New paper shows overlooked pollutants are responsible for about 15% of current global warming

June 11, 2026.  In a new paper published today in Science, leading scientists and climate policy experts show that approximately 15% of current global warming (about 0.3°C) from human emissions stems from pollutants that fall outside most existing climate policy frameworks. Most of these overlooked pollutants are called "indirect greenhouse gases," and include carbon monoxide, non-methane volatile organic compounds, nitrogen oxides, and molecular hydrogen. 

The authors of the paper include several prominent figures in climate science and policy, including Steven Hamburg, Chief Scientist at the Environmental Defense Fund; Rick Duke, former U.S. Deputy Special Envoy for Climate; Phil Duffy, Chief Scientist at Spark Climate Solutions and former White House climate science advisor; Ilissa Ocko, Senior Climate Scientist at Spark Climate Solutions and former Senior Advisor to the U.S. Special Presidential Envoy For Climate, as well as other key experts from leading institutions including Clean Air Fund and Three Cairns Group.

Unlike traditional greenhouse gases such as carbon dioxide, the warming impact of indirect greenhouse gases does not come from trapping heat directly. Instead, these gases trigger chemical reactions in the atmosphere that can increase the abundance of methane, ozone, and other greenhouse gases, producing warming as a result. Scientists have studied these effects for decades, but the gases have never been incorporated into key climate policy frameworks, such as the Paris Agreement or the underlying UN Framework Convention on Climate Change. This means most countries are not accounting for the impacts of these gases in their climate targets and policies or developing strategies to reduce their warming impacts despite their significance. 

The exclusion of indirect greenhouse gases traces back to the Kyoto Protocol, drafted nearly 30 years ago, which established the "greenhouse gas basket" that continues to shape climate policy today. At the time, the climate impacts of indirect greenhouse gases were not well understood, but scientific understanding has advanced considerably since then.

“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,” said the paper’s lead author Ilissa Ocko, a Senior Climate Scientist at Spark Climate Solutions and former Senior Advisor to the U.S. Special Presidential Envoy For Climate.

“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,” said former U.S. Deputy Special Envoy for Climate Rick Duke.

“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,” said Steven Hamburg, Chief Scientist, Environmental Defense Fund. 

Indirect greenhouse gases come from a variety of combustion and industrial sources, many of which remain overlooked with current climate action plans, such as solvents, small-scale biomass and coal combustion, open biomass and waste burning, and biogenic emissions from land use and agriculture. While many indirect greenhouse gases are also air pollutants, air quality measures alone are often insufficient for addressing their climate impacts.

“We already know that many indirect greenhouse gases are themselves harmful air pollutants, and contribute to the formation of toxic ground-level ozone,” said Tom Grylls of Clean Air Fund, another of the paper’s co-authors. “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.”

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About Spark Climate Solutions

Spark Climate Solutions is a science-driven, philanthropically funded non-profit that accelerates progress on unsolved climate challenges. We focus on blind spots that pose big climate risks or offer big climate opportunities—like major sources of unabated emissions—and work to speed up the development of the fields needed to address them.