The Anthropogenic Greenhouse Effect and Key Gases

The anthropogenic (human-caused) greenhouse effect is the extra warming of Earth’s surface that results from greenhouse gases released by human activities, mainly carbon dioxide from burning fossil fuels and clearing forests, together with methane, nitrous oxide and fluorinated gases. In brief: the natural greenhouse effect keeps Earth about 33 °C warmer than it would otherwise be. Since the 19th century, human emissions have raised the carbon dioxide concentration from about 280 ppm to more than 420 ppm, and global average temperature has risen by about 1.1 to 1.2 °C compared with 1850–1900. The Intergovernmental Panel on Climate Change (IPCC) states that it is unequivocal that human influence has warmed the climate.

Understanding how greenhouse gases work and where they come from is the basis for climate policy. This guide explains the mechanism, the key gases, their sources and warming strength, compares the gases in a table, works through emission and warming-potential calculations, outlines the history of the science and corrects common mistakes.

How does the greenhouse effect work?

Sunlight passes through the atmosphere and warms the surface. The surface sends the energy back as infrared radiation. Greenhouse gases, whose molecules absorb infrared radiation, take up part of this energy and re-emit it in all directions, including downward, so the lower atmosphere and the surface are warmer than they would be without them. Nitrogen and oxygen, which form 99% of the air, hardly absorb infrared, so the trace gases are what matter. Adding more greenhouse gas makes the atmosphere more opaque to heat, so the planet has to warm up until it again radiates as much energy as it absorbs. The term “greenhouse” is a loose analogy, since a real greenhouse stays warm mainly because it stops air from mixing.

What are the key greenhouse gases?

• Carbon dioxide (CO₂): the most important human-emitted gas, responsible for the largest share of the added warming. It stays in the atmosphere for centuries to millennia, because it is exchanged with the oceans and the biosphere.
• Methane (CH₄): from livestock, rice fields, landfills, wetlands, and oil, gas and coal production. It is much stronger per molecule but lasts only about a decade in the air. Its concentration is more than two and a half times higher than the pre-industrial level.
• Nitrous oxide (N₂O): from fertilizers and manure, industry and burning. It is long-lived (over a century) and strong.
• Fluorinated gases (HFCs, PFCs, SF₆): synthetic gases from refrigeration, air conditioning and industry, present at very low concentrations but with thousands of times the warming effect per unit of CO₂. The Kigali Amendment (2016) to the Montreal Protocol limits HFCs.
• Water vapor: the strongest natural greenhouse gas, but humans do not directly control its amount. It acts as a feedback: warmer air holds more water vapor (about 7% more per °C), which amplifies the warming caused by other gases.
• Ozone: in the lower atmosphere it is a greenhouse gas and a pollutant.

To compare gases, scientists use the global warming potential (GWP), the warming effect of one tonne of a gas relative to one tonne of CO₂ over a set period, usually 100 years. In the IPCC’s Sixth Assessment Report, the 100-year GWP of methane is about 27–30 and that of nitrous oxide about 273.

Where do the emissions come from?

The burning of fossil fuels (coal, oil and gas) for electricity, heating, transport and industry is the largest source and releases about 37 billion tonnes of CO₂ per year (as of the early 2020s). Land use change, mainly deforestation, adds more. Agriculture contributes methane and nitrous oxide. Industrial processes, such as cement and steel production, release CO₂ from chemical reactions, and waste and leaks from fossil-fuel systems release methane. By sector, energy supply, industry, transport, buildings and agriculture and land use are all major contributors. Emissions per person differ greatly between countries, and historical emissions have been highest in Europe and North America, while China is now the largest annual emitter in total.

How do we know humans are responsible?

Several independent lines of evidence point to human causes. The increase in CO₂ matches the amount of fossil fuel burned: roughly half of the emitted CO₂ stays in the atmosphere, and the rest is absorbed by the oceans and land. The carbon in fossil fuels has a distinctive isotopic signature (no carbon-14 and more carbon-12), and this signature is seen in the atmosphere’s carbon. The oxygen level is falling slightly as it is used in burning. The upper atmosphere is cooling while the lower atmosphere is warming, a pattern expected for greenhouse warming and not for a stronger Sun. Satellites measure less infrared escaping to space at the wavelengths absorbed by CO₂. Natural factors, such as the Sun and volcanoes, cannot explain the warming observed since 1950.

Key gases compared

Worked example: emissions and warming potential

1. CO₂ from carbon: one tonne of carbon becomes 44 ÷ 12 = 3.67 tonnes of CO₂ when it burns, because CO₂ has a molar mass of 44 and carbon of 12.
2. Burning fuel: a car uses 1 liter of petrol that has a mass of about 0.74 kg and a carbon share of about 86%. The carbon is 0.74 × 0.86 = 0.64 kg, and the CO₂ is 0.64 × 3.67 = 2.3 kg per liter. A car using 7 liters per 100 km emits 7 × 2.3 = 16.1 kg of CO₂ per 100 km, which is 161 g per km.
3. Electricity: a household using 3,500 kWh a year from a grid that emits 0.4 kg CO₂ per kWh causes 3,500 × 0.4 = 1,400 kg of CO₂, or 1.4 tonnes.
4. CO₂ equivalent: a dairy farm emits 60 tonnes of methane and 2 tonnes of nitrous oxide a year. With GWP values of 28 and 273, the CO₂ equivalent is 60 × 28 + 2 × 273 = 1,680 + 546 = 2,226 tonnes.
5. Concentration change: an increase from 280 to 420 ppm is 140 ppm, or 50%. About 2.1 billion tonnes of carbon (7.8 billion tonnes of CO₂) raise the atmospheric concentration by 1 ppm. Global emissions of about 37 billion tonnes of CO₂ would raise the level by 37 ÷ 7.8 = 4.7 ppm a year if all stayed in the air. With about 45% staying, the rise is 0.45 × 4.7 = 2.1 ppm a year, which is close to the observed rise of around 2 to 2.5 ppm per year.
6. Warming: if global temperature has risen by 1.2 °C since 1880 and by 0.8 °C of that since 1970, then 0.8 ÷ 1.2 = two thirds of the warming occurred in the last 50-odd years.

How did the science develop?

Joseph Fourier described in the 1820s how the atmosphere warms the planet. John Tyndall measured the heat absorption of CO₂ and water vapor in 1859. In 1896, the Swedish chemist Svante Arrhenius estimated that doubling CO₂ would warm the Earth by several degrees. In 1958, Charles David Keeling began precise measurements of CO₂ at Mauna Loa in Hawaii, which show a steady rise with annual cycles (the Keeling curve). A US scientific report in 1979 (the Charney report) estimated a warming of about 3 °C for a doubling of CO₂, a range that has held up. The IPCC was set up in 1988, and its Sixth Assessment Report of 2021 concluded that it is unequivocal that human influence has warmed the atmosphere, ocean and land.

Which mistakes are common?

Mistake 1: Confusing the greenhouse effect with the ozone hole

The ozone hole is caused by chemicals such as CFCs that destroy stratospheric ozone and let in more ultraviolet light. The greenhouse effect concerns heat trapping. They are separate problems, though some CFC substitutes are also greenhouse gases.

Mistake 2: Saying that water vapor is the main cause of current warming

Water vapor amplifies warming, but its amount is controlled by temperature, and so it is a feedback, not the initial driver. The additional CO₂ started the change.

Mistake 3: Thinking that CO₂ is a tiny gas and cannot matter

Even small concentrations of gases that absorb infrared have large effects. Increasing from 280 to over 420 ppm is a rise of 50%.

Mistake 4: Comparing gases only by quantity

The global warming potential accounts for how strong the gas is and how long it lasts. A tonne of methane warms much more than a tonne of CO₂ over 100 years.