Energy transitions and climate: Why transitions are slow

2.1 The fossil system is the largest machine humanity has built

Start with scale. The world now emits about 38 billion tonnes of CO₂ a year from fossil fuels and cement, according to the Global Carbon Budget's estimate for 2025, which corresponds to roughly 10 billion tonnes of carbon. Behind that figure stand thousands of coal mines and oil and gas fields, hundreds of refineries, hundreds of thousands of kilometres of pipelines, thousands of power stations, more than a billion road vehicles, tens of thousands of ships and aircraft, and billions of boilers, furnaces and stoves.

Smil's point is not that this system cannot change, but that any replacement has to be built at a comparable scale. A technology that grows 50-fold from a tiny base can still be small next to a system this big. Between 2000 and about 2020, new renewables (wind, solar and modern biofuels) grew roughly 50 times over, yet the fossil share of world primary energy slipped only from about 87% to about 85%, and much of that slippage came from more hydroelectricity.

Scale also shows up in materials. Wind turbines, solar farms, transmission lines and batteries need steel, concrete, copper, aluminium, glass, silicon and battery minerals in large quantities. Most of those materials are still produced with fossil energy, so the new system is being built with the old one. That is not a contradiction; every transition has worked this way, and early railways ran on iron smelted with charcoal and rails laid by horse-drawn carts. But it means the build-out has its own emissions and its own supply constraints, including the concentration of mineral processing in a few countries.

The practical lesson for anyone assessing a plan is to translate it into physical quantities. How many gigawatts a year, how many tonnes of steel and copper, how many kilometres of line, how many trained workers? If the implied build rate is many times anything achieved before, the plan needs a very good explanation of what will be different this time.

• Smil, How the World Really Works: A Scientist's Guide to Our Past, Present and Future (2022) · via Smil Mastery Course, M9 'Why transitions are slow'
• Smil, Energy Transitions: Global and National Perspectives (2017) · via Smil Mastery Course, M9 'Why transitions are slow'
• 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/

2.2 Long-lived assets and locked-in networks resist early retirement

Energy assets are built to last. Coal and nuclear stations run for 40 to 60 years, hydro dams for a century, refineries and pipelines for decades, and buildings, which set much of the demand for heating and cooling, for a century or more. A car lasts 15 years or so, an industrial boiler 30. When a coal plant is closed after 15 years, its owner writes off most of the capital, and somebody, often electricity consumers or taxpayers, pays for it.

This bites hardest in fast-growing economies. A large part of the coal fleet in China and India was built in the past two decades, so on normal lifetimes much of it could run into the 2050s or 2060s. Closing it early, or running it for fewer hours as backup to renewables, raises hard questions of finance and fairness: who compensates the owners, the lenders, the workers and the coal-mining regions?

The second source of inertia is lock-in. Fuels only work with matching converters, and converters only work with matching networks. Petrol cars need refineries and filling stations; gas boilers need a gas grid; blast furnaces need coke ovens and coking coal; airports need jet fuel supply chains. Changing one part means changing the others, and nobody wants to build charging stations before there are electric cars, or buy the cars before there are stations. Each piece of the old system is cheap partly because the rest of it already exists and has been paid for.

Lock-in also covers institutions: grid codes written for large spinning generators, tariff designs, utility business models, skills and supply chains. These can change faster than steel and concrete, and good policy targets them. India's switch to competitive auctions for solar and wind contracts in the late 2010s, which drove bid prices down sharply, is an example of institutional change unlocking physical change.

• Smil, Energy Transitions: Global and National Perspectives (2017) · via Smil Mastery Course, M9 'Why transitions are slow'
• Smil, Energy Myths and Realities: Bringing Science to the Energy Policy Debate (2010) · via Smil Mastery Course, M9 'Why transitions are slow'
• 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

2.3 Rising demand absorbs much of the new clean supply

The third brake is growth. A transition is a race between new clean supply and rising total demand. If demand grows faster than clean supply, fossil use still rises.

For most of the past two decades that is what happened. World energy use kept climbing, mainly in Asia, and new wind and solar met only part of the increase. Fossil fuels filled the rest, which is why global CO₂ emissions kept setting records even as clean energy investment boomed.

In electricity the race is now tipping. Ember's Global Electricity Review 2026 found that in 2025 world electricity demand rose by about 849 TWh, while low-carbon generation rose by about 887 TWh, more than covering the increase. Fossil generation fell slightly, by 0.2%, the first year without growth since the pandemic year of 2020. Solar alone added 636 TWh, a rise of 30% in a single year, and overtook wind. Renewables reached 33.8% of world electricity, edging past coal at 33.0%.

Two cautions keep this in proportion. First, electricity is only about a fifth of final energy. Heat for industry and buildings and fuel for transport are still mostly fossil, and in those uses the race has barely begun. Second, demand growth itself is uncertain. The IEA's World Energy Outlook 2025 expects energy demand to keep rising to 2050 in all its main scenarios, led by emerging economies, above all India and Southeast Asia, with cooling, industry and transport as the big drivers. Data centres and AI attract the headlines but account for under a tenth of projected growth in global electricity demand, though more in the United States.

The lesson is to watch two numbers together: how fast clean supply grows in absolute terms, and how fast total demand grows. Only when the first is larger does fossil use fall, and that is now happening in power but not yet across the whole energy system.

• 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 2025 (2025) · Overview and key findings, https://www.iea.org/reports/world-energy-outlook-2025/overview-and-key-findings
• 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/
• Smil, How the World Really Works: A Scientist's Guide to Our Past, Present and Future (2022) · via Smil Mastery Course, M9 'Why transitions are slow'

See it in the table

Check your understanding

Between 2000 and about 2020 new renewables grew roughly 50-fold. What happened to the fossil share of world primary energy?

In 2025, according to Ember, why did world fossil-fuel generation stop growing?

Which statement about lock-in is most accurate?