A greenhouse gas so overlooked it barely appears in climate policy has been warming the planet by 0.3°C all on its own—and almost nobody has noticed.
In a paper published in Science, a team of leading climate scientists and policy experts revealed that indirect greenhouse gases—pollutants like carbon monoxide, nitrogen oxides, and molecular hydrogen—are responsible for 15% of current global warming, yet remain almost entirely absent from major climate frameworks including the Paris Agreement. This blind spot traces directly back to the Kyoto Protocol, drafted nearly 30 years ago, when the warming effects of these gases were poorly understood. Since then, scientific knowledge has expanded dramatically, but international climate policy has not caught up.
Unlike traditional greenhouse gases such as carbon dioxide, which trap heat directly, indirect greenhouse gases trigger chemical reactions in the atmosphere that increase the abundance of methane, ozone, and other warming agents. The list of sources is sprawling: solvents, biomass and coal combustion, waste burning, and emissions from agriculture and land use all contribute. Yet because these gases fall outside the historic "greenhouse gas basket" established by Kyoto, most countries do not measure their impact or develop strategies to reduce it.
Ilissa Ocko, lead author of the study and senior climate scientist at Spark Climate Solutions, underscored the scale of this oversight: "Among all human-caused emissions that warm the climate, indirect greenhouse gases collectively rank as the third-largest contributor to the warming we experience today after carbon dioxide and methane—ahead of nitrous oxide, hydrofluorocarbons and black carbon. This is a significant contributor to warming that has been left out of climate policy discussions for far too long."
The research team includes Steven Hamburg, chief scientist at the Environmental Defense Fund; Rick Duke, former U.S. deputy special envoy for climate; and Phil Duffy, chief scientist at Spark Climate Solutions and former White House climate science adviser, alongside experts from the Clean Air Fund and Three Cairns Group. Together they make a case that is both urgent and practical: addressing these gases is not just scientifically necessary but also politically feasible.
The case for action becomes even stronger when viewed through a public health lens. Many indirect greenhouse gases are also air pollutants that contribute to toxic ground-level ozone formation, meaning countries do not need to start from scratch. Tom Grylls of the Clean Air Fund noted that existing air quality monitoring systems and policies could be leveraged to reduce these emissions. "Doing so would deliver immediate air quality and public health benefits, while also tackling an often overlooked source of global warming," he said.
Some of these gases present particular challenges ahead. Molecular hydrogen, increasingly touted as a clean energy solution, can easily escape infrastructure due to its tiny molecular structure—potentially amplifying its warming impact even as decarbonization efforts expand. This suggests that a transition to hydrogen energy, while necessary, must be paired with rigorous measures to prevent leakage.
For the world aiming to limit warming to 1.5°C above preindustrial levels, the implications are clear. Rick Duke emphasized that "if we are going to effectively and efficiently slow the rate of warming, we need to consider all sources of warming, not just the traditional basket of greenhouse gases." The science has moved on. Now climate policy must follow.