Famines, not heatwaves: how to think clearly about climate change

We demonstrate that depending on scenarios of population growth and warming, over the coming 50y, 1 to 3 billion people are projected to be left outside the climate conditions that have served humanity well over the past 6,000y.¹

Who exactly has two years to save the world? The answer is every person on this planet.²  

Wading through a flurry of news about COP29, it has forced me to reflect on the near-constant chattering about climate change that we're all subject to, and how little tends to go in. It's as if hyper-awareness is producing greater ignorance, such that more statistics and more news stories become equivalent to increases in the volume of noise.  

For my money, I'd put this down to a lack of an accurate narrative of what climate change will actually look like, one that would allow us to actually make sense of what's happening and what's at stake. Our foggy understanding is that climate change means increases in temperatures that go up but can come down just as far, with temperature-effects following carbon emissions pretty closely, such that temperatures will stop rising as soon as we stop emitting - and in any case, even though it's horrible that whole regions will be underwater or suffering drought in 100 years' time, and it will be a great loss to lose the coral reefs, the ice sheets and the Amazon, but we'll still muddle through, won't we? 

Turns out that almost every aspect of that narrative is wrong, in one way or another. It would be far more accurate to say that if annual net carbon emissions were already at zero in 2024, we'd still be guaranteed to breach 1.5ºC (by some margin) and stay over it for many years, relying solely on immediate and rapid removal of CO2 (and other greenhouse gases) to limit this period of overshoot, but risking at every moment during this period the severe impairment of our global food systems through permanent freshwater depletion and irreversible changes in weather patterns (extreme heat or cold, devastatingly intensive rainfall followed by severe drought).¹ Even in a scenario that stays only 1.5ºC hotter than preindustrial, there are points of no return that will be crossed, given time - and every 0.1ºC increases this risk.^{3 4} 

There are all sorts of additional worries, like worsened air pollution, the destruction of cities by storms and rising sea levels, poverty, and increased pandemics of deadly diseases.⁵ But to be blunt, all of these challenges will just be adding insult to injury in a starving world. 

That’s what climate change really is: an ongoing and stubbornly irreversible degradation of our food systems.

Geophysical dynamics

In order to really get to grips with climate change, we need to have a sense of the crucial geophysical dynamics that define both the climate crisis and our ability to counteract it. These are: thermal inertia, irreversibility and the maximum sequestration rate. 

Here’s a quick primer on how these dynamics interact. For reasons we’ll discuss later, much of what happens as the climate changes is largely irreversible, so protecting our food systems depends significantly on limiting how much of this change happens. Whilst geoengineering can decelerate the rate of aspects of this irreversible change, thermal inertia constrains our ability to properly stop it. Finally, our maximum sequestration rate constrains our ability to reduce atmospheric GHG concentrations in time to avoid further heating resulting from thermal inertia. 

From here on out, these are the parameters that you need to keep in mind when assessing the quality of your government's actions, not this year's emissions or current average temperatures. How we choose to act within these limits will decide what happens to our food. So, it might be worth understanding what they are. 

Thermal inertia

Getting to net zero won't stop warming 

Typically, explanations of global warming are limited to a reductive understanding of the greenhouse effect, whereby greenhouse gases (like carbon dioxide, methane, nitrous oxide, ozone, water vapour and certain fluorinated gases) act like the glass panes of your greenhouse, preventing solar heat from radiating out into space. But the climate is a self-regulating, complex system, closer to a microprocessor than a common all-garden greenhouse. So, how does it really deal with solar heat? 

Because the ocean is dark, it absorbs lots of solar radiation, and because it can absorb more heat without a rise in temperature than can air, lots of the new atmospheric heat that has accumulated with greater GHG concentration has transferred to the ocean; observations from buoys, ships, and underwater sensors (like the Argo float network) have demonstrated that over 90% of the excess energy trapped by greenhouse gases since the 1970s has been absorbed by the oceans.⁶  

On top of this greater heat capacity, the oceans continually redistribute heat in such a way as to delay rises in surface temperature. Besides surface currents that disperse heat so that the atmosphere is heated more uniformly (and therefore, more slowly) across the planet, different depths of ocean water are systematically re-shuffled so that the lowest layers become the surface layers through the interaction of waters with differing temperatures and salt contents (colder, saltier water sinks, while hotter, fresher water rises). In effect, this stores heat in the deep sea (below approximately 700 meters), so that it only resurfaces sometime later, either through resurfacing as part of the thermohaline circulation or by gradual vertical mixing as a result of tides, turbulence and internal wave activity.

In addition to all of this, when heat is transferred from warmer air to colder seas, the resultant evaporation absorbs more heat than is required to raise the water's temperature in order to effectuate a phase change; this 'extra' heat is called latent heat. When this water vapour is blown into some colder region, it undergoes a different phase change, condensation, which releases this heat, warming the air - in essence, this latent heat effect is another facet of the oceans' slow redistribution of heat. 

In effect, then, the nature of the oceans means that there is global warming that has already happened but has yet to actually warm the atmosphere. All of these systems working in tandem constitute the climate's thermal inertia.

Most worryingly, at least some of our future warming could be driven by solar radiation from early in the last century, i.e., before GHG concentrations really sky-rocketed, that hasn't worked its way through the system yet and unlike in previous centuries where it would have radiated back out into space, there is now a massive influx of greenhouse gases that will trap it. But, of course, most of the future warming will be from our activities in the last few decades.

Plenty of research has been done – particularly by the IPCC – on how long it takes for the Earth's climate system to reach its new equilibrium after a rise in GHG concentrations; unsurprisingly, it's not easy to make exact predictions. But the IPCC has established two different figures: the equilibrium climate sensitivity (ECS), which is the long-term average surface temperature resulting from a doubling of atmospheric carbon dioxide (relative to pre-industrial concentrations), measured in hundreds and thousands of years, though up to three-quarters of ultimate warming manifests within decades; and the transient climate response (TCR), which is the near-term average temperatures resulting from a doubling, measured in single-digit years.⁷ While TCR is primarily calculated in the context of a gradual 1% annual increase in CO₂ concentrations leading to a doubling over 70 years, it can be used to roughly estimate what temperatures will be within 1-5 years of particular CO₂ concentrations being reached. By the way, this understanding is why plenty of scientists at this year’s COP have said that 1.5ºC is ‘deader than a doornail’. 

But of course, this works both ways. If concentrations of GHG reduce, some of the re-emitted heat from the oceans will simply radiate into space when it emerges further down the line. So, whilst keeping concentrations the same promises continued warming, rapid reductions can actually avoid further warming, and all of the irreversible change this could bring. 

Irreversibility

Why most climate change is permanent 

If there's anything that you take away from this discussion, let it be that, for the most part, climate change is irreversible.  

Firstly, we have already permanently altered the acidity of the oceans and the functioning of the cryosphere, with no consequential restoration possible - and by, respectively, limiting the quantities of carbon dioxide that the oceans can absorb as well as reducing the ice-albedo of the planet, these irreversible changes directly constrain our ability to cool the planet down. Sea-level rise up to the present, too, is mostly irreversible. Whilst this does not entail that meaningful climate action is impossible, it does mean that we cannot entertain fantasies of total geophysical restoration – and unfortunately, the irreversibility of further climate change is more profound even than warming so far. 

Every moment that we spend warmer by 1.5ºC than pre-industrial times, we're in a state of what the IPCC term 'overshoot', following work done on planetary boundaries by Johan Rockström’s team⁸; scientifically, this is a preliminary transitional phase, a precursor to a 'sudden' and complete shift in the nature of the climate that will radically reduce the viability of our contemporary food systems. At 1.5ºC warmer than pre-industrial, we'll be permanently at risk of starting this change because we'll be likely to set off a handful of irreversible processes that, far from being discrete mechanisms that operate parallel to each other, reinforce each other to produce a total transformation in our climate system. 

To understand this aspect of climate change's irreversibility, we should distinguish between first-order and second-order irreversibility in Earth's geophysical system, with the former leading to the latter. The forms of first-order irreversibility are ocean acidification, the destruction of the cryosphere, the dieback of the Amazon Rainforest, the release of water vapour, and the thawing of permafrost; and the forms of second-order irreversibility are the destruction of coral reefs, the collapse of the Atlantic meridional overturning circulation (AMOC) and the slowing of the polar jet stream.  

Let's start with the first-order irreversibility. As we've mentioned, you can't meaningfully de-acidify the ocean once you’ve made it more acidic – and as it gets more acidic, it absorbs less CO2, which leaves more of it in the atmosphere. Then, the dieback of the Amazon Rainforest is a self-sustaining negative feedback loop because the Rainforest generates its own water supply, trillions of litres that it would be basically impossible to supply artificially. This is believed to happen between 20-25% of the Amazon’s deforestation - by most estimates, we're between 17% and 20%.⁹ Once the Amazon’s gone, of course, one of the keystones of our current climate system will have been dislodged, never to return. 

Similarly, the melting of ice sheets and glaciers is also self-sustaining, a result of lowering albedo that progressively increases the ice’s heat absorption whilst warming the atmosphere and meltwater lubricating regions so that they slip into the sea, which further reduces albedo. With no Amazon Rainforest and a severely degraded cryosphere, we’ll have far less of a chance to reduce GHG concentrations whilst needing to reduce these concentrations by a lot more than their historical levels to get to lower temperatures, because the planet will reflect less sunlight and absorb more heat. This isn’t to mention the fact that glaciers are crucial sources of freshwater for billions, so their loss or reduction will permanently reduce our freshwater supplies. 

Permafrost thaw, too, once it begins, is a self-sustaining process: all of the carbon dioxide and methane that thawing releases amplifies the warming that continues the thaw, whilst melting away ice and snow covers that reduce the permafrost’s absorption of heat and forming new features (thermokarst lakes) that trap heat. 

Lastly, increased evaporation from warmer oceans increases atmospheric concentrations of water vapour, a greenhouse gas, further increasing temperatures that warm oceans, releasing more water vapour of which warmer air, in accordance with the Clausius-Clapeyron relation, can hold greater quantities - and so on. This isn't the end of it, however: as we've already seen, eventual condensation of water vapour itself warms the atmosphere, which means that water vapour both traps heat as a gas and releases heat when it condenses, causing further evaporation, and so on.

But climate change’s second-order irreversibility is the real kicker, locking in catastrophic reductions in the food supply.  

First off, the destruction of coral reefs is a negative feedback loop, because warmer oceans cause corals to expel symbiotic zooxanthellae algae, reducing their robustness to pathogens, and more acidic oceans decreases the availability of compounds used by corals to strengthen their skeletons, leaving them extremely vulnerable to storms. Then, when either breakage or disease leads to their death, corals release stored carbon, adding to the oceans acidity and further decreasing the chances of survival for the rest of the reef. Coral reefs are essential habitats for fish, they're where they feed, breed and find shelter, so they're decline directly reduces the availability of seafood, which, according to research by the UN in 2024, provides roughly 40% of the global population with 20% of their animal protein.¹⁰

Another keystone of our current climate system, the Atlantic meridional overturning circulation (AMOC) is a crucial component of the global thermohaline circulation that takes place in the North Atlantic, where cold, salty water sinks to the deep sea and is moved southwards, where it will eventually resurface somewhere in the Southern Hemisphere. This overturning will severely weaken as a warmer, less icy Arctic achieves ocean temperatures more equal to the North Atlantic, thereby disabling the temperature difference that drives part of the overturning, and further malfunctioning will be driven by the ongoing influx of freshwater from the melting of the Greenland ice sheet as well heavier precipitation, which makes the water less salty and therefore, lessens the density that causes sinking. When the AMOC’s fully collapsed, a 'cold blob' will descend over Ireland, the UK, Scandinavia and Western Europe, totally decimating these regions’ agricultures as radically reduced average temperatures and massively increased precipitation freeze and drown crops.^{11 12} On average, northern England and Scotland will be hit by –10ºC drops, with southern England and Ireland suffering between –3ºC and –6ºC falls; coastal Norway would be -5ºC to -10ºC cooler, with inland Norway and the whole of Sweden -3ºC to -5ºC cooler; Denmark drops by -5ºC to -7ºC;  Northern France by -5ºC to -8ºC and Southern France by -3ºC to -5ºC; the Netherlands and Belgium will suffer -5ºC to -8ºC drops in average temperatures; and Northern Germany -5ºC to -8ºC cooler. But the Southern Hemisphere, which now can’t shift its heat using the AMOC, will flip the other way, with expected warming of approximately 1ºC–2ºC - that’s above whatever temperature carbon emissions have achieved by then! It could even spell the end of South Asia’s monsoon, yet another catastrophic blow to freshwater supplies.¹³ All in all, this would secure the melting of the West Antarctic ice sheet, with all of the further warming and sea-level rise that would bring. It’s worth noting that some of the most up-to-date studies have warned that this collapse may be either imminent or far closer than was believed, with the aforementioned effects to follow in the coming decades.^14 15 16 

Finally, there’s the fact that the polar jet stream, which is driven by the temperature gradient between the Arctic and mid-latitudes, will slow as the Arctic warms, thereby trapping weather systems and causing prolonged heatwaves, droughts, storms, and cold spells across the Northern Hemisphere. This would wreak havoc on agriculture in the Northern Hemisphere, decimating the real breadbaskets of Europe that will avoid the cold blob formed by the AMOC’s collapse, as well as China and North America. By certain estimates, that’s about 45-50% of all global calories under threat. 

Make no mistake: the collapse of the AMOC, the loss of coral, and the slowing of the jet stream, all of which are progressing irreversibly as part of ongoing global warming, will cause mass starvation on a global scale.

Plenty of studies have been conducted which demonstrate that for certain crops, including rice but particularly wheat, the negative effects of increased heat can be completely counteracted by increased CO₂ fertilisation, such that yields for these crops can actually increase.¹⁷ But as we've seen, the real threat to agriculture is that our weather system is mutating under the influence of that very same CO₂, radically shifting precipitation patterns whilst depleting the cryosphere’s stores of freshwater. In our new climate, far more intense storms and rainfall will migrate slowly along the jet streams, alternating long weeks of heavy rain with long weeks of total drought, locking both hot and cold spells over each regions for far longer than happens today. The rest of the planet, meanwhile, will be pretty much bone-dry year-round, as almost all surface water completes its evaporation from today’s already much-reduced state; massive surface water reservoirs like the Aral Sea and Lake Chad, for instance, have already been basically wiped off the face of the Earth – in fact, Wikipedia now lists the Aral Sea as a historical lake. 

Of course, in this post-apocalyptic scenario, we’d try to adapt – but what would that look like? In short, a total revolution in the geography and techniques of food production, requiring monumental new infrastructure to contain, collect, filter, use, store and transport rainwater over vast distances, huge reconfigurations of the coast to protect precious freshwater rivers and lakes from rising saline seas, and unprecedentedly massive desalination projects, as well as historic shifts in where we live and what sort of buildings we live in. And even if we managed a successful transition to this entirely new way of life, requiring massive amounts of money and well-organised labour to achieve in a far more chaotic, pressurised world, we will still end up with far less freshwater than we have today, because we’ll have much-reduced glaciers (or none at all, of course), and far scarcer seafood. Either way, no matter what happens, given the warming we’ve got locked in, we’ll have to start amending agriculture along these lines in the coming years.

As the years go by, we’re progressively leaving our current climate niche and at an ever-increasing pace, entering a new system that will struggle to support us. What climate change looks like today, and what it will look at every point in the future, isn’t just sinking coasts, forest fires, chunks of ice calving into the sea, or heatwaves – it’s starvation. So, what can be done to avoid as much of this as possible? 

Sequestration rates

Climate action’s speed-limit 

A critical but often overlooked factor is the maximum sequestration rate—the rate at which we can remove CO₂ from the atmosphere. While industrial carbon capture technologies are being developed and tried out, there’s nothing to really go on just yet – for that reason, it’s best to stick with the natural processes that have been sequestering carbon for the long millennia before Homo sapiens arrived. Assessing these, we can come up with a quick and loose figure of our yearly sequestration capacities as things stand today – and from there, how quickly we can wind down the thermostat. 

Calculating the speculative maximum sequestration rates 

Based on various natural and technological methods, the estimated maximum annual sequestration rates are: 

• Afforestation and reforestation: Planting new forests can sequester approximately 10–15 gigatonnes of CO₂ (GtCO₂) per year. 

• Soil carbon sequestration: Improving agricultural practices to increase soil carbon can sequester about 2–3 GtCO₂ per year. 

• Ocean-based methods: enhanced algae growth could sequester around 1 GtCO₂ per year, though warming oceans may limit this potential; and large-scale seaweed cultivation might sequester 1–3 GtCO₂ per year, but this too is constrained by ocean conditions. 

Total (contemporary) maximum sequestration potential: approx. 18 GtCO₂ per year. 

Current atmospheric CO₂ levels are around 420 parts per million (ppm), equivalent to about 3,200 GtCO₂. To reduce CO₂ levels from 420 ppm to 350 ppm—a reduction of 70 ppm—we would need to remove about 560 GtCO₂ from the atmosphere. 

At a maximum sequestration rate of 18 GtCO₂ per year, it would take over 30 years to achieve this reduction, assuming we can reach and maintain these sequestration rates, which is optimistic given the challenges. 

Constraints:

• Land availability: Afforestation requires vast tracts of land, potentially competing with agricultural needs. 

• Climate feedbacks: Warming temperatures can impair the very systems we rely on for sequestration. For instance, heat stress can reduce tree growth, and warmer soils may release more CO₂. 

• Technological limitations: Carbon capture technologies are still in development and require significant energy and investment. 

Therefore, while carbon sequestration is a critical component of climate mitigation, our current capacities provide a sort of a speed-limit that has to be factored in when assessing our prospects of avoiding further warming.  

It seems obvious, then, that geo-engineering will be required, things like pumping out the meltwater that’s causing ice sheets to break apart (using it to thicken surface layers of these sheets) and spraying aerosolised sulfates into the air to reflect solar radiation - but this is only a pause button, with a lot of side-effects and only a short life-span. We can stop basal sliding but we can’t stop basal or overall melt, for instance – and as soon as the aerosolised sulfates wear out, temperatures snap right back into place. The only real solution is a reduction in atmospheric GHG concentrations. There is simply no other way. 

So, when you’re reading about climate change from now on, remember: climate change is a (mostly) irreversible process and its most consequential impacts are famines, not heatwaves. 

¹ Future of the human climate niche. Xu et al. Environmental Sciences. 2020.  

² UN climate chief presses for faster action, says humans have 2 years left ‘to save the world’.  

³ Exceeding 1.5°C global warming could trigger multiple climate tipping points. McKay et al. Climate Change. 2022.  

⁴ Climate tipping point interactions and cascades: a review. Wunderling et al. Earth System Dynamics. 2024.  

⁵ The Uninhabitable Earth: Annotated Edition. David Wallace-Wells. New York Magazine. 2017.

⁶ Recent acceleration in global ocean heat accumulation by mode and intermediate waters. Li et al. Nature Communications. 2023.  

⁷ The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. 2021.  

⁸ Planetary boundaries: exploring the safe operating space for humanity. Rockström et al. Ecology and Society. 2009. 

⁹ Can Amazon Countries Save the Rain Forest? Diana Roy. Council on Foreign Relations. 2024. 

¹⁰ The State of World Fisheries and Aquaculture 2024. Food and Agriculture Organisation of the United Nations. 2024.

¹¹ Shifts in national land use and food production in Great Britain after a climate tipping point. Ritchie et al. Nature Food. 2020.  

¹² Running AMOC in the farming economy. Tim G. Benton. Nature Food. 2020. 

¹³ Penultimate deglaciation Asian monsoon response to North Atlantic circulation collapse. Wassenburg et al. Nature Geoscience. 2021. 

¹⁴ Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Peter D. Ditlevsen and Susanne Ditlevsen. Atmospheric and Oceanic Physics. 2023.  

¹⁵ Weakening of the Atlantic Meridional Overturning Circulation driven by subarctic freshening since the mid-twentieth century. Garbiel M. Pontes and Laurie Menviel. Nature Geoscience. 2024. 

¹⁶ Open Letter by Climate Scientists to the Nordic Council of Ministers. 2024. 

¹⁷ How will climate change affect crop yields in the future? Hannah Ritchie. Our World in Data. 2024.