How Indirect Greenhouse Gases Are Quietly Fueling Climate Change

Unlike carbon dioxide or methane, which directly absorb infrared radiation, these gases warm the planet through atmospheric chemistry. The main indirect greenhouse gases are:

• Carbon monoxide (CO): Released largely from incomplete fossil fuel and biomass combustion, CO reacts with hydroxyl radicals (OH) in the atmosphere. This reaction reduces the availability of OH, which is the primary sink for methane, thereby extending methane’s atmospheric lifetime. CO also contributes to the formation of tropospheric ozone .
• Nitrogen oxides (NOₓ): Produced by fuel combustion in vehicles, power plants, and industrial processes. In the presence of sunlight and volatile organic compounds, NOₓ catalyzes the photochemical production of ozone in the troposphere .
• Non-methane volatile organic compounds (NMVOCs): Emitted from solvents, paints, fuel evaporation, and vegetation, NMVOCs participate in complex reactions that generate ozone and secondary organic aerosols .
• Molecular hydrogen (H₂): Although not a direct greenhouse gas, hydrogen leakage into the atmosphere can react with OH, similarly increasing methane concentrations and contributing to ozone formation .

The UK’s national greenhouse gas inventory already tracks four indirect gases—NOₓ, CO, NMVOCs, and sulfur dioxide—explicitly because of their role in producing tropospheric ozone and affecting radiative forcing .

How They Drive 15% of Global Warming

The warming mechanism involves two interrelated pathways.

Pathway 1: Formation of Tropospheric Ozone

Tropospheric ozone is a short-lived greenhouse gas and a harmful air pollutant. It is not emitted directly but forms through photochemical reactions involving NOₓ, CO, NMVOCs, and methane in the presence of sunlight . The IPCC AR5 report estimated tropospheric ozone radiative forcing at 0.40 W/m², with a total ozone forcing of 0.35 W/m², largely attributed to anthropogenic emissions of methane, nitrogen oxides, carbon monoxide, and NMVOCs .

A NASA-linked modeling study attributed pre-industrial-to-present-day tropospheric ozone radiative forcing to increased emissions of methane (44±12%), nitrogen oxides (31±9%), carbon monoxide (15±3%), and NMVOCs (9±2%) .

Pathway 2: Extending Methane’s Lifetime

Methane's primary removal mechanism is oxidation by hydroxyl radicals (OH) in the troposphere. When CO and NMVOCs are emitted, they consume OH, reducing the atmosphere’s capacity to break down methane. This feedback increases methane’s perturbation lifetime by a factor of approximately 1.3–1.4, enhancing its warming impact . IPCC assessments have noted that indirect effects from CO and NMVOCs increase methane’s global warming potential significantly .

The combined effect of these pathways is substantial. The new Science study estimates that these indirect effects collectively account for about 0.3°C of the warming experienced to date—approximately 15% of total human-caused warming .

Why They Remain Outside Global Climate Agreements

Despite their significant warming contribution, indirect greenhouse gases have been largely omitted from major international climate treaties.

The Kyoto Protocol’s gas basket focuses on a defined set of direct greenhouse gases: CO₂, CH₄, N₂O, HFCs, PFCs, SF₆, and NF₃. Although parties are required to report indirect gases like NOₓ and CO in their national inventories, there are no binding reduction targets . The UNFCCC has acknowledged this limitation, noting that black carbon and most tropospheric ozone precursors will be included in the 2027 IPCC Methodology Report on Inventories, signaling a potential shift .

The Paris Agreement frames its temperature goals around broader emissions reductions but does not explicitly incorporate indirect greenhouse gases as a distinct mitigation category. Its focus remains on CO₂ and the direct Kyoto gases .

Researchers argue this oversight stems from historical complexities: the warming effects of indirect gases depend on nonlinear atmospheric chemistry and regional emission patterns, making them harder to quantify and regulate than well-mixed gases like CO₂ . Yet as scientific understanding has matured, the policy gap has become increasingly untenable.

Rapid Benefits of Reducing Indirect Greenhouse Gases

Because tropospheric ozone has an atmospheric lifetime of only a few weeks—compared to centuries for CO₂—reducing its precursors would produce a rapid decline in radiative forcing. The Science study emphasizes that cutting CO, NOₓ, and NMVOC emissions could reduce near-term warming faster than CO₂ reductions alone, while simultaneously improving air quality and public health .

Tropospheric ozone is a respiratory irritant linked to asthma, reduced lung function, and premature mortality. Many of the sources of indirect greenhouse gases—vehicle exhaust, industrial solvents, fossil fuel combustion—also emit co-pollutants that degrade local air quality. Mitigation measures would therefore provide dual climate and health benefits.

Integrating indirect greenhouse gases into climate frameworks represents a pragmatic, high-impact opportunity. As the Science study authors contend, fixing this blind spot could be one of the most effective near-term levers for slowing global warming and protecting public health—without requiring new technologies, only updated accounting and policy attention .