What is the main source of carbon dioxide emissions from human activity?

The main source of carbon dioxide emissions from human activity is burning fossil fuels.

Coal, oil, and natural gas contain carbon that was stored underground for millions of years. When these fuels are burned for electricity, transport, heating, industry, or other energy uses, the carbon combines with oxygen and forms carbon dioxide. This transfers carbon from long-term geological storage into the atmosphere much faster than natural systems can remove it.

Fossil-fuel combustion is distinct from industrial processes such as cement production, which also release CO₂ but through chemical reactions in materials. Agriculture can emit methane and nitrous oxide, while deforestation releases carbon by burning biomass and allowing carbon-rich vegetation and soils to decompose. Those activities matter, but they are not the largest overall source in this question’s standard global framing.

The distinction between “emissions” and “concentration” is useful: emissions are gases released each year, while atmospheric concentration is the amount accumulating in the air after natural sinks such as oceans and ecosystems absorb part of it.