Feeling the Heat: Global Warming Potentials and 20- vs. 100-year Time Horizons

Greenhouse gases (GHGs) such as carbon dioxide and methane have different lifespans in the Earth’s atmosphere, ranging from a century to a millennium with the former to about 12 years for the latter. Consequently, the potency of their impact on climate change (i.e., their global warming potential) will vary, not just based on their heat-trapping qualities, but also depending on what time horizon you consider.

Comparing GHGs by their global warming potential

Global warming potential (GWP) is a metric developed to make it easier to compare the climate change effects of different GHGs. Carbon dioxide — the most abundant GHG — is the reference gas, so its GWP is always 1 on all time scales. Other gases such as methane and nitrous oxide each have their own GWPs relative to carbon dioxide.

For example, nitrous oxide — another long-lived GHG, but one present in far smaller amounts than carbon dioxide — has a GWP of 273 on a 100-year time frame, meaning that its effect on Earth’s climate is 273x greater than carbon dioxide over that period. By contrast, methane’s 100-year GWP is about 30. However, it is a shorter-lived GHG. If we instead look at its 20-year GWP, methane roughly triples to about 80.

Thus, accurately comparing the climate impacts of different GHGs requires looking at their respective GWPs on consistent time horizons. This approach is also the basis for reporting all gases in a single, combined metric of carbon dioxide equivalent (i.e., CO2e or CO2eq).

Based on the latest GWPs from the IPCC’s AR6 report, CO2 has a GWP of 1 across all time. Shorter-lived GHGs such as CH4 have stronger GWPs on shorter time scales, but taper significantly over time. Longer-lived but less-abundant GHGs such as N2O have very strong GWPs that make them important for climate action despite being present in much smaller quantities in the atmosphere.

The importance of 20- vs. 100-year GWPs

GWPs for major GHGs are most often listed for 20-, 100-, and 500-year time frames. The UN IPCC (among many other organizations) uses 100-year GWPs as the standard. The 100-year time horizon does a good job balancing the combined impacts of short- and long-lived GHGs, and is on a time scale for meaningful climate action.

In recent years, though, 20-year GWPs have gained increasing attention. The 20-year perspective gives stronger weighting to potent but shorter-lived GHGs such as methane whose impacts otherwise get ‘diluted’ in 100-year GWPs. As the climate crisis worsens and accelerates, this 20-year view helps countries, corporations, and other emitters prioritize near-term, high-impact opportunities to reduce GHG emissions. By the same token, however, a 20-year GWP that focuses attention on shorter-lived GHGs such as methane could inadvertently cause emissions reductions efforts to de-prioritize longer-lived, more-abundant gases such as carbon dioxide.

Although gases such as methane and nitrous oxide make up just 16% and 6% of global GHG emissions, respectively, their higher GWPs make them important focuses for climate action. Although carbon dioxide’s GWP is much lower, its sheer abundance— 76% of global GHG emissions across sectors — and long-lived nature make it crucial for emissions reductions.

Evolving metrics — looking beyond pure GWP

Given the important of balancing short-term and long-term climate impacts of different gases, there’s been talk of dual 20- and 100-year reporting. Scientists have also been looking at a new era of climate change metrics for GHGs.

One such metric is the global temperature-change potential (GTP). Whereas GWP measures how much energy the atmosphere absorbs, GTP calculates how much warmer the Earth will get as a result of that absorbed energy (i.e., how much mean global temperature will go up). As with calculating GWPs for specific gases vs. a combined metric of CO2e, so too can GTP be calculated for GHGs individually as well as for them in aggregate via a combined GTP number (CGTP).

Another new metric under consideration is GWP*. It is an attempt to more-accurately reflect the climate influence of a time-series of short-lived GHGs such as methane, the incremental impact of increases or decreases in their emissions, and the tradeoffs between calculating their potency across different time scales.

How Climate TRACE reports GHGs

Since launching our emissions inventory dashboard in September 2021, Climate TRACE has reported CO2e-100 numbers for countries, sectors, and subsectors using GWPs from the IPCC’s Fifth Assessment report. As of July 2022, we’ve updated our CO2e-100 numbers with GWPs from the IPCC’s newer Sixth Assessment report.

Plus, we’ve added more detail and options to the beta version of our updated emissions inventory user interface (UI): you can now toggle between 20- and 100-year time horizons, and you can view combined CO2e or you can examine specific gases such as carbon dioxide, methane, and nitrous oxide. These UI updates will be reflected on the main Climate TRACE emissions inventory UI later this year.

A password-protected 2022 beta of an updated Climate TRACE dashboard UI includes the ability to toggle between 20- and 100-year GWPs, as well as choose between gases CO2e, CO2, CH4, and N2O. These features are expected to be available in the main public UI later this year.

We invite you to explore the data for yourself.

Ann Marie Gardner is an editor, journalist, and Climate TRACE coalition member.