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Integrating indirect greenhouse gases into climate frameworks
About 0.3°C of current warming — roughly 15% — comes from compounds largely ignored by global climate policy. Most of this warming is from “indirect greenhouse gases” that have minimal direct climate effects, but trigger chemical reactions that can warm the planet.
The scientist behind this explainer
Dr. Ocko has spent her career translating cutting-edge climate and atmospheric science into actionable insights for policymakers — making her uniquely positioned to explain why indirect greenhouse gases belong in climate frameworks.
What are indirect greenhouse gases?
Indirect greenhouse gases have minimal direct climate effects, but they trigger chemical reactions in the atmosphere that can increase greenhouse gas concentrations — especially ozone and methane.
Why now?
- As we decarbonize fossil fuels, some indirect greenhouse gases emissions may actually increase — for example, leaks from hydrogen infrastructure. Accounting for these effects is critical to avoiding unintended climate consequences.
- Indirect greenhouse gases increase tropospheric ozone, a deadly air pollutant linked to hundreds of thousands of premature deaths each year. Reducing these emissions would save lives and reduce warming.
Indirect greenhouse gases sit "beyond the basket" — so are often left out of climate action frameworks.
What you need to know about indirect greenhouse gases
Key concepts behind where indirect greenhouse gases come from, how they can warm the planet, and why they remain outside most policies and climate accounting systems.
Where do indirect greenhouse gases come from?
Human activities have considerably increased indirect greenhouse gas emissions beyond natural levels. Natural sources can also be perturbed by land use change, climate change, and rising carbon dioxide levels.
How do indirect greenhouse gases affect the climate?
Forming new greenhouse gases
Indirect greenhouse gases trigger chemical reactions in the atmosphere that produce tropospheric ozone and stratospheric water vapor — potent greenhouse gases that are typically not directly emitted.
Extending methane's lifetime
Many indirect greenhouse gases consume hydroxyl radicals — the main atmospheric sink that breaks down methane. Fewer hydroxyl radicals means methane persists longer in the atmosphere, amplifying its warming effect.
Producing carbon dioxide
As indirect greenhouse gases oxidize in the atmosphere, they produce small additional amounts of carbon dioxide. This effect is considerably smaller than the first two pathways.
How long do indirect greenhouse gases affect the climate?
Indirect greenhouse gases persist in the atmosphere for only hours to a few years. However, ongoing emissions of these pollutants are so high that the warming impact is still currently 0.25°C.
They primarily influence the climate through short-lived greenhouse gases that last months to a decade.
This means that targeted reductions in indirect greenhouse gas emissions could lead to near-immediate climate benefits and help slow the rate of warming.
| Indirect greenhouse gas lifetimes | |
|---|---|
| Carbon monoxide | 1–4 months |
| Non-methane volatile organic compounds | Hours–months |
| Molecular hydrogen | ~2 years |
| Nitrogen oxides | Hours–days |
| Greenhouse gases they influence | |
|---|---|
| Ozone | Hours–weeks |
| Stratospheric water vapor | 1–2 years |
| Methane | ~1 decade |
| Carbon dioxide | Centuries |
Why are indirect greenhouse gases excluded from most climate frameworks?
The Kyoto Protocol established a "greenhouse gas basket" in the 1990s — a set of gases that continue to shape climate policy today. The basket is made up of prominent greenhouse gases emitted from human activities that are driving climate change, including carbon dioxide, methane, nitrous oxide, and fluorinated gases.
But there are more emissions that are warming the planet, most prominently indirect greenhouse gases. They weren't included in the basket largely because of the state of the science around 30 years ago. But since then, we have a much clearer understanding of their important role in climate change.
Continuing to exclude indirect greenhouse gases from climate frameworks like goals, policies, plans, and accounting systems risks missing opportunities to slow and eventually stop warming.
Meet the gases beyond the basket
Click each gas to learn how it forms, where it comes from, and its estimated warming or cooling effect.
Carbon monoxide is mainly produced by incomplete combustion — fossil fuel burning, residential biomass burning, and wildfires. It has a minimal direct greenhouse effect, but it reacts with hydroxyl radicals (OH) in the atmosphere, reducing the oxidative capacity that breaks down methane, thus extending methane’s warming lifetime.
Carbon monoxide also acts as a precursor to tropospheric ozone, a potent short-lived greenhouse gas. Together, carbon monoxide and NMVOCs drive about 60% of tropospheric ozone's warming impact.
NMVOCs are a family of hydrocarbons — benzene, ethylene, propylene, and hundreds of others — released from solvents, paints, fuel combustion, industrial processes, and also vegetation. Like CO, they consume OH (extending methane’s lifetime) and generate tropospheric ozone.
NMVOCs also ultimately oxidize to carbon dioxide, contributing a small additional warming effect. NMVOCs can also affect aerosol formation, leading to warming or cooling climate effects. Many NMVOCs are also toxic air pollutants, making their control doubly beneficial.
Nitrogen oxides lead to both warming and cooling effects. In the presence of carbon monoxide and NMVOCs, they help produce tropospheric ozone — contributing to warming. But nitrogen oxides also generate hydroxyl radicals (OH), which destroy methane — causing cooling. The balance depends on geography, altitude, and atmospheric conditions, and can be either warming or cooling depending on where it is. Globally, nitrogen oxides’ impact is estimated to be a net cooling. Nitrogen oxides can also lead to the formation of aerosols, which is another source of cooling.
Research during COVID-19 lockdowns showed that temporary nitrogen oxide drops reduced hydroxyl radical levels, letting methane accumulate — contributing to the surge in atmospheric methane observed in the early 2020s.
Molecular hydrogen is the “emerging” indirect greenhouse gas. It’s naturally present in the atmosphere, but human activities have been increasing both its emissions and its production in the atmosphere through chemical reactions from other emitted compounds – like methane. While its current impact is estimated to be minor, emissions may grow considerably as hydrogen systems expand as a replacement for fossil fuels. This leads to more opportunities for hydrogen to escape infrastructure, and it is also routinely released through standard operations. Molecular hydrogen indirectly warms the climate in three ways: it extends methane’s lifetime (by consuming hydroxyl radicals), contributes to tropospheric ozone formation, and produces water vapor that in the stratosphere has a significant radiative effect.
How much warming do indirect greenhouse gases cause?
Present-day temperature contributions are taken from data in the IPCC Sixth Assessment Report (WGI Chapter 6 Supplemental Material) and estimated relative to 1750.
Incorporating indirect greenhouse gases into climate frameworks
Three categories of action — spanning policy, accounting, and research — could unlock near-term climate benefits at low cost.
Incorporate indirect greenhouse gases into national and international climate processes — targets, standards, plans, and accounting systems — to uncover mitigation opportunities that current frameworks systematically overlook.
- Include indirect greenhouse gases in NDCs and Biennial Transparency Reports under UNFCCC
- Use forthcoming IPCC Methodology Report on Short-Lived Climate Forcers for guidance
- Break down silos between air-quality and climate-change departments
Integrate indirect greenhouse gases emissions into climate inventories and climate impact assessments — leveraging data already collected by air-quality monitoring systems worldwide.
- Leverage existing air-pollution emissions datasets (many already measure carbon monoxide, nitrogen oxides, NMVOCs)
- Use already-available IPCC emissions metrics from previous assessment reports (provided in the supplementary materials in Ocko et al. Science 2026)
- Conduct multipollutant assessments that account for warming and cooling co-emitted species
Targeted research can provide important information to aid in decision-making.
- Improve simple tools to assess climate impacts of indirect greenhouse gases
- Determine what sectors and regions indirect greenhouse gas mitigation would have the largest benefits for health and climate
- Assess how much mitigation may be achieved from air quality and decarbonization measures, versus what sources need targeted measures
