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

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Could Four Billion People Die at 3°C?

Measuring the Human Cost of Abrupt Climate Warming

Impacts & Risks

Risk Analysis

Climate Extremes

Food & Water

Health

Human Security

By<br>David Spratt

Monday 27 July, 2026

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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.

Summary<br>Mortality depends on exposure, vulnerability and adaptive capacity, making precise death tolls impossible to model.<br>The four billion deaths at 3°C figure is not a forecast or modelled projection.<br>The estimate of two billion deaths at 2°C is implausible.<br>The timeframe for the 3°C / four billion deaths estimate is undefined.<br>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.<br>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?<br>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.<br>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 definitions

The 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.<br>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.<br>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.<br>So it is important to understand what these numbers are, and are not, and where they came from.<br>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”.<br>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).<br>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.<br>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.<br>I understand there have been suggestions that a...