Energy transitions and climate: Carbon dioxide, climate and the paths ahead

4.1 Humans now rival the planet's natural flows of carbon, nitrogen and sulfur

Life depends on a few elements that circulate between the air, water, rocks, soils and living things. After water, the three most disturbed by human activity are carbon, nitrogen and sulfur. Carbon is the backbone of every organism and every fossil fuel. Nitrogen is in every protein and is the nutrient that most often limits plant growth. Sulfur is also part of proteins, and is released in large amounts when coal and oil are burned and metal ores are smelted.

Each cycle has reservoirs, such as the atmosphere, the oceans, soils and sedimentary rocks, and flows between them, such as photosynthesis, respiration, weathering, volcanic emissions and the bacterial fixation of nitrogen from the air. For most of history, human activity added little to those natural flows. That changed in the twentieth century. Burning fossil fuels now moves around 10 billion tonnes a year of carbon that was locked in rock for millions of years into the air. Synthetic fertiliser made by the Haber-Bosch process, plus nitrogen oxides from combustion, has roughly doubled the rate at which reactive nitrogen enters land ecosystems. At their peak, fuel burning and smelting released more sulfur than all natural sources combined.

In the 1980s Smil ranked the three problems. Sulfur, he judged, would be the most tractable, because it can be removed from fuels and flue gases with known technology at modest cost. Nitrogen would be the most pervasive, because it is tied to feeding the world. Carbon would be the hardest, because of its sheer scale and the lack of cheap substitutes for fossil fuels. Events have borne this ranking out.

Two further points matter. The cycles are linked: burning coal releases CO₂, sulfur dioxide and nitrogen oxides together, so one change, such as replacing a coal plant, can ease several problems at once. And the cycles have long memories. A substantial part of the CO₂ emitted today will still be in the atmosphere and oceans for centuries, so delay has lasting consequences.

• Smil, Carbon-Nitrogen-Sulfur: Human Interference in Grand Biospheric Cycles (1985) · via Smil Mastery Course, M12 'Interfering with the grand cycles'
• Smil, Cycles of Life: Civilization and the Biosphere (1997) · via Smil Mastery Course, M12 'Interfering with the grand cycles'

4.2 The physics of CO2 warming has been known for over a century

The basic science is old. In 1896 the Swedish chemist Svante Arrhenius estimated that doubling the CO₂ in the air would warm the Earth by several degrees. In 1958 Charles Keeling began continuous measurements of atmospheric CO₂ at Mauna Loa in Hawaii, producing the most famous curve in climate science.

The rise is unambiguous. For centuries before 1800, CO₂ stayed close to 270 to 280 parts per million. It reached about 290 ppm by 1900 and nearly 375 ppm by 2000. The Global Carbon Budget puts the 2025 level at about 425.7 ppm, some 52% above pre-industrial levels and higher than at any time in at least 800,000 years. Fossil fuel combustion is the dominant cause, with cement and deforestation adding more. Methane and nitrous oxide, from fossil fuel supply chains, farming and landfills, add further warming.

Not all emitted CO₂ stays in the air. Land plants and the oceans absorb roughly half of each year's emissions. The Global Carbon Budget warns that climate change itself is weakening these sinks, and that this weakening explains about 8% of the rise in atmospheric CO₂ since 1960. A weaker sink means more warming for the same emissions.

The IPCC's Sixth Assessment found that the world had warmed by about 1.1 C between pre-industrial times and 2011-2020, and that the most likely long-term warming from a doubling of CO₂ is about 3 C, within a likely range of 2.5 to 4 C. Smil's own treatment, written long before climate dominated headlines, is wary of both denial and catastrophism. He surveys the consequences, including higher temperatures, sea-level rise, ocean acidification and shifting rainfall, and treats the remaining uncertainties about clouds, regional effects and timing as reasons for prudence, not for waiting.

• Smil, Carbon-Nitrogen-Sulfur: Human Interference in Grand Biospheric Cycles (1985) · via Smil Mastery Course, M12 'Carbon dioxide and climate change'
• Smil, Global Catastrophes and Trends: The Next Fifty Years (2008) · via Smil Mastery Course, M12 'Carbon dioxide and climate change'
• Smil, How the World Really Works: A Scientist's Guide to Our Past, Present and Future (2022) · via Smil Mastery Course, M12 'Carbon dioxide and climate change'
• 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/
• 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/

4.3 The IEA's 2025 scenarios span roughly 1.5 to 3 C of warming

The IEA's World Energy Outlook is the most widely used set of long-term energy scenarios. The 2025 edition, published in November 2025, frames the future around three main paths, plus a new one focused on energy access.

The Current Policies Scenario assumes governments keep only the measures already in force. In it, oil demand is about 11% above today's level by 2050 and gas about 31% higher, while coal declines before 2030. Warming reaches about 2.9 C by 2100 and is still rising. The Stated Policies Scenario adds policies that governments have announced and the IEA judges likely to happen. In it coal is at or near its peak already, oil levels off around 2030 and gas grows until about the mid-2030s, helped by a wave of new liquefied natural gas exports. Solar generation more than quadruples by 2035, wind nearly triples and nuclear grows by almost two-fifths, and renewables become the largest source of energy in the early 2040s. Warming reaches about 2.5 C by 2100. The Net Zero Emissions by 2050 Scenario shows what reaching net zero would take; even then warming peaks at about 1.65 C around mid-century and falls back to 1.5 C only through active removal of CO₂.

In every scenario, world energy demand keeps growing, led by India and Southeast Asia, and electricity demand rises fast, largely because of air conditioning. About 730 million people still lack electricity and nearly 2 billion cook with polluting fuels, which is why the new access scenario sets universal electricity by 2035 and clean cooking by 2040.

For India the gap between scenarios comes down to concrete policy choices. The Current Policies case assumes, for example, that the share of electric cars in sales never rises from today's low level, that the planned carbon market is not implemented, and that efficiency rules for buildings and appliances are not extended. Each of these is a decision still being made, and each changes the investment map for the next two decades.

• 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

See it in the table

Check your understanding

Which of the three element cycles did Smil judge the most tractable, and why?

According to the Global Carbon Budget 2025, how long would the remaining 1.5 C carbon budget last at 2025 emission rates?

In the IEA's 2025 Stated Policies Scenario, roughly how much warming is reached by 2100?