Each year humans burn enough fossil fuel and clear enough forest to release roughly ten gigatonnes of carbon into the atmosphere — about half stays airborne (driving the Keeling-curve rise from 280 ppm pre-industrial to 420 ppm in 2024), about a quarter dissolves into the ocean, and about a quarter is taken up by the terrestrial biosphere. The airborne fraction has held at ~45% for decades, but it is not guaranteed: as ocean and biosphere sinks weaken, more of each new tonne would remain in the air. The carbon cycle — the global circulation of carbon among atmosphere, ocean, biota, and rock — is the single most consequential biogeochemical system in the climate, and industrial combustion is a brutal short-circuit of a process normally operating on timescales of tens to hundreds of millions of years.
There are really two carbon cycles operating at very different speeds, and the climate problem lives in the gap between them. The fast cycle turns over on years to centuries: photosynthesis fixes about 120 GtC/yr on land and 50 GtC/yr in the ocean's surface, respiration and decomposition return roughly the same amount, CO₂ dissolves into cold high-latitude oceans and outgasses from warm tropical ones, and a biological pump carries a small fraction of surface carbon to the deep ocean for centuries. The slow cycle turns over on 10⁵ to 10⁸ years: silicate weathering — atmospheric CO₂ reacting with rain and silicate rock to produce dissolved bicarbonate — is temperature-sensitive and acts as Earth's long-term thermostat, while marine organisms bury calcium-carbonate shells and a tiny ~0.1% of organic carbon escapes decomposition into sediment, building over hundreds of millions of years the fossil-fuel reservoirs humans now mine. The reason fossil-fuel combustion is so consequential is this timescale mismatch: carbon removed from the atmosphere over ~10⁸ years is being returned to the fast cycle in ~10² years, roughly a million times faster than the geological burial rate — and the atmosphere and surface ocean cannot equilibrate that quickly, so the airborne fraction stays high and the climate warms. The isotopic fingerprint clinches it: fossil-fuel carbon is depleted in carbon-14 (decayed during burial) and slightly depleted in carbon-13 (photosynthesis prefers ¹²C), and atmospheric ratios shift in exactly the direction fossil-fuel emissions predict — independent confirmation that the rising CO₂ is anthropogenic.
Annual emissions in 2024 ran about 41 GtCO₂, cumulative since 1750 about 2,500 GtCO₂, and the remaining budget consistent with limiting warming to 1.5 °C is roughly 250 GtCO₂ — under seven years at current rates. The land sink is showing signs of stress (Amazon and boreal forests under fire and drought, permafrost thaw releasing both carbon and methane), and although the Global Carbon Project still finds the airborne fraction holding, IPCC scenarios reaching 1.5 °C assume gigatonne-scale carbon-dioxide removal by mid-century — against current deployment of about two megatonnes a year, four orders of magnitude short.