Overlooked Pollutants Account for 15% of Global Warming: What You Need to Know

Carbon monoxide (CO) and volatile organic compounds (VOCs) not only pollute the air but also accelerate atmospheric chemical reactions, contributing to global warming.

Since the pre-industrial era, roughly 15% of global warming can be attributed to these non-greenhouse gas emissions, mainly CO and VOCs. This impact is twice that of nitrous oxide, which ranks as the third most prevalent greenhouse gas, following carbon dioxide and methane.

Regrettably, many countries overlook these prevalent “indirect greenhouse gases” in their emissions reduction strategies.

“A category of overlooked climate pollutants significantly impacts current warming; integrating them into climate policy could drastically mitigate future warming,” explains Ilyssa Okko, a co-author from Spark Climate Solutions.

Released primarily from fossil fuel usage, CO and VOCs interact with other atmospheric compounds to form harmful ozone. While stratospheric ozone protects against UV rays, ground-level ozone traps heat, further exacerbating warming.

Moreover, indirect greenhouse gases cause additional warming by reacting with highly reactive hydroxyl radicals, which typically clean the atmosphere by removing pollutants like methane. When hydroxyls engage with CO and VOCs, fewer are available for methane decomposition, resulting in significant heat retention—about 80 times more than CO2 over the short term.

Alongside black carbon (soot)—a pollutant not factored into climate strategies—indirect greenhouse gases have contributed to a warming of approximately 0.3°C. Some of this warming is mitigated by aerosols (e.g., sulfur dioxide) that block sunlight, while nitrogen oxides may produce low ozone levels yet are generally considered to have a net cooling effect due to hydroxyl radical production.

Unlike CO2, which lingers in the atmosphere for centuries, and methane, which remains for decades, indirect greenhouse gases degrade within hours or a few years. Thus, reducing these emissions can lead to a rapid decrease in their warming effects.

“If we reach a tipping point in climate change, targeting these pollutants could serve as a crucial strategy for preventing catastrophic impacts,” notes Alex Archibald from Cambridge University.

Carbon monoxide primarily results from incomplete fossil fuel combustion in appliances like gas boilers and old vehicles, while agricultural practices, including grassland and forest burning in regions like the Amazon, further exacerbate emissions. VOCs encompass various hydrocarbons released from fossil fuels, paints, and cleaning products.

In countries such as the UK, stringent air quality regulations reduce indirect greenhouse gases by implementing emission standards for vehicles and industrial processes and limiting VOC levels in paints. However, many nations maintain looser regulations, prioritizing ground-level pollution reduction over comprehensive atmospheric health.

Recently, the U.S. Environmental Protection Agency introduced regulations that scientists criticize as weakening nitrogen oxide emissions controls from gas power plants.

Okko advocates for countries to incorporate indirect greenhouse gases into their UN climate action plans under the Paris Agreement and set specific reduction targets for these pollutants.

Failing to address indirect greenhouse gases might hinder decarbonization efforts or increase their emissions, warns Alastair Lewis from York University.

With hydrogen being the smallest molecule, it often leaks, leading to hydroxyl radical consumption that results in ozone and water vapor formation in the atmosphere.

Projected emissions and leaks of hydrogen may raise global temperatures by an additional 0.1°C by 2100, particularly if countries adopt ambitious plans to replace fossil fuels with hydrogen in industries like steel and fertilizer production. Combustion of hydrogen in aircraft fuels also emits nitrogen oxides and water vapor.

“Using low-carbon fuels without battery integration might not significantly improve carbon accounting, but in terms of air pollution and indirect greenhouse impacts, it can make a considerable difference,” cautions Lewis.