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Energy transitions and climate: Key terms, quiz and sources

Understanding Energy / Module 13: Energy transitions and climate Wrap-up

Energy transitions and climate: review

Key terms

Energy transition
A sustained change in the mix of primary energy, the prime movers that convert it, or the ways energy is used.
Prime mover
A machine or animal that converts energy into mechanical motion, such as a steam turbine, an engine or an electric motor.
Substitution method
An accounting convention that credits non-fossil electricity with the fuel a thermal power station would have burned to produce it.
Physical energy content method
An accounting convention that counts wind and solar electricity at its actual energy content of 3.6 MJ per kWh.
Hydrogen to carbon ratio
The number of hydrogen atoms per carbon atom in a fuel; higher ratios mean less CO2 per unit of heat.
Carbon intensity
The CO2 released per unit of energy or per unit of economic output.
Relative decarbonisation
A fall in carbon intensity, which can happen while total emissions still rise.
Absolute decarbonisation
A fall in the total quantity of CO2 emitted.
Lock-in
The way fuels, converters, networks and institutions depend on one another, making any single part hard to change.
Stranded asset
An asset that loses its value before the end of its expected life because it can no longer run profitably.
Hard-to-abate sectors
Activities such as steel, cement, ammonia, plastics, aviation and shipping whose emissions cannot be removed simply by switching to clean electricity.
Process emissions
CO2 released by the chemistry of a production process rather than by burning fuel, as when limestone is heated to make cement.
Calcination
The breakdown of limestone into lime and CO2 when heated: CaCO3 -> CaO + CO2.
Parts per million (ppm)
The number of molecules of a gas per million molecules of dry air; the usual unit for atmospheric CO2.
Carbon sinks
Natural processes in plants, soils and oceans that absorb CO2 from the atmosphere, currently about half of annual emissions.
Carbon budget
The total CO2 that can still be emitted while keeping a stated chance of holding warming below a given level.
Current Policies Scenario
An IEA scenario that keeps only the energy and climate measures already in force.
Stated Policies Scenario
An IEA scenario that adds announced policies the agency judges likely to be implemented.
Net Zero Emissions by 2050 Scenario
An IEA pathway showing what the energy system would need to do to reach net-zero CO2 by mid-century.

Try it in the table

  • Open the Steam turbines and thermal power plants tile and find the typical efficiency it gives for a coal plant. Use it to work out how many MJ of fuel the substitution method credits to 1 kWh of wind, and compare with the 3.6 MJ physical content.
  • Open the Thermal coal commodity tile and look at which countries dominate consumption. Set that against Part 1's point that coal's share falls while its tonnage can still rise.
  • Open Energy-intensive industry and list which of the four pillars (ammonia, steel, cement, plastics) it covers. For each, note whether the main emissions come from fuel or from the process itself.
  • Open Electrolysers and then the Ammonia chemical tile. Find how much electricity is needed per kilogram of hydrogen, and estimate the clean electricity needed to make one tonne of ammonia without natural gas.
  • Open Lithium iron phosphate batteries and Pumped hydro storage and compare how many hours each can typically discharge for. Which would you use to cover a windless week, and which a solar evening peak?

Final quiz

Coal reached about 5% of world primary energy around 1840. Roughly when did it reach 25%?

It took about 35 years to go from 5% to 25%, and coal did not supply half of world energy until around 1900.

What happens to a non-fossil source's share of primary energy if a statistician switches from the physical energy content method to the substitution method?

Each kWh is credited with about 9 to 10 MJ of avoided fuel instead of 3.6 MJ.

Which pair of fuels is listed correctly from higher to lower CO2 released per unit of heat?

Coal releases about 95 kg of CO2 per GJ, oil about 73 and gas about 56, because of rising hydrogen to carbon ratios.

According to Ember, what was notable about world electricity in 2025?

Low-carbon output rose about 887 TWh against demand growth of about 849 TWh; renewables reached 33.8%, edging past coal at 33.0%.

In Smil's estimate, the production of ammonia, steel, cement and plastics causes roughly what share of fossil CO2 emissions?

They take about 17% of primary energy and cause about a quarter of fossil CO2, partly because of process emissions.

Why does a fast-growing economy's young coal fleet create a financial problem for decarbonisation?

Long asset lives mean early retirement or low running hours leave owners, lenders or consumers with unrecovered costs.

The Global Carbon Budget 2025 puts atmospheric CO2 at about 425.7 ppm. Roughly how does this compare with pre-industrial levels?

Pre-industrial levels were close to 280 ppm, and the 2025 value is about 52% above that.

Which statement about the IEA's 2025 scenarios is correct?

CPS reaches about 2.9 C. STEPS has oil levelling off around 2030, and even the net-zero case overshoots to about 1.65 C. The IEA stresses that scenarios are not forecasts.
Next module: Energy, the economy and the future →

Sources for this module

  • Smil, Energy Transitions: Global and National Perspectives (2017) · via Smil Mastery Course, M9 'What an energy transition is', 'The grand fuel sequence and its pace', 'National transitions', 'Why transitions are slow', 'Decarbonisation so far'
  • 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', 'Why transitions are slow', 'Energy myths and realities'
  • 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', 'The four pillars and the hard parts of decarbonising'; M12 'Carbon dioxide and climate change'
  • Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M9 'What an energy transition is', 'National transitions'
  • Smil, Grand Transitions: How the Modern World Was Made (2021) · via Smil Mastery Course, M9 'Decarbonisation so far'
  • Smil, Energy at the Crossroads: Global Perspectives and Uncertainties (2003) · via Smil Mastery Course, M9 'Energy myths and realities'
  • Smil, Making the Modern World: Materials and Dematerialization (2014) · via Smil Mastery Course, M9 'The four pillars and the hard parts of decarbonising'
  • Smil, Still the Iron Age: Iron and Steel in the Modern World (2016) · via Smil Mastery Course, M9 'The four pillars and the hard parts of decarbonising'
  • Smil, Carbon-Nitrogen-Sulfur: Human Interference in Grand Biospheric Cycles (1985) · via Smil Mastery Course, M12 'Interfering with the grand cycles', 'Carbon dioxide and climate change'
  • Smil, Cycles of Life: Civilization and the Biosphere (1997) · via Smil Mastery Course, M12 'Interfering with the grand cycles'
  • Smil, Global Catastrophes and Trends: The Next Fifty Years (2008) · via Smil Mastery Course, M12 'Carbon dioxide and climate change'
  • IEA, World Energy Outlook 2025 (2025) · Overview and key findings, https://www.iea.org/reports/world-energy-outlook-2025/overview-and-key-findings
  • Carbon Brief, 'IEA: Fossil-fuel use will peak before 2030 unless stated policies are abandoned' (November 2025) · summary of IEA World Energy Outlook 2025 scenario results, https://www.carbonbrief.org/iea-fossil-fuel-use-will-peak-before-2030-unless-stated-policies-are-abandoned/
  • ACCR, Investor bulletin: IEA World Energy Outlook 2025 (13 November 2025) · comparison of WEO 2025 scenarios with the 2015 and 2019 editions, storage capacity by scenario, https://www.accr.org.au/downloads/13112025-investor-bulletin-weo-2025.pdf
  • 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/
  • 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
  • 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
  • IPCC, 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 2 Energy, Table 2.2 (2006) · default CO2 emission factors for combustion, https://www.ipcc-nggip.iges.or.jp/public/2006gl/vol2.html
  • IPCC, Climate Change 2021: The Physical Science Basis, Summary for Policymakers (2021) · observed warming 2011-2020 and equilibrium climate sensitivity, https://www.ipcc.ch/report/ar6/wg1/
  • IEA, Global EV Outlook 2025 (2025) · 2024 electric car sales above 17 million, share above 20%, https://www.iea.org/reports/global-ev-outlook-2025/executive-summary
  • 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
  • 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