The world owes a great debt to Extinction Rebellion. XR’s heyday protests in London helped deliver a UK net zero commitment, forcing many other countries to adopt similar targets. XR has already changed the world – if governments stick to these targets, we have a significantly higher chance of avoiding climate catastrophe and many lives and ecosystems will be saved.

More latterly, Just Stop Oil has taken up the mantle. Its protests have been inconvenient for motorists and sometimes unpopular, but as the UK government backs away from its climate commitments and allows new oil drilling civil disobedience becomes more and more both a moral and a political necessity. Roger Hallam, as one of the key originators of both groups, is someone I admire and respect both as a much-needed voice of reason and a strategist.

Although I have not attended XR or JSO protests, I have worked to assist in legal support for activists who have put their liberty on the line in bringing attention to the climate emergency. I am constantly in awe of their commitment and bravery, and am honoured to assist in any way I can. That people will risk significant jail time in the wider service of humanity and protecting the living planet is a testament to the very best of human nature.

One way of supporting these essential movements that I hope I and others can offer is to help ensure that the science on which they act is as robust as possible, so that all the demands they make are evidence-led and cannot easily be dismissed by politicians and the media.

It is particularly important to be fully aligned with the science when we are asking people to put their freedom on the line by taking direct action. Risking arrest and a possible prison sentence is a momentous thing in peoples’ lives, and should be based on as accurate an assessment as possible of the situation. Moreover, if people are going to rely on climate science in any legal defence this must accurately represent the state of the science or we risk undermining activists’ legal cases.

I have never met Roger Hallam, and I hope he will excuse my unsolicited intervention and even welcome my thoughts on this. Roger has been a bold and strategic thinker and innovator, and has already achieved remarkable successes both with XR and JSO. That is why it is so important that we all, and Roger in particular as such a crucial voice, clearly represent the current state of climate science.

Roger was interviewed by the BBC’s Nick Robinson recently, in a show called ‘Political Thinking with Nick Robinson – the Roger Hallam one‘. I was not impressed with Robinson’s performance. His questions were parochial, trivial and poorly informed. Some were especially cheap shots or attempts at gotchas, such as when he brought up the issue of whether Roger’s daughter supports XR or not. But Robinson’s chief failure lay in the other direction – he completely failed to challenge or even examine Roger’s starting premise, which was a straightforward and very compelling statement about the science.

Roger set the scene by reading a quote from a paper. He claimed there was “nothing particularly unique about this”, presumably meaning that in his opinion the paper is roughly representative of the current state of the science. He was at pains to emphasise also that the paper had been “peer-reviewed” and therefore was scientifically rigorous. The quote went as follows: “If warming reaches or exceeds two degrees centigrade, mainly richer humans will be responsible for killing roughly 1 billion mainly poorer humans”.

Roger then added, in response to Robinson’s rather lame attempts at clarification, that this was a “death project” whose “tool” was the “destruction of the climate”. He added that it was merely an “interesting intellectual digression to split hairs over what degree of murder it is” but that would merely be a “displacement activity” – instead we should allow ourselves an emotional reaction and “feel the terror and horror of what I’ve just said”.

Strikingly Robinson at no point challenged this statement or asked for more detail about how these numbers – stated with confidence and precision – were generated. Nor did he ask Roger to make clear whether he believed that this paper accurately represents the current state of climate science overall.

This is very important for the rest of the interview, because Roger’s premise is vital. If he is correct that a billion people are about to be killed, then presumably more than just civil disobedience might be warranted. Why not violence? In my moral calculus it would certainly be justifiable to injure and kill if this would avoid a billion deaths. I think killing Nazis was evidently justified in order to shorten the Holocaust (and indeed later in the interview Roger stated his belief that the climate deaths would be “worse” than the Holocaust “because of the scale”.) Why restrict oneself to non-violent civil disobedience, except for merely tactical reasons?

Indeed, when you are dealing with numbers this large, it is not even clear how a moral calculus would work. One of the few pieces of mainstream science work I’m aware of that does present a quantified death toll was published by the WHO a few years back and projects an annual death toll of 250,000 people a year between 2030 and 2050, which sums to 5 million over those two decades. Is that not already enough to justify an emergency approach to climate activism? Or does it need to be 50 million or 500 million? There is no good answer to this question; no clear threshold either side of which certain forms of action are or are not justified. In other words, even if we could accurately predict the number of deaths a given level of heating would cause, this would not help us to resolve the question of how we should respond. Citing unverifiable numbers does nothing to help our moral case, while jeopardising our intellectual case.

Robinson failed to pursue this line of enquiry, and indeed said explicitly that they were not there to talk about the science, which was – for some strange reason – “for another time”. He therefore had no option but to accept Roger’s premise, and flailed accordingly through the next 40 minutes as Roger – quite reasonably – insistently wondered why Robinson personally was not prepared to act accordingly given that “twenty times more people than were killed in World War Two” were about to die. Robinson could not even object when Hallam stated that the UK government will be guilty of “mass murder” on the basis of these numbers, and that its stance is “treasonous” as a result of its failure to defend the fundamental right to life of the British people.

Roger then went on to say that these 1 billion deaths are “effectively locked in” and therefore cannot be avoided by any realistic mitigation of greenhouse gas emissions. Crucially Roger also said that he had come into the interview to “say something that is fundamentally truthful”, and that when he helped to initiate XR and Just Stop Oil “my fundamental rule is to say the truth and act as if it’s real”. He added: “What we are dealing with here is the objective reality of physics” that if “the elites” continue pumping out greenhouse gases, “then billions of people will die “.

Robinson thereafter tried to change the discussion back to the tactics of civil disobedience, but as Roger rightly asserted, he was merely missing the point. Here Hallam had been very precise: at least a billion people are unavoidably going to die due to the climate emergency, this is an objective truth as undeniable as physics, and people should therefore feel morally compelled to act accordingly. This was all admirably clear, and if Robinson was going to accept the premise, he should accept the moral implications of doing so.

I am obliged to Roger for stating his source so clearly. This is good practice, is admirably transparent, and allows an examination of the scientific basis of his claims. I looked up the paper he referred to, which contains the assertion of 1 billion deaths. I felt motivated to do so because I knew that this is not an assertion which represents the mainstream opinion in climate science. Indeed I cannot think of a time when I have heard an actively-publishing climate scientist make such a claim. Nor have I come across a figure of one billion deaths at two degrees – or indeed any quantified projected death toll figure for overall climate impacts – throughout my more than two decades researching and publishing climate books and advising governments in the Global South on climate impacts. Nor is this something you will find in the reports of the IPCC, which are widely accepted as representing the scientific consensus.

So I already felt that Roger’s claim that this 1 billion deaths figure represented objective scientific truth was suspect, and I wanted to evaluate the source for myself. The paper Roger referred to is entitled ‘Quantifying Global Greenhouse Gas Emissions in Human Deaths to Guide Energy Policy‘, published in Energies journal on 19 August 2023. The abstract of this paper does indeed include the claim that Hallam read out in his BBC interview, which in full reads: “If warming reaches or exceeds 2 °C this century, mainly richer humans will be responsible for killing roughly 1 billion mainly poorer humans through anthropogenic global warming, which is comparable with involuntary or negligent manslaughter.”

The paper’s lead author is Joshua Pearce, an electrical engineer at Western University in London, Ontario, Canada. Pearce’s other papers seem to be mainly about solar photovoltaic power systems, so his career academic expertise is not directly relevant to the subject of the paper. This to me is a bit of a red flag: I get skeptical when big climate claims are published by people who are experts in a different field than climate science. Pearce’s co-author is Richard Parncutt, and indeed it a previous paper by Parncutt which – in reference 59 – is the source for the claim in the body of the paper that “2C of anthropogenic global warming… will cause roughly a billion future premature deaths spread over a period of very roughly one century”.

Self-referencing is my second red flag, particularly when it comes to extraordinary claims such as this. Ideally other authors would have written on the subject and a wider body of work could be referenced to buttress the case. The third red flag is Parncutt’s own expert credentials: his affiliation is simply given as “University of Graz, Austria”, but when I looked him up it appears that he is a musicologist, not a climate expert. As I said, reference 59 leads to an earlier paper by Parncutt, published in 2019 in a journal called Frontiers in Psychology.

Frontiers in Psychology is, unsurprisingly, not a climate journal (a fourth red flag), and is therefore an odd place for a paper such as this, suggesting that the peer-review process might have been somewhat sub-optimal. Two peer reviewers are listed on the paper (this kind of transparency is very useful in science for precisely these reasons, and is to be commended): one is a professor in educational research methodology in Spain, and the second is a Scottish expert in soil carbon sequestration. I mean no disrespect to the peer reviewers, but I do not think their experience is directly relevant to the arguments that are presented in Parncutt’s paper.

Still, all of these are arguably ad-hominem issues, so let’s look instead at the substance of and evidence for Parncutt’s figures. This can be found on pages 7 to 10 of the 2019 paper, which is entitled ‘The Human Cost of Anthropogenic Global Warming: Semi-Quantitative Prediction and the 1,000-Tonne Rule‘. I must go into the weeds here, but this is at the heart of it, so please do not skip. There is no TL;DR here, and this is precisely the point.

Parncutt rightly acknowledges that claims about climate impacts in the future are inherently uncertain. He cautions that his are “no more than order-of-magnitude estimates” (OMEs), and that “the uncertainty of an OME is itself uncertain”. Parncutt originates his billion-deaths figure by the following method: he decides that “the expected value of the total global death toll caused by 2°C AGW lies logarithmically midway between a likely best-case scenario of 3 × 108 and a likely worst-case scenario of 3 × 109.” (Just to be clear, 10 to the power of 8 is 100 million, and 10 to the power of 9 is 1 billion.)

But how are these OMEs derived? For the best case scenario, Parncutt provides a list of climate impacts, including higher temperatures, longer heatwaves, more flooding and drought, more intense tropical cyclones, higher sea levels, dying coral reefs, species extinctions, food insecurity, spreading disease and so on. All of these are undoubtedly climate-relevant issues, and for all of them there is a voluminous literature, summarised indeed in several IPCC reports.

But for none of these issues is there any evidence cited for a quantified death toll figure. Parncutt’s calculation is that the issues he lists (and I’m quoting in full here): “are consistent with a best-case scenario in which AGW increases the current death rate in connection with poverty (roughly 107 per year) by 30% to 1.3 × 107 per year, the difference between the two rates (with and without AGW) remaining roughly constant for a century. The total death toll due to AGW of 2°C would therefore be 100 × (3 × 106) = 3 × 108.”

This all sounds very mathematical, but the important thing is that there is no evidential basis for the numbers. Parncutt says he is merely giving a “zeroth-order OME”, but he goes on to use this number very precisely in the overall conclusion of the paper, which is that two degrees of global warming will cause a billion premature deaths spread over one to two centuries. This to my mind is a sleight of hand. If there are no numbers to start with, a “zeroth-order OME” is just a clever way of saying a “guess”. This is a very different thing from a scientifically-valid quantification.

And this was the ‘best case’. To come up with a ‘worst case’ number, Parncutt gives an even more extensive list of climate impacts, from the food security implications of biodiversity loss to the water supply implications of the loss of glaciers. I am the first to admit that any and perhaps all of these will have impacts which lead to premature deaths – but the question here, if we are going to attempt a quantification, is how many. None of the citations include a specific number to my knowledge (I have read some of them but not all), and Parncutt does not quote any numbers in his list.

Similar to his best-case scenario calculation, Parncutt then states: “In a zeroth-order estimate, one or more of these points could alone cause 107 deaths per year for a century—a total of 109 deaths each. If that is true, a worst-case estimate of 3 × 109 for the worst-case AGW death toll may be realistic.” Parncutt then puts his best-case and worst-case scenario numbers into a “mathematical model”, using a Gaussian probability distribution to generate a figure roughly halfway between the two.

But this is bogus. However impressive one’s mathematical model, putting two guesses in as inputs can only generate another guess as an output. An oft-quoted adage within the climate modeller community is that garbage in equals garbage out. At best Parncutt’s conclusion seems to be an example of false precision, where a precise number is generated by flawed means which disguise the inherent uncertainties and claim a false level of confidence in the result. To be fair, Parncutt does introduce lots of caveats, but few people in reality read beyond the abstract of a paper, and this paper includes the claim prominently in the abstract that climate change of 2 degrees will kill 1 billion people.

I apologise if the above is complicated, but complexity is part of the problem here, because of the way it can be used as in this case to surround a flawed calculation with false precision. If you have got this far, thank you – but the question here is a more important one: how many Just Stop Oil or XR activists will read the entirety of Parncutt’s paper (and follow the references), and how many will take Roger Hallam’s word for it? How many activists may risk arrest, expecting to use the Parncutt paper as part of their defence against incarceration? As a climate expert I have given evidence in court to defend XR activists on more than one occasion, but I would never rely on numbers this flawed while under oath.

Roger may respond that this is only one paper, which would be fair enough. But it is the only one he cited, that he implied was representative of the state of the science overall, and that he relied on to make his moral case in the BBC interview. It is important to look at the numbers presented in the paper and determine whether or not they are justified. They are clearly not in this case.

None of this should be read as undermining the case for acting against the climate emergency, and there are many pieces of science that could be cited in support of the urgency of our predicament. Tim Lenton and co-authors, for example, recently presented new figures quantifying the “human cost of global warming” in a May 2023 paper in the (very relevant) journal Nature Sustainability. Crucially, this paper does not attempt a projected death toll (which is likely an impossible task) but instead uses a climatologically meaningful concept of the ‘human climate niche’, and calculates how many humans will be exposed to “unprecedented heat” by the end of the century in different emissions scenarios. There are many more papers I could cite.

Getting the science right will strengthen rather than weaken the case for climate activism, both in the public mind and in court. The problem is not that the science is insufficiently clear and urgent to make the moral case for activism, it is that governments and people refuse to recognise the implications of the science. Somehow getting them to do so is one of the roles of non-violent direct action, alongside the moral clarity of – as Roger Hallam tried his best to tell Nick Robinson – simply being on the right side of history.

Being on the right side of science, and the right side of history: that would be a moral stance that is truly unassailable, and that might continue to generate the radical change that XR and Just Stop Oil – and Roger Hallam himself – have so brilliantly begun.