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Climate Change

A Matter of Time: Three Ways to Clarify Emissions Data

Typical emissions metrics are akin to putting a sprinter and marathoner in the same race – but clearer alternatives can help us cut climate impacts now and later.

Who is the world’s fastest runner? It depends on timing — and whether we’re deciding based on speed, stamina, or something in between.

Climate pollutants have a similar story (as described by the Institute for Governance and Sustainable Development, or IGSD). Each pollutant that warms our planet has a different potency and lifetime in the atmosphere — not unlike the pace and endurance of different runners.

Yet for most climate emissions metrics, it’s like putting a sprinter and marathoner in the same race — with an arbitrary distance that could suit some better than others. Efforts to improve these metrics have faced decades of inertia, but science-backed alternatives could accelerate action across the pollutants.

The CO2e Metric: Issues & Inertia

For simpler decision-making, emissions are often reported in carbon dioxide equivalent (CO2e) — where each pollutant is related to CO2 based on its global warming potential (GWP) factor. If one ton of emissions causes 10 times as much warming as one ton of CO2, that pollutant would have a GWP factor of 10.

However, these factors can vary drastically over time. As we see below, different metrics can present vastly different pictures of methane’s impact.

Picking one time-specific CO2e metric is like asking all runners to compete in a 5k, no matter their strengths. But that’s exactly what global climate authorities have done — choosing 100 years, or GWP100, as the default for decades.

Global Warming Potential (GWP) and “Curve” values from Environmental Defense Fund based on the IPCC Sixth Assessment Report, for biogenic methane. Values for fossil methane are slightly higher (e.g., 83 for GWP₂₀, 30 for GWP₁₀₀, and 135/8 for GWP*).

Efforts toward alternatives are nearly as old — from a 2009 scientist panel to many different research studies. More and more sources are highlighting the underappreciated importance of non-CO2 emissions, from a recent Shayle Kann podcast to a Bloomberg explainer and groups like RMI, IGSD, Environmental Defense Fund, Climate Central, Clean Air Task Force, and the Climate & Clean Air Coalition. Yet the 100-year CO2e values remain ubiquitous in business and country commitments as well as key technology scenarios — discounting or disregarding the opportunity to cut emissions of other pollutants.

Fortunately, there are many ways to clarify this opportunity. Here are three suggestions.

Recommendation 1: Track Separate Climate Pollutants (& Their Temperature Impacts).

Though long-lived CO2 has caused the most warming, other pollutants have contributed nearly half of the total. Many pack the short-term punch of a sprinter — particularly methane (CH4), black carbon (from soot), and some hydrofluorocarbons (HFCs). Even hydrogen has a brief warming effect when the H2 gas is leaked — though “green hydrogen” from renewable energy is much cleaner than fossil fuels.

Curbing climate change demands rapid reductions of all pollutants, both long- and short-lived. While CO2 is often the focus, cutting short-lived pollutants could avoid up to 0.6oC of warming by 2050 — keeping global climate goals within reach, saving millions of annual air pollution deaths, and helping to avoid near-term tipping points like melting ice caps. As a result, leading climate scientists have called for separate tracking of separate pollutants — just as sprinters and distance runners have separate races.

Source: IPCC AR6 WG1 (Table 6.SM.2, Figure 7.7, Table 7.15, Table 7.SM.7); Cooper et al. (2020); Ocko and Hamburg (2022) Get the data  Download PDF

Recommendation 2: Two Timeframes for Inventories

When emissions must be combined, GWP100 is far from the only option. Some decision makers (such as in New York State) have begun to use a 20-year GWP, which is long for some transient pollutants (like black carbon) but more suitable for methane and HFCs. Reporting both values (GWP20 and GWP100) could emphasize both short-term and long-term climate impacts — similar to rating runners by their 800-meter dash time as well as the 5k.

Leading researchers have suggested this, and some sources have taken heed — like a major report on coal mine methane from Global Energy Monitor. Other tools allow users to toggle between GWPs, such as RMI’s OCI+ tool (Oil Climate Index plus Gas) and affiliated Climate TRACE data. While not perfect, this recommendation expands on existing practices and is especially useful for emissions inventories, for countries and corporations alike.

GWP*: Accuracy with an Asterisk

Other approaches aim to avoid picking specific timeframes. A key example is GWP-star (GWP*), which improves on single-GWP temperature models by incorporating all timeframes into a short-term and long-term component (or for runners, a “short distance” and “long distance” rating).

However, GWP* has scientific and ethical issues for emissions inventories, with different values for a ton of emissions depending on past trends. The math is better for narrower cases of emissions reductions, which focus on the emissions change rather than the total inventory. But given the potential for misuse and confusion, we recommend other alternatives that are more broadly applicable.

Recommendation 3: The Full Curve for Context

Sometimes, it’s better to have the full picture. Fortunately, many existing tools can help visualize the climate impact of each pollutant over time — akin to plotting a runner’s rating across every possible distance. This option could support analyses where nuance matters, such as the timing in buildings’ embodied and operating emissions or temporary carbon storage.

Above all, you can’t manage what you don’t measure — and curbing climate change now and later requires dual action for both timescales. Dual action requires dual metrics — helping decision makers see the value in cutting short-lived emissions from oil and gas, landfills, or air conditioners as well as long-lived CO2.

To do so, we need separate emissions data for each climate pollutant — not in pre-defined CO2e as is often the case. Like with runners, it’s time for pollutants to get their separate tracking — and there is no time to waste.

By Will Atkinson, © 2021 Rocky Mountain Institute. Published with permission. Originally posted on RMI.

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Since 1982, RMI (previously Rocky Mountain Institute) has advanced market-based solutions that transform global energy use to create a clean, prosperous and secure future. An independent, nonprofit think-and-do tank, RMI engages with businesses, communities and institutions to accelerate and scale replicable solutions that drive the cost-effective shift from fossil fuels to efficiency and renewables. Please visit for more information.


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