should know by now not take these budgets too seriously or with too much of a sense of finality… but, like the temperature targets, they exist as guidelines, some reasonably-arrived-at number to pin our efforts towards.
That 1.5C budget was 420GtCO2 in the SR1.5 report was from the start of 2018. In 2018 and 2019, global CO2 emissions were about 40 billion tonnes a year. I’m writing this in June 2020; COVID-19 means that any prediction of what CO2 emissions have been for the first half of this year are probably massively wrong, as it’s not been a normal year, so I’ll just go up to the start of 2020. That would then mean that we have around 340GtCO2 left in the 1.5C budget. Collectively, since 1750, humanity has emitted around 1.5 trillion tonnes of CO2, or 1500GtCO2, according to Our World In Data. To date, around 350GtCO2 has been emitted by the EU-28 countries, 457GtCO2 by North America, 457GtCO2 by Asia, and so on.

In other words, if we want to stick to this 1.5C budget, then we have already emitted 82% of the CO2 that we can ever allow ourselves to emit as humans… and our emissions are higher than they’ve ever been, so we’re chewing up the tiny 18% remainder of that budget faster than every before. It’s not looking good.

The budget for 2C is substantially higher, it’s worth saying — around 1170GtCO2, or 1090GtCO2 after the last two years — so that would amount to about 26 years of current emissions to have a 66% chance of staying below 2C of warming. Arguably, then, this target is a little bit easier to achieve — providing we can get emissions to zero not too much later than 2050, we should be able to achieve it, although obviously, given that emissions have grown exponentially throughout human history (coronavirus and major recessions apart), that’s easier said than done.

What can you do with 340 billion tonnes of CO2? You could have another 8 and a half years like 2019. Since the average per capita emissions of a US citizen are 16 tonnes a year, if the Earth’s 7 billion citizens all emitted as much per capita as the US does, we could last 3 years before blowing that budget to pieces; the numbers are similar for Australia. For the UK, that would be around 9 years. If we all emitted as much carbon per person as China did, it would be about 6 years.

If you imagine dividing this up on an even, per-person basis, from tomorrow, then each of the 7 billion people alive today would get about 50 tonnes of CO2 to emit for the rest of their lives, prior to hitting net zero. I dislike this kind of personal framing of emissions because, of course, there are structural factors that we don’t have personal control over that influence CO2 emissions. But just for fun; that 50 tonnes of CO2 would let you fly from London to New York 50 times, or from London to Perth 15 times. A typical US vehicle emits 4.6 tonnes of CO2 per year, with a fuel economy of 22mpg and driving 11,500 miles on average, according to the EPA. A few years of driving, then, would take a significant chunk out of your own personal budget.

When you look at this, then, you can see why we haven’t done the obvious thing. One might conclude that the reason for calculating a carbon budget might be to say: okay, this is the total CO2 that we can emit before exceeding the Paris target — let’s find some way of dividing up the remaining budget between every country, or even everyone, and we can all be sure that we do our part not to exceed it.

Maybe the Paris Agreement can involve binding targets on each country, so that if they do exceed their share of the carbon budget, they are responsible for sucking CO2 out of the atmosphere — or they can trade it with countries that successfully cut their emissions faster. We’d be treating the remaining “permissible” CO2 emissions as what they are — a scarce resource!

But you can see immediately why this would be a total non-starter. Not only would people end up horribly squabbling over how to fairly allocate the budget — by population? By GDP? By the nature of the industries that are dominant in that country? Do you take into account the dependence of certain countries on fossil fuel exports? Should developing countries get a larger slice of the pie as they focus on economic growth, and because they are less able to invest in decarbonisation? How do you account for the fact that China emits CO2 producing products that are ultimately consumed in the West? What do you do about international aviation and shipping — do you share their emissions evenly between both destination and source? But there would also be incredible squabbling over how to actually define the budget, with the different ways of interpreting it becoming way more political than they already are. It would also probably lead to regulations so stringent that most countries simply wouldn’t sign up to them.

Then you’re left with this fundamental (and depressing) irony in the middle of action on climate change. Everyone says that they are committed to the Paris Agreement. But no one is telling you how their actions are actually going to add up to achieving the Paris Agreement. No one is telling you what slice of the remaining carbon budget they are willing to take; and no-one is credibly demonstrating how they will reduce their emissions to stay within their slice, or making a coherent argument that the slice of the pie they’re taking is fair.

Let’s take my favourite example — the UK! The UK is one of the few countries that actually has legally binding carbon budgets, based on the Climate Change Act 2008, for each 5 year period. Although these budgets haven’t yet been updated, and the independent Committee on Climate Change says that we are not on track to meet some of the more stringent ones, we can still look at them and determine how large a slice of the carbon pie the UK’s own internal climate targets allocate to itself.

It’s a little hard to be precise here, because the UK recently changed the law so that the goal is now Net Zero by 2050 and not 80% reductions in CO2 emissions by 2050. However, our more recent carbon budgets have not yet been updated to reflect this, and the ones up until 2032 are already set in law.

I assumed that only the post 2032 budgets will be updated, causing CO2 emissions to linearly fall to zero by 2050 from 2032, and then estimate what this means using some back of the envelope maths. This calculation shows that the UK’s legislation allows it to emit about 9.3 billion tonnes of CO2 from 2018 until its emissions hit net zero.

So the UK’s legislation, from a 2018 perspective, has effectively allocated us with 2.2% of the world’s remaining carbon budget for 1.5C. This is despite the fact that we have just 1% of the world’s population. Interestingly, we amount to almost exactly 2.2% of the world’s GDP; I wonder if this is a coincidence, or not?

If this is not a coincidence, and those in charge are consciously working towards this idea that a “fair share” of the world’s remaining carbon budget can be allocated by GDP, then you have to ask about the questions of fairness and equity implied here. The UK’s GDP is about on a par with that of India — India are probably overtaking the UK in GDP as I write this. So allocating future emissions out to 2050 by GDP means that the 70 million people of the UK get a “pass” to emit as much as 1.4 billion people who live in India at the moment. You can definitely argue about whether that’s fair!

And, given that the UK has some of the most stringent climate targets in the world, and is generally considered a leader in mitigation compared to a lot of other countries, you can see the issue here: even one of the developed economies that is relatively good on climate and has been pretty successful so far is trying to take an arguably unfair slice of the pie for 1.5C in its rules, and this is more stringent than many other people are willing to be. It’s hardly even possible for me to work out what many other countries are intending to take as their “slice of the pie” in this world where we try and tackle climate change by dividing up the carbon budget, as many of them don’t have legally-binding targets. I suppose you could estimate it by taking the countries that have net zero targets and assuming their emissions will reduce at a constant rate to get there, but that seems a bit optimistic.

So, although carbon budgets aren’t iron-clad and never will be, they can illustrate — in cases like this — where current policies are unfair, lacking, or likely incompatible with the Paris Agreement on the world stage.

So this concludes the episode on carbon budgets — what they are, how they can be defined scientifically, how they influence climate policy, how it’s possible to spend hundreds of hours arguing over their precise definition, why they should be taken with a grain of salt, and what they can tell us about how much CO2 we can emit to stay below certain levels of warming.

They are a useful tool, but don’t let them — or misleading framings around 12 years left to save the climate — distract you from the simple message: the faster we act on climate, the less damage there will be, and the less dangerous things will be. We know that acting on climate will save us vastly more money in the long run than it will cost to fix the problem; we know that it will save lives and prevent environmental disasters from damaging the lives of millions of people around the world; we know that the consequences of failing to act will end up being disastrous, disproportionately impacting the most vulnerable, and making life in the future and for future generations far worse than it otherwise could be; we know that many of the measures we can take will result in a more sustainable, cleaner, healthier society; we know that in the midst of a Great Depression there’s a lot to be said for having a big, collective investment project to work on, to stimulate the economy and give people something useful to do; and we know that doing so will wean us off fossil fuels that will eventually run out, with volatile prices, other nasty health side effects, and geopolitical tensions and wars endlessly being fought over the supply… so… budget or no budget, what are we waiting for?

Thanks for listening etc.