Chapter 01 · Climate & transitionClimate & Greenhouse Gas Emissions
Carbon dioxide (CO2)
Definition
A colourless gas (one carbon, two oxygen atoms) produced by combustion, respiration and calcination, and absorbed by photosynthesis and ocean dissolution. It is the most consequential anthropogenic greenhouse gas: NOAA measurements show global atmospheric CO₂ reached 422.8 ppm in 2024 and set a record annual jump of 3.72 ppm into 2025, with Mauna Loa readings averaging 432.3 ppm in May 2026 — more than 50% above pre-industrial levels (~280 ppm).
References
2024 global mean 422.80 ± 0.10 ppm; record 3.72 ppm increase into 2025; measurement network.
>50% increase since pre-industrial times; fossil-fuel attribution; Mauna Loa and ice-core records.
Overview
What it means
CO₂'s dominance in climate change comes from scale and longevity: it is emitted in the largest volumes and a substantial fraction persists in the climate system for centuries to millennia, making cumulative emissions the controlling variable for peak warming. It is also the accounting yardstick: all other greenhouse gases are converted into "CO₂ equivalent" (CO₂e) using global warming potentials.
Its principal anthropogenic sources are fossil-fuel combustion, cement production (process emissions from limestone calcination) and land-use change.
How it is used
CO₂ concentrations and emissions are tracked by NOAA's global network and the Mauna Loa record (continuous since 1958); emissions targets, carbon prices, footprint metrics and removal technologies are all denominated in CO₂ or CO₂e.
Why it matters
Stabilising the climate requires net CO₂ emissions to fall to zero; the gas is simultaneously the problem's measure, its main driver, and the currency in which all solutions are counted.