Indirect Greenhouse Gases Make up 12% of Global Warming: The Impacts of Carbon Monoxide, Non-Methane VOCs, Nitrogen Oxides, and Hydrogen

     The 1997 Kyoto Protocol set up reporting requirements for seven ‘direct’ greenhouse gases, with significant global warming potentials (GWPs). These are listed below.

     Six of the seven of the gases, the ones aside from CO2, have much higher GWPs than CO2, which is the standard by which they are measured.

     Recently, a group of scientists argued that indirect greenhouse gases (iGHGs), which are responsible for 12% of global warming, merit more attention. These gases are not addressed under any climate treaty.

     Mediafeed reports:

“Carbon monoxide, non-methane volatile organic compounds (NMVOCs), nitrogen oxides and hydrogen are considered indirect greenhouse gases, because they do not absorb heat themselves, but they cause reactions in the atmosphere that affect the climate,” says Tianyi Sun, a senior climate scientist at Environmental Defense Fund, a global nonprofit.

“In a recent paper in the journal Science, Sun, EDF Chief Scientist Steven Hamburg and eight other climate experts argue that it’s time to include indirect greenhouse gases in the international agreements that shape what types of climate pollution countries measure, target and reduce.”

     Indirect greenhouse gases do cause global warming directly, but do so indirectly through chemical reactions in the atmosphere. Most iGHGs come from burning fossil fuels and from industrial processes like chemical manufacturing. Emissions of them come from many sectors.

“Thirty years ago, our understanding of indirect greenhouse gases was in its early stages,” explains Sun. “Now, our understanding has matured, but the policy hasn’t caught up.”

     The IPCC is developing technical guidance for countries on how to include a wider range of climate pollutants in their national inventories in its next report, scheduled to be published in 2027. I wonder how this will affect trajectories. Have we been underestimating total greenhouse gas emissions by as much as 12% by not accounting for iGHGs? While the article suggests 15%, the abstract of the paper in Science notes that iGHGs make up 80% of the unaccounted 15% of global warming. That makes them potentially responsible for as much as 12% of global warming in future assessments.

     The authors of the paper note that addressing three of these indirect GHGs, carbon monoxide, VOCs, and nitrogen oxides, also addresses them as concerning pollutants and/or precursors to ground-level ozone.  

“Even though monitoring and enforcement of these pollutants isn’t uniform and needs to be scaled up across the world to deliver maximum results, existing management programs give policymakers a head start, says Sun.

     The table below shows the GWPs of the four iGHGs as well as their global temperature change potential (GTP).

 

References:

 

The forgotten 15% of global warming pollution. Anne Marie Borrego. Mediafeed. August 8, 2026. The forgotten 15% of global warming pollution

Integrating indirect greenhouse gases into climate frameworks: These substances have minimal direct climate effects but trigger chemical reactions that can lead to warming. Ilissa Ocko, Jean-Francois Lamarque, Jonathan M. Moch, Sam Abernethy, Tianyi Sun, Tom Grylls, Arindam Roy, Steven P. Hamburg, Rick Duke, and Philip B. Duffy. Science.11 Jun 2026. Vol 392, Issue 6803. pp. 1134-1137. Integrating indirect greenhouse gases into climate frameworks | Science

Kyoto Protocol Gases. Green Calculus. August 2026. The 7 Kyoto Protocol Gases: The GHGs You Must Report