A landmark study published in Science reveals that a class of pollutants largely absent from global climate frameworks is responsible for about 15% of current human-caused warming. Researchers say fixing this blind spot could be a major lever for slowing climate change.
A new study published June 11 in the journal Science is drawing attention to a category of pollutants that have quietly been warming the planet while escaping the reach of nearly every major climate policy on the books. The research finds that so-called “indirect greenhouse gases” — a group that includes carbon monoxide, nitrogen oxides, non-methane volatile organic compounds, and molecular hydrogen — are collectively responsible for roughly 15% of current global warming, or about 0.3 degrees Celsius.
That number may sound modest, but scientists and policy experts behind the paper argue it represents an enormous and largely untapped opportunity to slow warming by addressing emissions that have been systematically excluded from international frameworks like the Paris Agreement and the underlying UN Framework Convention on Climate Change.
What Are Indirect Greenhouse Gases?
Unlike carbon dioxide or methane, indirect greenhouse gases do not trap heat on their own. Instead, they set off chains of chemical reactions in the atmosphere that boost concentrations of methane, ozone and other warming agents. Scientists have understood these mechanisms for decades, yet the gases were never folded into the core accounting systems that countries use to track and reduce their emissions.
The roots of that exclusion go back nearly 30 years. When the Kyoto Protocol was drafted in the late 1990s, it established a “greenhouse gas basket” — a defined set of gases that would be counted in national climate targets. At the time, the warming effects of indirect greenhouse gases were poorly understood, and the omission was baked into every subsequent policy framework that followed.
Third-Largest Driver of Warming — and Largely Ignored
Lead author Ilissa Ocko, a senior climate scientist at Spark Climate Solutions and a former senior advisor to the U.S. Special Presidential Envoy for Climate, framed the stakes bluntly.
“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,” Ocko said in a news release.
Co-author Rick Duke, a former U.S. Deputy Special Envoy for Climate, emphasized that fixing this gap is not optional if the world hopes to limit the worst near-term temperature spikes.
“Measuring and driving down indirect greenhouse gases is essential if we want to fully address climate change, including minimizing midcentury overheating above 1.5°C,” Duke said in the news release.
Steven Hamburg, chief scientist at the Environmental Defense Fund, pointed to molecular hydrogen as a particularly urgent example. As governments and industries ramp up hydrogen as a clean-energy carrier, any leaks from pipelines or infrastructure could quietly undermine climate gains.
“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. Climate impacts of indirect greenhouse gases are already significantly impacting the climate and for some, like hydrogen, we could see their impact grow considerably in the future. Hydrogen is a powerful tool in our efforts to decarbonize, but as a tiny, slippery molecule, hydrogen can easily escape infrastructure and its potential benefits diminish if we ignore the indirect effects,” Hamburg said.
Where These Emissions Come From
Indirect greenhouse gases originate from a wide range of everyday activities and industrial processes — including solvent use, small-scale burning of biomass and coal, open burning of waste, and land-use-related biological emissions. Many of these sources are already inadequately tracked under current national climate action plans.
Why It Matters — and a Path Forward
For students studying environmental science, public policy, engineering or sustainability, this research highlights a structural flaw in the architecture of global climate governance that will need to be addressed in the coming years. It also points to a genuinely actionable solution: many indirect greenhouse gases are already regulated as air pollutants, meaning the monitoring infrastructure to tackle them partially exists.
Co-author Tom Grylls of the Clean Air Fund noted that many of these substances are already recognized as harmful to human health because they contribute to ground-level ozone formation.
“We already know that many indirect greenhouse gases are themselves harmful air pollutants, and contribute to the formation of toxic ground-level ozone,” he said. “That means most countries do not need to start from scratch. There are opportunities to build on existing air quality policies and monitoring systems to reduce these pollutants and their climate effects. Doing so would deliver immediate air quality and public health benefits, while also tackling an often overlooked source of global warming.”
The study calls for integrating indirect greenhouse gases into national climate frameworks — a move that could simultaneously improve public health outcomes, reduce smog, and shave crucial fractions of a degree off projected warming. For a generation of students who will inherit the consequences of today’s policy choices, understanding this gap is a critical first step toward closing it.
Source: Spark Climate Solutions