Muscles, wood, water and wind: energy before fossil fuels: Wood, charcoal and the limits of the organic economy

4.1 Wood was the universal fuel, and straw and dung filled the gaps

Until coal, nearly every fire in the world burned plant matter. Air-dried wood holds about 15 to 17 MJ per kilogram, roughly half the energy of good coal. It fuelled cooking, heating, baking, brewing, salt making, glass making and the firing of pottery, bricks and lime. A preindustrial town needed a constant flow of firewood from the surrounding countryside, and the radius of that supply area grew with the town.

Because trees grow slowly, the power density of a managed woodland is low. A well-tended coppice, cut every decade or two, might yield 5 to 10 tonnes of dry wood per hectare per year, around 0.3 to 0.6 W/m2 (calculated at about 18 GJ per dry tonne). A city of 100,000 people, with households burning a few tonnes of wood a year plus the demands of bakers, brewers and kilns, needed well over ten thousand hectares of managed woodland, and the transport costs of bulky wood made any shortfall acute.

Where trees were scarce, on crowded plains or arid lands, people burned crop residues, such as straw, stalks and husks, and dried dung. These are bulky, smoky and less energy-dense. Smil's Biomass Energies documented households in India, China and Africa relying on them, and pointed out a hidden cost: burning residues and dung removes organic matter and nutrients that would otherwise return to the soil.

The way the fuel was burned also mattered. An open three-stone fire transfers only about 10 to 15% of the fuel's energy into the cooking pot; the rest escapes as hot gases and smoke. Indoor smoke from such fires is a major killer, as the final section shows. In 1980 Erik Eckholm, writing in Energy in the Developing World, called fuelwood scarcity "the other energy crisis": quieter than the oil shocks, but touching far more people.

• Smil, Biomass Energies: Resources, Links, Constraints (1983) · via Smil Mastery Course, M4 'Wood, charcoal and other biomass fuels'
• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M4 'Wood, charcoal and other biomass fuels'
• Smil, Energy in the Developing World: The Real Energy Crisis (ed. with W. Knowland, 1980) · via Smil Mastery Course, M4 'Wood, charcoal and other biomass fuels'

4.2 Charcoal made high temperatures possible, at a heavy cost in forests

Charcoal is wood upgraded by slow heating with little air. Water and volatile compounds are driven off, leaving almost pure carbon with about 29 to 30 MJ per kilogram. It burns hot and nearly smokeless, which made it the preferred fuel for smiths, glassmakers and smelters. The upgrade was expensive. Traditional earth-covered kilns turned only 15 to 25% of the wood's mass into charcoal. Since charcoal has about twice the energy per kilogram, that meant losing roughly half to three quarters of the wood's energy in the making (calculated).

Metallurgy was the most voracious user. Copper was smelted from about 5000 BCE and bronze, an alloy of copper and tin, from about 3300 BCE, both in small charcoal furnaces. Iron ore is far more abundant, but early bloomery furnaces could not melt iron; they produced a spongy mass of iron and slag that smiths hammered into shape. The Iron Age began in the Near East around 1200 BCE. China was far ahead in one respect: by the 5th century BCE its blast furnaces were producing liquid cast iron, more than 1,500 years before Europe. High-quality steel, such as the crucible wootz steel of South India used for Damascus blades, was made in tiny batches and was precious.

Every kilogram of iron required several kilograms of charcoal, and so tens of kilograms of wood. As iron output grew in early modern England, Sweden and Russia, charcoal demand stripped forests over wide areas and pushed ironworks deeper into woodland. Smil notes that the shift to coke, from coal, in 18th-century Britain released iron production from this constraint, a theme continued in Still the Iron Age.

• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M4 'Metals and warfare in the organic age'
• Smil, Still the Iron Age: Iron and Steel in the Modern World (2016) · via Smil Mastery Course, M4 'Metals and warfare in the organic age'
• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M4 'Wood, charcoal and other biomass fuels'

4.3 Building, fighting and the ceiling of an economy on current sunlight

Preindustrial monuments show both the reach and the limits of organic energy. The Great Pyramid needed a modest workforce and two decades. Gothic cathedrals took generations of skilled masons, with treadwheel cranes lifting stone. China's Great Wall and Grand Canal mobilised huge numbers of conscripted labourers over centuries. In each case the achievement came from organisation, persistence and craft rather than concentrated power.

Warfare was bounded by food and fodder. An army could move only as far and stay only as long as it could gather or carry grain for its soldiers and feed for its horses. Cavalry was the fastest and most powerful force on any battlefield, but each horse needed several kilograms of feed a day. Gunpowder, invented in China and used in Europe from the 14th century, changed the energetics of war. It released stored chemical energy in a fraction of a second, so a few kilograms of powder could hurl a cannonball with more destructive power than any team of archers. Medieval castle walls lost their value.

Stepping back, the organic economy faced a single underlying limit: everything depended on the current flow of photosynthesis and the modest mechanical flows of water and wind. Food, feed, fuel, building timber and fibre all came from the same land, so more of one meant less of another. Power was available only in small units of a few kilowatts at most, and only where water fell or wind blew. Population and output could grow only slowly.

Coal broke that limit first in Britain, where it replaced wood for heating and then for iron making, and powered steam engines. Coal is concentrated, stored photosynthesis from millions of years, available in amounts that no forest could supply. The next modules follow that transition.

• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M4 'Moving and building before engines'
• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M4 'Metals and warfare in the organic age'
• Smil, Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World (2020) · via Smil Mastery Course, M4 'Moving and building before engines'
• Smil, Energy in World History (1994) · via Smil Mastery Course, M4 'Metals and warfare in the organic age'
• Wrigley, Energy and the English Industrial Revolution, Cambridge University Press (2010) · about 15 million acres of woodland needed to match the energy Britain got from coal in 1800; review in Times Higher Education, https://www.timeshighereducation.com/books/britons-who-saw-coal-fuel-nations-rise-and-rise/193623.article

4.4 About two billion people still cook with polluting fuels, mostly biomass

The preindustrial fuel system is not only history. According to the World Health Organization, about 2.1 billion people, roughly a quarter of humanity, still cook over open fires or inefficient stoves burning wood, charcoal, crop residues, dung, coal or kerosene. The joint Tracking SDG7 report puts the number without access to clean cooking at 2,087 million in 2023, down from about 3 billion in 2010. Asia accounted for about 983 million and Africa about 975 million.

The two continents are moving in opposite directions. In Asia, the number has fallen steeply since 2010, driven largely by programmes promoting liquefied petroleum gas, together with clean air policies; The IEA expects China to reach universal access by 2028. India still had about 430 million people without clean cooking in 2023, according to the IEA, a number projected to fall to about 285 million by 2030 on current policies. India's main tool has been the Pradhan Mantri Ujjwala Yojana (PMUY), launched in May 2016, which gives poor households a free LPG connection; it had about 10.33 crore (103 million) connections by July 2025. Getting families to keep buying gas has been harder. Average refills per PMUY household rose from about 3 a year in 2019-20 to about 4.5 in 2024-25, and in 2025-26 the government pays Rs 300 towards each 14.2 kg cylinder for up to nine refills a year. Four or five cylinders a year suggests that many of these homes still cook partly on wood or dung.

In sub-Saharan Africa, by contrast, population growth has outpaced progress, and the number without access has risen. On current trends about 1.7 to 1.8 billion people worldwide will still lack clean cooking in 2030.

The costs are heavy. WHO estimates about 2.9 million deaths in 2021 from household air pollution, including more than 300,000 children under five; the IEA, using its own modelling, gives about 2.5 million premature deaths a year. The two figures differ in method and coverage, not in the picture they paint: household smoke is one of the largest environmental causes of death in the world. Households without clean cooking spend on average about four hours a day collecting fuel and cooking, mostly women and children. Traditional cooking also emits about 1.2 Gt CO₂-equivalent a year through direct emissions and forest degradation.

This energy is also partly invisible in statistics. The headline world primary energy total of 592 EJ for 2024 covers commercially traded fuels and leaves out most of the firewood and dung gathered by households.

• World Health Organization, 'Household air pollution' fact sheet (16 December 2025) · about 2.1 billion people cook with open fires or inefficient stoves using solid fuels or kerosene; about 2.9 million deaths in 2021, including over 309,000 children under five; https://www.who.int/en/news-room/fact-sheets/detail/household-air-pollution-and-health
• IEA, IRENA, UN Statistics Division, World Bank and WHO, Tracking SDG7: The Energy Progress Report 2025 (data analysis, SEforALL) web ↗ · 2,087 million people without clean cooking in 2023 (Asia 983 million, Africa 975 million), down from 2,999 million in 2010; 1.7-1.8 billion projected for 2030; https://www.seforall.org/system/files/2025-12/SEforALL%20SDG7_TRACKING_2025-data_analysis.pdf
• IEA, SDG7: Data and Projections, 'Access to clean cooking' (2024) · India about 430 million without access in 2023, projected 285 million by 2030; China 146 million; progress in developing Asia driven largely by LPG programmes; https://www.iea.org/reports/sdg7-data-and-projections/access-to-clean-cooking
• IEA, Clean Cooking in Africa 2026, Executive summary (2026) · nearly two billion people still cook with wood, charcoal and other polluting fuels; about 2.5 million premature deaths a year; about four hours a day spent collecting fuel and cooking; 1.2 Gt CO2-eq a year from traditional cooking; https://www.iea.org/reports/clean-cooking-in-africa-2026/executive-summary
• Energy Institute, Statistical Review of World Energy 2025 web ↗ · 2024 world primary energy 592 EJ, covering commercially traded fuels; https://www.energyinst.org/statistical-review
• Smil, Energy in the Developing World: The Real Energy Crisis (ed. with W. Knowland, 1980) · via Smil Mastery Course, M4 'Wood, charcoal and other biomass fuels'
• Government of India, Cabinet decision on PMUY targeted subsidy for 2025-26 (PM India / PIB, 8 August 2025) · PMUY launched May 2016; about 10.33 crore connections as of 1 July 2025; Rs 300 subsidy per 14.2 kg cylinder for up to 9 refills a year; average refills per PMUY household about 3 (2019-20), 3.68 (2022-23), about 4.47 (2024-25); https://www.pmindia.gov.in/en/news_updates/cabinet-approves-continuation-of-targeted-subsidy-for-pradhan-mantri-ujjwala-yojana-consumers-for-2025-26-at-rs-12000-crore/

See it in the table

Check your understanding

Why was charcoal preferred over wood for smelting, despite being costly to make?

Roughly how many people lacked access to clean cooking in 2023?

What single underlying limit constrained the preindustrial organic economy?