Could Four Billion People Die at 3°C? - Safe Climate Australia

The claim that four billion people could die in a 3°C warmer world has become a powerful climate narrative. But what does the science actually support, and where does evidence end and speculation begin?A patient receives emergency treatment at a Karachi hospital during a life-threatening heatwave on June 27, 2024.SummaryMortality depends on exposure, vulnerability and adaptive capacity, making precise death tolls impossible to model.The four billion deaths at 3°C figure is not a forecast or modelled projection.The estimate of two billion deaths at 2°C is implausible.The timeframe for the 3°C / four billion deaths estimate is undefined.The four billion figure is an educated worst-case estimate, not a robust scientific projection.The proposition that four billion people or more would be dead if — and more likely, when — global warming reaches 3 degrees Celsius (°C) has gained some currency, mainly due to the 4DB (four billion dead) website and associated activities.Now, that is half the current global population, and on current warming trends of 0.3-0.35°C/decade, the world will hit 3°C around 50 years from now, perhaps earlier, in part because there is little prospect of a rapid decline in fossil fuel emissions.  So that’s a mind-blowing average of 80 million people a year dying due to climate impacts every year from now to 2075? Is that realistic?The number of greater than four billion dead at 3°C first appeared in a 2025 report on Planetary Insolvency: Finding our balance with nature, published by the UK Institute and Faculty of Actuaries (IFoA) and the University of Exeter.The number appears in Figure 12: Planetary solvency risk impact and likelihood definitions (illustrative) on page 32 of the report.  In this figure, the scale of impacts listed for 2°C of warming include mortality of “>2 billion deaths” and GDP losses of 25%, as well as “>4 billion deaths” and GDP of 50% for 3°C of warming (Figure 1).Figure 1:  Planetary solvency risk impact and likelihood definitionsThe 2°C estimate is even more startling, because Earth has already hit 1.5°C for all practical purposes, with the average for 2023-25 above 1.5°C, and a strong El Nino likely on the way for later this year which could push annual warming towards 1.7°C for 2026-27. At the current, accelerated warming rate, the Earth will reach 2°C around 2040, just fifteen years from now.So the figure of two billion dead at 2°C would mean an average of 130 million people a year dying every year from now till 2040 due to climate change?  To my mind, that is simply not credible, and nothing I have read tells me that is a remotely-likely estimate.Likewise, the risk matrix gives a figure of 1-5% dead (80 to 400 million deaths) for 1.5°C, a level of warming Earth has already reached. Estimates of actual mortality are difficult due to the direct and second- and third-order impacts, such as: hotter-climate > drought > food-shortage > displacement > conflict > mortality, and so on. The 2025 report of the Lancet Countdown on Health and Climate Change estimated heat-related mortality has increased to an average 546,000 deaths per year, though not all are specifically related to climate heating, and that is only one element of the story.So it is important to understand what these numbers are, and are not, and where they came from.There is no qualitative indication in the report as to their source. On page 27, the report acknowledges that “very limited research has been carried out on the potential for large-scale loss of life in relation to these interconnected risks on which to base an assessment”.I asked a colleague in the UK who had worked with the authors of the report, and he was told that the figures came from the Climate Endgame paper. But its author, Luke Kemp, said this was not the case, and “he confirmed that he does not make any forecasts about mortality in the paper” (emphasis added).As the caption to Figure 12 says, it is a risk assessment matrix that illustrates general levels of risk. I was told that the authors have now privately clarified that the mortality numbers “are absolutely NOT forecasts or predictions and we don’t use them as such” (emphasis added). But this is now the way that sites like 4BD are using it.One interpretation would be that the risk matrix was simply indicating orders of magnitude rather than specific projections. So either it was too subtle in its distinctions, and/or it was a bit of a stuff-up in that it was assumed that the four billion figure came from somewhere, but it did not. So there is still a question of how the four billion figure was derived.I understand there have been suggestions that a correction or clarification be issued, at least saying that the figures “are absolutely NOT forecasts or predictions”. And I had conversations with people working on the 4BD project outlining the story above and why some nuance was necessary in using such numbers, but the die had been cast.So how can we think about this? One guess is the table was saying that at 3°C, an expert elicitation would find that mortality would likely be in the billions, rather than tens or hundreds of millions. Or alternatively, this was a plausible worst-case scenario, but not derived from models.  As discussed below, that is a reasonable proposition based on other literature. As for two billion dead at 2°C, I can see almost no credible evidence that is even in the ballpark, just half a degree warmer than at present.Another, likely interpretation is that Figure 12 had used some numbers for GDP loss (in column 1) and had simply applied the same number for mortality (in column 2), so 50% loss in GDP equals 50% mortality. This in itself is a brave assumption: in one case, during the Great Depression — when, for example, US GDP fell 30% between 1929 and 1933 — mortality rates did not increase, and in some cases improved, though there were later adverse outcomes for children born at that time.So there is no mortality analysis in the table at all, and no epidemiology; it simply flows from GDP impacts.So where did the 50% decrease in GDP at 3°C come from? An earlier IFoA report in 2023, The Emperor’s New Climate Scenarios, whilst recognising that “climate change is complex, nuanced and characterised by deep uncertainty”, provided a chart of damage functions relating temperature and GDP loss (Figure 9, page 25), and asks “at what point do we expect 50% GDP destruction – somewhere between 2070 and 2090 depending on how you parameterise the distribution”, and that’s around 3°C. Depending on assumptions about when GDP hits zero (called the ruin parameter), the figures could be higher (80%) or lower (30%) because these are abstract models of possible damage functions. So it looks like a 50% loss of GDP was a figure of choice.So what is the basis of the damage functions? The Emperor’s New Climate Scenarios says that  “Insurance leaders have unequivocally stated that if climate change raises average temperatures to 4˚C above pre-industrial levels most assets will be uninsurable”, and that “without insurance, investment, finance, business slow to a halt – we will no longer have an economy.” (page 27). I find this assumption highly problematic. As insurance premiums rise right now, in some cases dramatically due to extreme climate impacts, many households and small businesses are making decisions to continue to operate without asset insurance.The only footnote to these statements is a view by one insurance CEO, Thomas Buberl of AXA at a Davos panel, reported by Bloomberg, that at 3-4°C “it’s not insurable anymore”, but what he said was much more specific: he was talking about basement retail premises in New York and Mumbai and that is what the headline said: “Climate change could make your basement uninsurable within a decade”. That is a very narrow and specific base on which to draw a global conclusion.Now, how much of an economy would be left at 3, 4 or 5°C is a good question, but the method used here is far too narrow. I remember a long time ago James Lovelock saying at 4 or 5°C there may be 500 million people left eking out a miserable existence at the poles, or words to that effect.To reiterate, it is almost impossible to put specific numbers on such future impacts, due to the radical uncertainty about the social impacts of a physical system that itself is non-linear in many important aspects, characterised by abrupt changes whose specific human consequences are somewhere between difficult and impossible to model, and where quantifying social impacts is of limited efficacy.Vulnerability and adaptationEven if future (non-linear) physical changes are well known, mapping their human impacts involves several more degrees of difficulty because the risk (potential damage) varies with exposure and vulnerability. There are three factors:Hazard: the physical changes in a climate system subject to abrupt change;Exposure: the presence of people, livelihoods and ecosystems in that physical space; andVulnerability: the propensity of these human systems to be negatively impacted due to their sensitivity and/or limited adaptive capacity.In the uber-rich, low-rainfall Gulf states, for example, whilst unlivable heat is becoming the norm, adaptation paid for by stupendous oil and gas revenue — desalination, 24/7 air conditioning, using flood-lit beaches at night rather than during the day, irrigating date palms, importing almost all the food and most of the labour — reduces vulnerability, even though the whole project seems a bit crazy.So mortality in a hotter climate will be affected a great deal by adaptation capacities to reduce vulnerability, and that is largely a product of national income, and international assistance.But there are also hard boundaries that cannot be easily adapted to, for example rice yields diminish once temperatures exceed 35°C at the time of flowering, and by 37°C the damage becomes critical.  By 2050, between one-quarter and two-thirds of rice production