Where carbon dioxide and methane come from

Carbon dioxide and methane have both risen since people began burning coal on a large scale, around 1750. Scientists measure both very precisely.

Snow that piles up on Antarctica traps bubbles of air as it turns to ice, so ice cores drilled there hold air from long ago. They show about 280 ppm of carbon dioxide in 1750. In 1958 Charles David Keeling began measuring carbon dioxide on Mauna Loa, a mountain in Hawaii far from cities and forests, and the measurements there have continued ever since.

The worldwide average in 2025 was about 426 ppm, half as much again as in 1750. The rise has sped up, from under 1 ppm a year in the 1960s to about 2.6 ppm a year from 2015 to 2025.

Burning carbon

Burning a fuel turns its carbon into carbon dioxide, as when methane burns on a gas stove. Coal, oil and natural gas hold carbon that spent millions of years underground. In 2025, burning them, together with making cement, gave out about 38 billion tonnes of carbon dioxide, and clearing forests added about 4 billion tonnes more. About half of it stays in the air. Over the last ten years the oceans have taken up about 29% of it, and plants on land about 21%.

Methane

Methane has risen from about 0.73 ppm in 1750 to about 1.94 ppm in 2025, more than two and a half times as much. Most of it is made by microbes that live where there is no oxygen: in the mud of wetlands and flooded rice fields, in rubbish buried in landfills, and in the stomachs of cattle, sheep and goats, which burp it out. Natural gas is mostly methane, so leaks from gas wells and pipelines add more, and so do coal mines.

Human activities give out about 65% of all methane. A methane molecule slowly reacts in the air, ending up as carbon dioxide and water, so it lasts about nine years on average. Kilogram for kilogram, methane traps about 80 times as much heat as carbon dioxide over its first 20 years in the air.