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FalseClimateLast updated: December 15, 2025

CO2 is not a greenhouse gas

CO2's properties as a greenhouse gas — absorbing and re-emitting infrared radiation — were established experimentally by physicist John Tyndall in 1859 and have been confirmed by countless laboratory measurements and satellite observations since. This is foundational physics with no credible scientific dispute.

What we know

A greenhouse gas is defined as a gas that absorbs longwave (infrared) radiation emitted by Earth's surface and re-emits it, trapping heat in the atmosphere. CO2 qualifies because its molecular structure — one carbon atom bonded to two oxygen atoms — creates asymmetric vibrational modes that couple with infrared photons at specific wavelengths, particularly around 15 micrometers. This was first demonstrated experimentally by physicist John Tyndall at the Royal Institution in London between 1859 and 1861.

The CO2 greenhouse effect has been confirmed by every independent measurement method available: laboratory spectroscopy, atmospheric absorption measurements, satellite observations of outgoing longwave radiation, and surface radiometer measurements. A 2001 satellite study directly measured the decrease in outgoing long-wave radiation at CO2 absorption wavelengths between 1970 and 1997, providing direct experimental evidence of the enhanced greenhouse effect. A 2015 study measured the surface radiative forcing from CO2 at a ground station in Oklahoma, confirming predictions.

NOAA's Annual Greenhouse Gas Index (AGGI) tracks radiative forcing from long-lived greenhouse gases, showing CO2 accounts for about 66% of the total forcing increase since 1990. Without CO2's greenhouse effect, Earth's surface temperature would be approximately 33°C colder, rendering the planet largely uninhabitable.

Claims that CO2 is not a greenhouse gas are sometimes based on misunderstandings of how the greenhouse effect operates at the molecular level, or on confusion between the direct radiative effect and feedback mechanisms. Neither creates any scientific doubt about CO2's fundamental infrared absorption properties, which are measurable with basic spectroscopy equipment.

Common claims

  • CO2 is too small a fraction of the atmosphere to affect temperatureNot supported — CO2's infrared absorption is strong enough that even trace concentrations produce measurable warming
  • Water vapor is the real greenhouse gas, not CO2Misleading — water vapor is the largest natural greenhouse gas, but CO2 is the key driver of human-caused warming
  • CO2 absorptions are 'saturated' so adding more makes no differenceIncorrect — while lower atmosphere absorption at some wavelengths is saturated, warming occurs at higher altitudes where it is not
  • Scientists have recently discovered CO2 does not absorb infrared as claimedFalse — recent claims of this kind have been based on misunderstandings of spectroscopy methodology

Evidence hierarchy

01NOAA Annual Greenhouse Gas Index — CO2 radiative forcing dataNOAA measures CO2 at 422.8 ppm in 2024; CO2 accounts for 66% of total greenhouse gas radiative forcing increase since 199002Tyndall's original experiments (1859–1861) — The Carbon Dioxide Greenhouse EffectAIP History of Physics traces CO2 greenhouse effect from Tyndall's experiments to modern climate science03Climate Feedback: Simple measurements demonstrate CO2 is a greenhouse gasScience Feedback review of claims that CO2 is not a greenhouse gas; confirms it is established by basic spectroscopy since 185904MIT Climate Science: Physical basis of the greenhouse effectMIT explains molecular physics of CO2 absorption; notes this is not controversial even among skeptics of climate projections

All sources

Related terms

Human-caused climate changeFlat EarthChemtrails