After water, carbon dioxide is arguably the most important molecule in human history. In 1896, Swedish scientist Svante Arrhenius published the first quantitative estimate of how changes in atmospheric carbon dioxide concentrations could alter Earth’s temperature. 1 Using basic principles of physical chemistry, he calculated that increasing carbon dioxide levels could raise global temperatures by several degrees Celsius—a foundational concept in modern climate science.
Carbon dioxide is a powerful and long-lived greenhouse gas. Atmospheric concentrations are now 50 percent higher than they were before the Industrial Revolution, meaning the amount of carbon dioxide in the atmosphere is 150 percent of its level in 1750. 2 Carbon dioxide is the single most important greenhouse gas related to human activities, accounting for about 64 percent of the warming effect on the climate. 3
There are two broad sources of anthropogenic carbon dioxide emissions: the combustion of fossil fuels and industrial activities such as cement production, and land use change such as deforestation. This article focuses on the first category of emissions.
Anthropogenic carbon dioxide emissions increased from about 9,000 kilotons in 1750 to approximately 36 million tons in 2022—a 4,000-fold increase. Economic growth and the shift to fossil fuels that accompanied the Industrial Revolution drove this rise in emissions. Between 1850 and 1900, there were only two years in which emissions did not increase from the previous year. Most emissions during this period resulted from the combustion of coal for railroad transportation, iron and steel production, steam-powered manufacturing, and residential and commercial heating. Coal was also converted into gas for street lighting and home illumination.
In the second half of the 19th century, new industrial activities emerged that released so-called “process emissions”—carbon dioxide produced by chemical reactions not involving direct fuel combustion. These activities included the production of cement, iron and steel, lime, aluminum, and various petrochemical and chemical products.
Sustained economic growth in the three decades following World War II was also characterized by rapid increases in carbon dioxide emissions. However, the energy price shocks that began in the mid-1970s slowed the rate of emissions growth by encouraging improvements in energy efficiency and dampening economic activity. By the early 2000s, low-carbon energy sources such as wind and solar began to emerge and contributed to reduced rates of emissions growth.
In the first half of the 20th century, there was a dramatic shift in carbon dioxide emissions from coal to liquid fuels derived from oil, such as motor gasoline, diesel fuel, and heavy oil. This transition reflected the sharp increase in demand for transportation services to move people and goods. Beginning in the 1960s, natural gas also became a major source of carbon dioxide emissions, as advances in pipeline and processing technology made it widely available for heating and power generation.
Coal’s share of total carbon dioxide emissions began to rise again in the late 1970s. Several factors contributed to this shift. First, coal became the primary fuel for electricity generation and industrial growth in emerging economies—especially China and India—where coal is cheap and abundant. Second, in response to the oil price shocks of the 1970s, many high-income countries, including those in the U.S. and Europe, reduced their reliance on oil, particularly for power generation. Much of this gap was filled by coal, leading to higher emissions due to coal’s higher carbon intensity (~95 kg CO₂ per million BTU) compared to oil (~73 kg CO₂ per million BTU).
There have been similar shifts in the relative importance of the economic sectors responsible for carbon dioxide emissions. Prior to 1900, iron and steel manufacturing were the dominant industrial source of coal combustion emissions. The blast furnace became widespread, particularly in the UK, Germany, and the U.S., and its use of coking coal resulted in large emissions. Industries such as glass, brick, cement, and chemical manufacturing, all of which used coal either as a fuel or as a feedstock. Residences were another major source of emissions due the use of coal for home heating and cooking, particularly in Europe and North America.
In the twentieth century the generation of electricity with fossil fuels steadily expanded until it was the single largest source of carbon dioxide. Emissions from the combustion of liquid fuels in transportation also increased, especially after World War II. Heat production outside of the residential sector also became an important source of carbon dioxide emissions from processes in cement, pulp and paper, ceramics, and chemicals. Commercial buildings also use heat from fossil fuels for space heating, water heating, cooking, and laundry.
By a wide margin, the United States has released more carbon dioxide than any other nation. This is largely a result of its sizable economy and population, along with a long-standing dependence on fossil fuels to produce goods and services and to support lifestyles characterized by high levels of material consumption. In recent decades, China has rapidly closed the gap due to a massive expansion of coal-fired electricity generation and rising levels of consumption.
In many countries, electricity generation is the primary driver of carbon dioxide emissions. Russia is a notable exception, where heat production accounts for about 40 percent of total emissions. Russia has one of the oldest and most expansive district heating systems in the world. Fossil fuels—primarily natural gas, but also coal and oil—are often burned in dedicated heat-only boiler plants that do not generate electricity. Much of the district heating infrastructure and building stock is old and inefficient. In addition, Russia’s long, cold winters create exceptionally high demand for heat.
Brazil presents another interesting case, where road transport is the dominant source of carbon dioxide emissions. Several factors contribute to this pattern. Over 90 percent of Brazil’s electricity is generated from low-carbon energy sources such as hydropower, wind, and solar. 4 Brazil is a large country with a vast road network used to move people and goods. Most freight is transported by trucks rather than rail, and vehicle ownership is high in urban centers.
Global energy-related carbon dioxide emissions set a record in 2024.5 The time profile of carbon dioxide emissions varies considerably among countries. Emissions in some countries have peaked and markedly declined, while in other countries emissions continue to rise. Countries can be categorized as follows:
Early peakers: United Kingdom (1973), Germany (1979), Belgium (1979), Russia (1989)
Mid-peakers: Finland, (2003), Japan (2004), United States (2007)
Later peakers: Mexico (2012), Brazil (2014), Botswana (2018)
No apparent peak: India, China, Nigeria, Saudi Arabia, Iran, Indonesia
Climate change is a threat to human well-being and planetary health, and there is a rapidly closing window of opportunity to secure a livable and sustainable future for all.6 Reducing carbon dioxide emissions is front and center in any strategy to stabilize climate. Global emissions remain at unsustainable levels due to historic emissions in developed countries and rising emissions in countries such as China and India.
The major pathways to reducing carbon dioxide emissions are well established: improving the efficiency of energy end use, electrifying electricity generation, transportation, and heating, and deploying low-carbon fuels in sectors where electrification is difficult or costly.
The barriers to emissions reductions are also well known. Markets often fail to reflect the human and environmental costs associated with climate change, and entrenched interests frequently wield enough political power to block government intervention. Many governments subsidize fossil fuel use, thereby encouraging continued consumption. In addition, many people are deeply attached to lifestyles characterized by high energy use. Finally, international cooperation and consensus on reducing carbon dioxide emissions have been uneven at best.
1 Arrhenius, Svante, “On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground” Philosophical Magazine and Journal of Science, Series 5, Volume 41, April 1896, pages 237–276, https://www.rsc.org/images/Arrhenius1896_tcm18-173546.pdf
2 National Aeronautics and Space Administration, “Vital Signs: Carbon dioxide,” accessed June 2, 2025, https://climate.nasa.gov/vital-signs/carbon-dioxide/?intent=121
3 World Meteorological Organization, “The State of Greenhouse Gases in the Atmosphere Based on Global Observations through 2023,” WMO Greenhouse Gas Bulletin, No. 20 – 28 October 2024, Link
4 Ember, Brazil, Brazil, updated 10 Apr 2025, accessed 2 Jun 2025, https://tinyurl.com/4bwdwmbn
5 International Energy Agency, “Global Energy Review 2025. CO2 Emissions,” accessed June 2, 2025, https://www.iea.org/reports/global-energy-review-2025/co2-emissions
6 IPCC, 2023: Summary for Policymakers. In: Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change Core Writing Team, H. Lee and J. Romero (eds.). IPCC, Geneva, Switzerland, pp. 1-34, doi: 10.59327/IPCC/AR6-9789291691647.001 Link
