CO2 is already in the atmosphere naturally, so why are emissions from human activity significant?
Human activities have significantly disturbed the natural carbon cycle by extracting long-buried fossil fuels and burning them for energy, thus releasing CO2 to the atmosphere.
In nature, CO2 is exchanged continually between the atmosphere, plants and animals through photosynthesis, respiration, and decomposition, and between the atmosphere and ocean through gas exchange. A very small amount of carbon (roughly 1% of the emission rate from fossil fuel combustion) is also emitted in volcanic eruptions. This is balanced by an equivalent amount that is removed by chemical weathering of rocks.
This natural cycling of carbon between the atmosphere and the land and ocean reservoir leads to no significant change in atmospheric concentration. This is known because the atmospheric concentration of carbon was stable throughout the last 10,000 years (the Holocene), until the start of the industrial revolution.
Burning of fossil fuels, along with deforestation, leads to a net release and accumulation of CO2 into the atmosphere. The CO2 level in 2025 was 50% higher than it was in the 19th century. Most of this CO2 increase has taken place since 1970, about the time when global energy consumption accelerated. Measured decreases in the fraction of other forms of carbon (the isotopes 14C and 13C) and a small decrease in atmospheric oxygen concentration (observations of which have been available since 1990) show that the rise in CO2 is largely from combustion of fossil fuels (which have low 13C fractions and no 14C). Deforestation and other land use changes have also released carbon from the biosphere (living world) where it normally resides for decades to centuries. Together, fossil fuel burning and deforestation release more than 40 billion tonnes of CO2 every year. This additional CO2 from fossil fuel burning and deforestation disturbs the balance of the carbon cycle, because the natural processes that would restore the balance are too slow compared to the rates at which human activities are adding CO2 to the atmosphere. As a result, a substantial fraction, about 50%, of the CO2 emitted from human activities accumulates in the atmosphere, where some of it will remain not just for decades or centuries, but for thousands of years. Comparison with the CO2 levels measured in air extracted from ice cores indicates that the current concentrations are now more than 50% higher than during the Holocene (see Question 6).
Page last updated: July 2026
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