Food and Energy: The Calories Behind the Calories
The food system accounts for roughly a quarter to a third of global greenhouse gas emissions and a significant share of energy use, and almost none of it is where people assume. Understanding the distribution changes which decisions matter and which are gestures.
The Process That Feeds Half the World
Plants need nitrogen and cannot use the vast quantity in the atmosphere, because the nitrogen molecule is held together by one of the strongest bonds in chemistry. Before 1913 the only routes were biological fixation by legumes and bacteria, or mined deposits of guano and nitrates, and both limited how much food could be grown.
The Haber-Bosch process breaks that bond industrially, combining atmospheric nitrogen with hydrogen at high temperature and pressure to make ammonia. It is among the most consequential inventions of the twentieth century, and it is energy-intensive precisely because the bond it breaks is strong. Ammonia production consumes on the order of one to two percent of world energy.
The hydrogen comes almost entirely from natural gas, which is why fertiliser prices track gas prices closely and why the 2022 gas crisis shut down ammonia plants across Europe and raised food costs worldwide. It is also why green hydrogen is often discussed for fertiliser first: the industry already uses hydrogen at scale and would need no new demand, only a different source.
The scale of dependence is worth stating plainly. Estimates suggest roughly half the nitrogen atoms in a typical human body arrived via Haber-Bosch. Current global population at current diets is not feedable without industrial nitrogen fixation by any method presently demonstrated, which places this process in a different category from most energy uses.
Why Food Miles Mislead
Transport accounts for a small share of the energy and emissions of most foods - commonly under a tenth, and for some products far less. The reason is the efficiency of bulk shipping: moving a tonne of grain across an ocean uses very little energy per kilogram of food.
The consequence is counterintuitive and well supported. What is eaten matters much more than where it came from. The difference between food categories is typically an order of magnitude or more, while the difference between local and imported within a category is usually a few percent.
There are specific exceptions, and they are consistent ones. Air-freighted produce - highly perishable items flown to arrive fresh - carries transport emissions that dominate its total, because air freight is roughly a hundred times more energy-intensive per tonne-kilometre than sea. Heated greenhouse production in a cold climate can also exceed the footprint of the same crop grown in season elsewhere and shipped.
The useful rule is therefore about method rather than distance. Something grown in season in a suitable climate and moved by sea is usually better than the same thing grown out of season locally under heat and light, and both are dwarfed by the difference between food types.
The Cold Chain
Refrigeration runs from the field to the plate: chilled storage after harvest, refrigerated transport, distribution warehouses, retail display cabinets and the domestic fridge. Each link runs continuously, and the total is a large and growing share of food-system electricity.
Retail display is the least efficient link and the most visible. Open-fronted chilled cabinets, which supermarkets favour because customers buy more from them, dump cold air into the store and then reheat it. Fitting doors reduces the energy use of a cabinet substantially and has been resisted on commercial grounds - a clear case where the efficient option and the commercial one point in different directions.
The cold chain is also what makes the food system work. It cuts spoilage dramatically, which is why post-harvest losses are far higher in countries without one. That produces a genuine trade-off: extending refrigeration to regions that lack it raises energy use and reduces waste, and the waste avoided typically embodies more energy than the refrigeration consumes.
Refrigerant leakage is a separate issue that is often larger than the electricity. Many refrigerants are potent greenhouse gases, and leaks from commercial systems have historically been substantial. The Kigali Amendment phases down the worst, and for a supermarket the refrigerant choice can matter as much as the electricity consumption.
The Third That Is Never Eaten
Roughly a third of food produced globally is not consumed. Every unit of energy that went into growing, fertilising, harvesting, processing, transporting and chilling it is wasted along with it, which makes waste reduction unusually leveraged - it saves the whole chain at once.
Where the loss occurs differs systematically by income. In lower-income countries most is lost between field and market, from inadequate storage, handling and the absence of a cold chain. In higher-income countries most is discarded at retail and in the home, after the entire energy investment has been made. The second kind is more expensive in energy terms because it happens at the end of the chain.
The interventions differ accordingly. Storage and cold chain investment addresses the first; date labelling, portioning and retail practice address the second. Neither requires new technology, and both are consistently identified among the cheapest available emissions reductions in the food system.
The broader point connects to the rest of this section. Food is one of several domains where the energy is almost entirely invisible to the person making the decision - nothing at the point of purchase indicates the fertiliser, the cold chain or the waste. That invisibility is the same structural problem as household energy, and it has the same implication: outcomes depend far more on how systems and labels are designed than on how conscientious individuals are.
Frequently asked questions
How much energy does food production use?
Modern agriculture spends several units of fossil energy per unit of food energy delivered, reversing the historical situation where farming was a net energy source. The food system overall accounts for roughly a quarter to a third of global greenhouse gas emissions.
Why is fertiliser the biggest energy item in food?
Because the Haber-Bosch process breaks the nitrogen molecule's bond, one of the strongest in chemistry, which requires high temperature and pressure. Ammonia production consumes on the order of one to two percent of world energy, and its hydrogen comes almost entirely from natural gas, which is why fertiliser prices track gas prices.
Do food miles matter?
Much less than assumed. Transport is commonly under a tenth of a food's footprint because bulk sea shipping uses very little energy per kilogram. What is eaten matters far more than where it came from - differences between food categories are typically an order of magnitude, while local versus imported is usually a few percent.
When does transport actually dominate?
For air-freighted produce, since air freight is roughly a hundred times more energy-intensive per tonne-kilometre than sea. Heated greenhouse production in a cold climate can also exceed the footprint of the same crop grown in season elsewhere and shipped, so the useful rule is about method rather than distance.
How much food is wasted, and why does it matter so much?
Roughly a third of what is produced globally. Waste destroys all the energy already invested in growing, fertilising, transporting and chilling it, which makes reduction unusually leveraged. Loss in lower-income countries happens mostly before market from poor storage; in higher-income countries mostly at retail and home, after the full energy investment.
