Chapter 01 · Climate & transitionClimate & Greenhouse Gas Emissions
Atmospheric concentration
Definition
The abundance of a constituent gas in the atmosphere, measured as a mole fraction — for carbon dioxide, parts per million (ppm). Continuous direct measurement began at Mauna Loa in 1958 (the Keeling Curve); global average CO₂ reached about 428–429 ppm in 2026, compared with roughly 280 ppm before industrialisation, and continues to rise by several ppm per year.
References
Definition of dry-air mole fraction (ppm); global monthly means (May 2026: 428.73 ppm); measurement method.
Longest continuous record (since 1958); July 2026: 429.12 ppm; seasonal cycle and trend.
Overview
What it means
Concentration — not annual emissions — is what drives radiative forcing and warming, because long-lived greenhouse gases accumulate. NOAA's Global Monitoring Laboratory maintains the reference records from a worldwide sampling network, reporting monthly and weekly means with the seasonal cycle (peaking in May, troughing in autumn) superimposed on the upward trend.
How it is used
Concentration targets and pathways underpin climate policy: stabilising temperatures requires stabilising concentrations, and carbon budgets translate concentration goals into allowable cumulative emissions. Milestones (400 ppm in 2013–2016, records above 430 ppm daily in 2025) serve as public markers of progress or failure.
Why it matters
Atmospheric concentration is the scoreboard of climate action: whatever policies achieve elsewhere, the number keeps rising until net emissions reach zero. It connects emissions accounting directly to physical climate outcomes.