The forgotten 15% of global warming
Leading scientists call for action on an overlooked group of climate pollutants.
You probably know that carbon dioxide is a greenhouse gas that is warming the atmosphere and changing the climate. You may even know about methane, which is responsible for about 30% of today’s warming. These climate pollutants are two of the seven greenhouse gases that nearly 200 countries agreed to regulate under the landmark 1997 Kyoto Protocol.
But did you know that 15% of global warming is caused by other gases that aren’t addressed under any climate treaty? A group of scientists argue that it’s time to pay attention to these overlooked climate pollutants.
“Carbon monoxide, non-methane volatile organic compounds (NMVOCs), nitrogen oxides and hydrogen are considered indirect greenhouse gases, because they do not absorb heat themselves, but they cause reactions in the atmosphere that affect the climate,” says Tianyi Sun, a senior climate scientist at Environmental Defense Fund, a global nonprofit.
In a recent paper in the journal Science, Sun, EDF Chief Scientist Steven Hamburg and eight other climate experts argue that it’s time to include indirect greenhouse gases in the international agreements that shape what types of climate pollution countries measure, target and reduce.
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What is an indirect greenhouse gas?
Indirect greenhouse gases do not warm the planet directly. Instead, when they are exposed to sunlight, they spark chemical reactions in the atmosphere that extend the lifespan of methane and produce more carbon dioxide, making warming worse.
These gases mostly come from burning fossil fuels and industrial processes like manufacturing chemicals.
“Thirty years ago, our understanding of indirect greenhouse gases was in its early stages,” explains Sun. “Now, our understanding has matured, but the policy hasn’t caught up.”
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Science can spur climate action
This isn’t the first time EDF scientists have brought an overlooked source of climate pollution into the spotlight. Scientists had known about the warming power of methane for decades, but the world focused on the big-ticket item — carbon dioxide pollution. Hamburg wondered if methane emissions might be a bigger problem than people realized, so he initiated a scientific campaign to find out.
In 2011, Hamburg and EDF spearheaded a major scientific research effort, in collaboration with more than 140 research and industry experts from 40 institutions and 50 companies, to better understand methane pollution. That effort resulted in in dozens of publications, an influential body of work that catalyzed policymakers around the world take action to drive down methane pollution. Methane is more than 80 times as powerful as carbon dioxide in the first 20 years after it’s released. Reducing methane pollution is the fastest way to slow the rate of global warming.
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The Global Methane Pledge, an international agreement signed by 159 countries plus the European Commission to cut methane pollution, was launched in 2021.
Addressing indirect climate pollutants is already gaining momentum, say Sun and Hamburg. The Intergovernmental Panel on Climate Change (IPCC), the world’s leading scientific assessment effort on climate change, is developing technical guidance for countries on how to include a wider range of climate pollutants in their national inventories in its next report, scheduled to be published in 2027.
“Accounting for these pollutants would be the first step toward managing them in climate policy,” Sun says.
One advantage is that some of these indirect climate pollutants are already regulated as air pollutants, because they create ground-level ozone, which creates smog, negatively impacts human health and plant productivity, and also traps heat. Adding indirect greenhouse gases to climate accounting can help address overlooked climate pollution and improve air quality too.
Even though monitoring and enforcement of these pollutants isn’t uniform and needs to be scaled up across the world to deliver maximum results, existing management programs give policymakers a head start, says Sun.
“The push for policy to keep up with science is strong,” she says.
