Overlooked 'Indirect' Greenhouse Gases Drive 15% of Global Warming, Major Study Finds

A new study published in the journal Science reveals that a group of "indirect" greenhouse gases is responsible for roughly 15 percent of current global warming—about 0.3 degrees Celsius. Yet, these pollutants remain almost entirely unregulated by international climate treaties. The research, led by scientists from Spark Climate Solutions and the Environmental Defense Fund, challenges the 30-year-old regulatory framework established by the United Nations. By quantifying the massive impact of these overlooked emissions, the authors argue that the global community is fighting climate change with an incomplete playbook.[1][2]

Unlike carbon dioxide or methane, these indirect pollutants—which include carbon monoxide, non-methane volatile organic compounds (VOCs), nitrogen oxides, and molecular hydrogen—do not trap heat directly. Instead, they act as atmospheric catalysts. Once released from fossil fuel combustion, industrial solvents, or agricultural waste, they trigger complex chemical reactions in the atmosphere. These reactions prolong the lifespan of existing methane and generate tropospheric ozone, a highly potent heat-trapping gas that sits near the Earth's surface.

Collectively, these indirect gases rank as the third-largest contributor to human-caused warming, outpacing well-known pollutants like nitrous oxide, hydrofluorocarbons, and black carbon. Carbon monoxide and volatile organic compounds alone are responsible for approximately 0.25 degrees Celsius of warming. Despite this outsized impact, the gases fall outside the United Nations Framework Convention on Climate Change (UNFCCC) and the Paris Agreement. Consequently, most countries do not include them in their national emissions reduction targets or official climate pledges.[1][3]

The exclusion of these gases traces back to the Kyoto Protocol, drafted in the 1990s. At the time, the scientific understanding of atmospheric chemistry was not advanced enough to accurately quantify the warming impact of indirect pollutants. Policymakers established a specific "basket" of direct greenhouse gases that continues to dictate climate policy today. Lead author Ilissa Ocko notes that while the science has evolved dramatically over the past three decades, the regulatory frameworks have remained stubbornly static, leaving a massive blind spot in global mitigation efforts.[2]

The research also highlights a looming, paradoxical threat in the green energy transition: molecular hydrogen. As industries and governments rush to adopt hydrogen as a clean alternative to fossil fuels, scientists warn that the infrastructure is highly prone to leaks. While burning hydrogen does not emit carbon dioxide, leaked molecular hydrogen reacts in the atmosphere to deplete hydroxyl radicals—the naturally occurring "detergent" molecules that break down methane.[4]

If hydrogen energy systems expand globally without stringent leak-monitoring protocols, the transition could inadvertently accelerate near-term warming. Energy transition planners are now faced with the reality that replacing one emission source with another requires a holistic understanding of atmospheric chemistry, rather than simply measuring what comes out of a tailpipe or smokestack.[4]

Beyond the climate implications, there is an urgent public health imperative to rein in these emissions. Many indirect greenhouse gases are primary precursors to tropospheric ozone, a toxic air pollutant. Ground-level ozone is linked to hundreds of thousands of premature deaths annually, primarily from respiratory and cardiovascular diseases. Because these gases have relatively short atmospheric lifespans—ranging from a few hours to a few years—targeted efforts to reduce them would yield near-immediate benefits for both global temperatures and local air quality.[3]

However, translating this science into policy will require navigating complex atmospheric trade-offs. Not all indirect emissions strictly warm the planet; certain nitrogen oxides, for example, can have localized cooling effects by reflecting sunlight or altering cloud formation. Furthermore, research during the COVID-19 lockdowns demonstrated that sudden drops in nitrogen oxide emissions actually allowed atmospheric methane to surge, illustrating the delicate balance of these chemical interactions.

As the United Nations prepares for the COP31 climate summit in Turkiye later this year, pressure is mounting on delegates to formally recognize the nuance of these "overlooked pollutants." Former U.S. Deputy Special Envoy for Climate Rick Duke, a co-author of the study, emphasized that minimizing midcentury overheating requires addressing all sources of warming. Integrating indirect greenhouse gases into the next generation of binding climate commitments may be the most crucial step toward keeping the 1.5-degree threshold within reach.[1][2]