Carbon monoxide and volatile organic compounds (VOCs) not only pollute our air but also accelerate atmospheric reactions, which in turn heat our planet.

Research shows that approximately 15% of the global warming observed since the pre-industrial era is attributed to emissions of non-greenhouse gases, particularly carbon monoxide and VOCs. This figure is double that of nitrous oxide, the third most prevalent greenhouse gas following carbon dioxide (CO2) and methane.

Despite their significance, few nations have integrated these “indirect greenhouse gases” into their emissions reduction strategies.

As noted by Ilyssa Okko of Spark Climate Solutions, California, we must pay more attention to these overlooked climate pollutants. Incorporating them into climate policies could significantly decelerate warming rates in the future.

Carbon monoxide and VOCs are released, in part, from fossil fuel usage, and these substances react with atmospheric compounds to create ozone. While ozone in the upper stratosphere naturally filters harmful ultraviolet (UV) rays, ozone formed at lower altitudes traps heat, exacerbating global warming.

Furthermore, indirect greenhouse gases contribute to warming by interacting with highly reactive hydroxyl radicals. These radicals play a role in removing various pollutants, including methane. As more hydroxyls engage with carbon monoxide and VOCs, fewer are available for methane breakdown, thus trapping significantly more heat than CO2 in the short term.

Alongside black carbon (soot), another pollutant frequently omitted from climate strategies, these indirect greenhouse gases account for approximately 0.3°C of global warming. While some cooling effects are provided by aerosols that block sunlight—such as sulfur dioxide—they still contribute to warming through the formation of low-level ozone and hydroxyl radicals.

Unlike CO2, which lingers in the atmosphere for centuries, and methane, which remains for decades, indirect greenhouse gases have much shorter lifespans, breaking down in mere hours or years. This rapid decay means that reducing these emissions could lead to swift decreases in their warming impact.

If we are approaching a climate tipping point, addressing indirect greenhouse gases presents a significant opportunity to avert catastrophic changes, as highlighted by Alex Archibald of Cambridge University.

The primary source of carbon monoxide is the incomplete combustion of fossil fuels found in appliances, such as gas boilers and stoves, as well as older vehicles. Agricultural practices, such as burning forests and grasslands in regions like the Amazon, also contribute to these emissions. VOCs, on the other hand, are released from various hydrocarbons, including fossil fuels, paints, and cleaning products.

Countries like the UK enact air pollution regulations to mitigate indirect greenhouse gases by enforcing emission standards for vehicles and limiting VOC content in paints. Yet, numerous nations have lax regulations, emphasizing ground-level exposure reduction over broader atmospheric management.

In January, the U.S. Environmental Protection Agency introduced regulations criticized for weakening nitrogen oxide emissions standards from gas power plants.

Okko suggests that countries should begin including indirect greenhouse gases in their action plans submitted to the UN climate agency under the Paris Agreement, ultimately establishing reduction targets for these gases.

Failure to do so may lead to ongoing or even increased emissions of indirect greenhouse gases, warns Alastair Lewis at York University, UK.

Hydrogen, as the smallest molecule, often leaks and can easily be vented into the atmosphere by manufacturers. In this process, it consumes hydroxyl radicals, forming ozone and water vapor.

If unchecked, hydrogen emissions and leaks could increase global temperatures by an estimated 0.1°C by 2100, particularly if countries push forward with plans to replace fossil fuels in industries like steel and fertilizer production. The combustion of hydrogen or synthetic aviation fuel also emits nitrogen oxides and water vapor.

As Lewis articulates, “Burning low-carbon fuels without batteries might not show a difference in carbon accounting, but in terms of air pollution and indirect impacts on greenhouse gas emissions, the variations could be immense.”