A team of scientists has identified a significant loophole in international climate policies in a new scientific analysis released today in the scientific journal Science— a significant portion of global warming is associated with pollutants that are not included in current international climate accounting systems.

In addition to the well-known greenhouse gases, such as carbon dioxide (CO₂) and methane, other pollutants, including carbon monoxide, nitrogen oxides, and volatile organic compounds (VOCs), contribute indirectly to warming by triggering atmospheric reactions that increase greenhouse gas concentrations of greenhouse gases, particularly tropospheric ozone, a potent greenhouse gas and air pollutant.

The scientists who conducted the study say these neglected pollutants account for about 15 percent of the warming that has taken place this century (about 0.3 °C) but are not governed by most of the climate goals set up nearly three decades ago, either nationally or internationally.

Perhaps one of the most surprising discoveries is the part molecular hydrogen – a clean energy carrier, much touted – plays in this. Hydrogen burns to produce water, but it can leak from infrastructure and indirectly contribute to warming through atmospheric chemical reactions. These effects add to the warming caused by methane, which is about 25 times more potent than CO₂ over a 100-year period.

In fact, the current “basket” of greenhouse gases used in the agreement, which was adopted in earlier climate treaties, such as the Kyoto Protocol and continued in the Paris Agreement, hasn't kept up with the latest climate research, the scientists report, creating a policy vacuum that could fail to adequately address “warming.

Many of these are already observed as pollutants in urban-air quality monitoring networks around the world and could thus potentially be integrated into these. It has been suggested that existing networks could be used for climate-relevant measurements, as many of these substances are already monitored as air quality pollutants in other cities globally. Due to these pollutants' dual climate and public health/air quality impacts, policies that seek to reduce these emissions represent both a public health and climate opportunity.