Energy transitions and climate: What a transition is and how fast transitions run

1.1 A transition changes fuels, machines and uses, and the way you count it matters

The everyday meaning of an energy transition is a change in the mix of primary energy: from wood and straw to coal, from coal to oil and natural gas, and now from fossil fuels towards wind, solar and other non-fossil sources. That is the change most charts show, but it is only one layer.

A second layer is the prime mover, the machine that turns energy into motion. Muscles gave way to water wheels and windmills, then to steam engines, and later to internal combustion engines, steam and gas turbines and electric motors. A third layer is electrification. A growing share of all primary energy is first turned into electricity, which can drive a motor, light a room or run a computer with very little waste at the point of use. A fourth layer is efficiency: an LED gives the same light as an incandescent bulb for about a sixth of the electricity, and a condensing boiler wastes far less heat than an open fire.

Counting is harder than it looks. Fuels are measured by the heat they release when burned, but wind, solar, hydro and nuclear electricity have no fuel to weigh. One convention counts their output at its actual energy content, 3.6 MJ per kWh. Another, the substitution method, counts the fuel a typical thermal power station would have needed to make the same kWh, roughly 9 to 10 MJ. The second approach makes non-fossil electricity look two to three times larger. Neither is wrong, but mixing them produces nonsense, so always check which one a chart uses.

Finally, a transition is a process, not an event. A start date and an end date say little. What matters for investors and for policy is the shape of the path between them: how quickly a new source gains share, where it stalls, and what it actually displaces.

• Smil, Energy Transitions: Global and National Perspectives (2017) · via Smil Mastery Course, M9 'What an energy transition is'
• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M9 'What an energy transition is'
• Our World in Data, 'The Energy Institute has changed how it measures primary energy' (2026) web ↗ · end of the substitution method in the Statistical Review, https://ourworldindata.org/primary-energy-measurement-change

1.2 Each new fuel needed decades to win a quarter of world supply

Smil's main historical argument rests on a simple yardstick: how long did each fuel take to grow from 5% of world primary energy to 25%? The answers are strikingly consistent.

Coal reached about 5% of global supply around 1840. It took about 35 years to reach 25%, and it did not supply half of world energy until around 1900. Crude oil reached 5% around 1915 and needed about 40 years to reach 25%. Natural gas reached 5% around 1930 and needed about 55 years to reach a quarter. Each had large advantages over what it replaced: coal is denser than wood, oil is denser still and liquid, gas burns cleanly and travels by pipe. Even so, each needed two generations to spread.

The reasons were physical and financial. A new fuel needed new mines or wells, new transport such as railways, tankers and pipelines, new converters such as steam engines, refineries, cars and gas furnaces, and new skills. All of that had to be financed and built while the old system kept working, so there was little pressure to scrap it early.

The present shift is harder in one way. Earlier transitions moved to fuels that were denser and more convenient than their predecessors. Wind and solar are cheap and clean, but diffuse and variable, so they need more land, more transmission, and storage and backup for each unit of reliable supply. They also replace fossil fuels mainly in electricity, which is only about a fifth of final energy use.

In another way it may be easier. Solar panels and batteries are factory products that get cheaper with every doubling of output, unlike mines and oilfields, which tend to get harder to develop over time. That is why solar has grown faster in recent years than any fuel did in the past. The open question is how far that speed carries beyond electricity into heat, industry and transport.

• Smil, Energy Transitions: Global and National Perspectives (2017) · via Smil Mastery Course, M9 'The grand fuel sequence and its pace'
• Smil, Energy Myths and Realities: Bringing Science to the Energy Policy Debate (2010) · via Smil Mastery Course, M9 'The grand fuel sequence and its pace'
• Ember, Global Electricity Review 2026 (April 2026) · 2025 data, https://ember-energy.org/latest-insights/global-electricity-review-2026/ and https://ember-energy.org/chapter/2025-in-review/

1.3 Old fuels lose share but rarely lose volume

A common picture of transitions is a relay race in which each runner hands over the baton and leaves the track. The record looks more like a crowd that keeps growing. The world burns more coal today than in 1900, when coal was dominant, and humanity still uses large amounts of wood, crop residues and dung, mostly in poor rural households. Coal's share of world energy has fallen for a century, yet its tonnage reached new highs in the 2020s, driven largely by China and India.

The explanation is the growth of total demand. World primary energy was about 592 EJ in 2024, according to the Energy Institute, many times what it was in 1900. New sources have mostly been added on top of the old ones rather than replacing them. When a fast-growing source captures only part of the increase in demand, older sources can keep growing in absolute terms even as their shares shrink.

This matters for climate, because the atmosphere responds to absolute emissions, not to shares. A falling coal share is good news only once coal tonnage itself goes down.

India shows the pattern today. In 2025, according to Ember, its renewable generation rose by a record 24% and its fossil generation fell by 3.3%. Yet coal still supplied about three-quarters of India's electricity in 2023-24, and the IEA expected the country to add around 60 GW of new coal capacity by 2030, even as older units retire. Both stories are true at once. A fast-growing source can win most of the new demand while the older source, already built and paid for, keeps running for decades.

• Smil, Energy Transitions: Global and National Perspectives (2017) · via Smil Mastery Course, M9 'The grand fuel sequence and its pace'
• Energy Institute, Statistical Review of World Energy 2025 (2025) web ↗ · 2024 world primary energy of 592 EJ and country totals, https://www.energyinst.org/statistical-review; per-person values are rounded calculations using UN population estimates
• Ember, Global Electricity Review 2026 (April 2026) · 2025 data, https://ember-energy.org/latest-insights/global-electricity-review-2026/ and https://ember-energy.org/chapter/2025-in-review/
• IEA, World Energy Outlook 2024 (2024), India findings as reported by PTI · Outlook Business, 17 October 2024, https://www.outlookbusiness.com/sustainability/india-set-to-lead-global-energy-demand-growth-coal-will-remain-central-iea
• Global Carbon Project, Global Carbon Budget 2025 (November 2025) · 'Fossil fuel CO2 emissions hit record high in 2025', https://globalcarbonbudget.org/fossil-fuel-co2-emissions-hit-record-high-in-2025/

1.4 Countries moved at very different speeds, set by resources, size and policy

Global averages hide sharply different national stories. Britain was the first coal economy. Coal already supplied about two-thirds of its heat by the mid-1600s and over 98% of its primary energy by 1850, and it still provided about 91% in 1950. Then North Sea oil and gas, found in the 1960s and developed in the 1970s, rebuilt the British system within a generation, and its last coal power station, Ratcliffe-on-Soar, closed on 30 September 2024.

The Netherlands shows how fast a country can move with the right resource. The giant Groningen gas field was found in 1959, deliveries began in 1963, and within about a decade gas supplied around half of Dutch primary energy, heating nearly every home through a new national network. France moved slowly from wood to coal, but after the 1973 oil shock the state chose nuclear power and built a fleet that came to supply roughly three-quarters of its electricity. That was a policy-driven transition, delivered by a single state utility using standardised reactor designs.

Large, resource-rich countries such as the United States and Russia moved from wood to coal to oil and gas on their own reserves. Resource-poor Japan built its modern economy on imported oil, coal and liquefied natural gas, plus nuclear power. China jumped from biomass straight to coal, became by far the largest coal user in the world, and then became the largest builder of hydro, wind, solar and nuclear plants and of electric vehicles.

India is in the middle of its own version. Household biomass is still widely used, coal supplies most electricity, oil is mostly imported, and solar is growing faster than any other source. Three lessons stand out from the national record. Abundant domestic resources speed transitions up. Small countries can switch faster than large ones. Policy can drive change, as France and Germany show, but it cannot override the physics and economics of the assets involved.

• Smil, Energy Transitions: Global and National Perspectives (2017) · via Smil Mastery Course, M9 'National transitions'
• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M9 'National transitions'
• Uniper, 'The end of an era: Ratcliffe-on-Soar power station ends coal generation' (30 September 2024) · web
• Pipeline & Gas Journal, 'Dutch Senate approves law to permanently close Groningen gas field' (April 2024); NL Times, 'Gas production in Groningen stopped after 60 years' (1 October 2023) · web

See it in the table

Check your understanding

A chart shows wind supplying 6% of a country's primary energy, while another shows 2.5%. Both use the same electricity data. What is the most likely explanation?

Roughly how long did oil and natural gas each take to go from 5% to 25% of world primary energy?

Why can coal use rise in absolute terms while its share of world energy falls?