Atmospheric CO₂ at Mauna Loa (Monthly Average)

## What this measures

The monthly average concentration of carbon dioxide in dry air at Mauna Loa Observatory, Hawaii, in parts per million (ppm). The record starts in March 1958 and is the longest continuous direct measurement of atmospheric CO₂.

## Why it matters

CO₂ concentration is the running total of what the world has emitted, less what oceans and land have absorbed. It is the physical quantity that climate policy, carbon pricing and transition capital are ultimately trying to bend.

## How to read it

It rises every year and carries a seasonal cycle: Northern Hemisphere plant growth draws it down through summer and it rebuilds through winter, peaking around May. Compare the same month across years, or use the de-seasonalized series (climate.co2.mauna_loa_trend_ppm) for the underlying trend.

## What it does not say

It is one station. It reads the well-mixed background atmosphere, not local or national emissions, and it does not respond quickly to changes in emissions: a year of lower emissions slows the rise, it does not reverse it. Missing months are interpolated by NOAA. After the Mauna Loa eruption, measurements from December 2022 to early July 2023 came from a site on Maunakea about 21 miles away.

## Source

NOAA Global Monitoring Laboratory, Trends in Atmospheric Carbon Dioxide (co2_mm_mlo.csv), https://gml.noaa.gov/ccgg/trends/. Data from March 1958 to April 1974 were obtained by C. David Keeling of the Scripps Institution of Oceanography. Cite: Dr. Xin Lan, NOAA/GML (gml.noaa.gov/ccgg/trends/) and Dr. Ralph Keeling, Scripps Institution of Oceanography (scrippsco2.ucsd.edu/). US federal data, public domain; NOAA asks that GML receive credit.