‘Forgotten’ climate pollutants drive 15% of global warming

indirect greenhouse – Carbon monoxide and volatile organic compounds—released from everyday combustion and land burning—are blamed for about 15% of warming since the pre-industrial era. Yet many countries don’t include them in climate targets, even as scientists warn that emissions

The next time the air smells like smoke from cleared fields, the climate math won’t stop at carbon dioxide.

Carbon monoxide and volatile organic compounds don’t just poison the air we breathe. They also push chemical reactions in the atmosphere that trap more heat. Together. they are among the emissions blamed for roughly 15 per cent of the global warming that has occurred since the pre-industrial era—about double the contribution of nitrous oxide. the third-most-common greenhouse gas after carbon dioxide and methane.

The problem is that most climate plans are built around greenhouse gases we already know how to count. But Ilissa Ocko. of Spark Climate Solutions. a non-profit organisation based in California. says there’s a set of “forgotten climate pollutants” that are strongly contributing to today’s warming—and that could slow the pace of warming in the future if governments start including them in their climate policies.

Her warning lands with special urgency because the chemistry moves fast. Carbon monoxide and VOCs react with other compounds to form ozone in the lower atmosphere. Ozone up in the stratosphere helps by filtering harmful ultraviolet rays. Lower down, the ozone traps heat that would otherwise radiate to space.

There’s another pathway too: indirect greenhouse gases warm the planet by reacting with hydroxyl radicals. a highly reactive “detergent” that scrubs the atmosphere clean of a wide variety of pollutants. including methane. When hydroxyl radicals get tied up breaking down carbon monoxide and VOCs. less is available to remove methane—an effect that matters in the near term. when methane traps around 80 times more heat than CO2.

Taken together with black carbon or soot—a pollutant not included in climate plans and national emissions data—these indirect greenhouse gases have caused 0.3°C of warming. A fraction of that warming has been compensated for by sun-blocking aerosols like sulphur dioxide, and also by nitrogen oxides. Those nitrogen oxides can warm Earth in some places by creating low-level ozone. but they are thought to have a net cooling effect overall because they generate hydroxyl radicals.

The timeline is part of why researchers keep calling this the “low-hanging fruit.” Carbon dioxide can stay in the atmosphere for centuries and methane survives for decades. Indirect greenhouse gases break down within hours, or at most a few years. That means the warming effect would quickly vanish if emissions are reduced.

“If we are heading for things like a [climate] tipping point or something like that, then this is the low-hanging fruit to prevent catastrophic change,” says Alex Archibald at the University of Cambridge.

Carbon monoxide comes from incomplete combustion of fossil fuels. largely in appliances like gas boilers and stoves. as well as in older vehicles. Another source is the burning of grasslands and forests for agriculture in places like the Amazon. VOCs cover a variety of hydrocarbons that evaporate from fossil fuels or from paint and cleaning solvents.

Some regulation has worked. In countries like the UK. air pollution rules have reduced indirect greenhouse gases by adopting emissions standards for vehicles. appliances and industry. and by limiting the VOC content in paints and varnishes. But many countries have looser rules, and they focus on reducing exposure at ground level rather than across the atmosphere.

The pressure isn’t only coming from climate targets—it’s also coming from policy choices that can move in the wrong direction. In January, the US Environmental Protection Agency issued a regulation that scientists say will weaken controls on emissions of nitrogen oxides by gas power plants.

There’s a second tension underneath the first. Alastair Lewis at the University of York, UK, argues that decarbonisation efforts could perpetuate or even increase some indirect greenhouse gas emissions.

Ocko says governments should start mentioning indirect greenhouse gases in the action plans they submit to the United Nations climate body under the Paris Agreement. and eventually set targets to reduce them. Lewis’s concern points to why those targets matter: hydrogen. often presented as a clean replacement in some industrial processes. can create new indirect pollution problems.

Hydrogen often leaks and is sometimes vented by manufacturers into the atmosphere. where it consumes hydroxyl radicals and forms ozone and water vapour. If countries carry out their most expansive plans to replace fossil fuels with hydrogen in industrial processes like steel-making and fertiliser manufacture. the venting and leakage of this gas could heat the globe by an additional 0.1°C by 2100. for instance. Burning hydrogen—or synthetic aviation fuels—in aircraft also produces nitrogen oxides and water vapour.

Lewis draws a sharp line between climate accounting and air-pollution reality. “If you burn a low-carbon fuel rather than use a battery. it may well be – from your carbon-accounting perspective – there’s no difference. but from an air pollution and indirect [greenhouse gas] point of view. it may be that there’s a big difference.”.

carbon monoxide volatile organic compounds VOCs indirect greenhouse gases ozone hydroxyl radicals methane black carbon black soot Paris Agreement climate policy hydrogen leakage nitrogen oxides