Global Warming Potential
Global Warming Potential (GWP) is an index that measures the relative impact of different greenhouse gases (GHGs) on global warming, compared to carbon dioxide (CO₂), the reference gas. It assesses the time-integrated effect of a specific GHG's emissions based on its ability to absorb infrared radiation, its atmospheric lifetime, and its concentration. The GWP is significant for international climate agreements, such as the United Nations Framework Convention on Climate Change (UNFCCC), where it helps quantify the greenhouse-enhancing potential of various gases and evaluate the effectiveness of GHG emission reductions.
One important aspect of GWP is the choice of time horizon for its calculations, commonly set at 100 years for climate policy assessments, but also varying between short-term and long-term analyses. For example, methane has significantly different GWPs over different time horizons, indicating that strategies for reducing short-lived gases may be undervalued in traditional GWP applications. While GWP serves as a useful tool, it has limitations, including the assumption of linear relationships in emissions effects and the potential misrepresentation of short-lived gases’ impacts, leading some scientists to advocate for alternative metrics, such as the modified GWP*. Understanding GWP can provide insights into the complexities of climate change mitigation and the relative urgency of addressing different GHGs.
Global Warming Potential
Definition
Global warming potential (GWP) is an index based upon the radiative (infrared-absorbing) properties of well-mixed greenhouse gases (GHGs). It can be used to estimate the relative potential future impacts of GHG emissions on the global climate. Specifically, the GWP of a given GHG is the time-integrated global mean from the instantaneous release of 1 kilogram of that GHG, relative to that of a reference GHG, usually carbon dioxide (CO2). The GWP is a function of a gas’s lifetime, concentration, and effectiveness at absorbing thermal infrared radiation.
Significance for Climate Change
GWP is the index used by parties to the United Nations Framework Convention on Climate Change (UNFCCC) to quantify the greenhouse-enhancing potential of trace atmospheric gases. The GWP is used to evaluate the effects of interference in the climate system and of GHG emission reductions. GWP is a purely physical index and does not take into consideration the costs and benefits of climate policy initiatives, cost discounting, or the location of emissions or regional climates, among other factors. Thus, GWP does not measure economic, cultural, or regional factors useful in determining climate- and GHG-related policies.
Other limitations of the GWP index exist. The choice of time horizon for integration ranges from short-term changes (such as responses of cloud cover to surface temperature changes) to long-term effects (such as changes in ocean level). For purposes of the first commitment period under the Kyoto Protocol, the parties to the UNFCCC agreed to a one-hundred-year time horizon for integration of GWPs as a mid-term balance between long-term (five-hundred-year) and short-term (twenty-year) climate effects. One-hundred-year GWPs for select GHGs were listed in Climate Change, 2007, a report prepared by the Intergovernmental Panel on Climate Change (IPCC). 2007 estimates of GWP were still used by some agencies and in some studies into the 2020s—for example, the United States Environmental Protection Agency (EPA) used these values in a 2020 inventory of US greenhouse gas emissions—although new research allowed the IPCC to calculate new values during this period. GWP metrics were also considered during the development of the 2015 Paris Climate Agreement, a landmark international treaty on addressing climate change.
The mid-term time horizon favors reductions in GHGs whose GWP is greatest over one-hundred-year periods, while diminishing the importance of reducing short-lived GHG emissions in the near future and long-lived GHGs over the long term. For example, GWPs for methane over twenty-year, one-hundred-year, and five-hundred-year time horizons are 72, 25, and 7, respectively. The value of reducing methane under analyses based on the short-term time horizon is nearly three times that of reducing it according to analyses employing the mid-term time horizon. Short-term analyses, however, have motivated the US to create the Methane to Markets initiative, which seeks to capture or reduce methane emissions to address near-term climate change cost-effectively.
Uncertainties in the GWP arise from assuming a linear radiative forcing function under a small emission pulse (1 kilogram). That is, GWP calculations assume that 100 kilograms of a GHG will have a GWP one hundred times that of 1 kilogram of that GHG. This is not necessarily the case, as emissions of a GHG may build on or interfere with each other, cause positive or negative feedback loops, or otherwise increase or decrease their effects on climate in a nonlinear manner.
In the decades after the Kyoto Protocols, some climate scientists questioned the usefulness of using GWP. For example, studies have shown that GWP tends to misrepresent the effects of short-lived gases, such as methane, that do not persist in the atmosphere as long as CO2 and have less of a long-term effect on global warming due to their relatively fast decomposition. To help account for these differences, scientists have proposed the use of a modified index known as GWP*.
Bibliography
Allen, Myles, Michelle Cain, John Lynch, and Raymond Pierrehumbert. “Demonstrating GWP*: A Means of Reporting Warming–Equivalent Emissions that Captures the Contrasting Impacts of Short– and Long–Lived Climate Pollutants.” Environmental Research Letters, Apr. 2020, vol. 15, no. 4, www.ncbi.nlm.nih.gov/pmc/articles/PMC7212016/. Accessed 9 Dec. 2024.
Cain, Michelle. “Guest Post: A New Way to Assess ‘Global Warming Potential’ of Short–Lived Pollutants.” CarbonBrief, 7 June 2018, www.carbonbrief.org/guest-post-a-new-way-to-assess-global-warming-potential-of-short-lived-pollutants/. Accessed 9 Dec. 2024.
“Global Warming Potentials (IPCC Second Assessment Report).” United Nations Framework Convention on Climate Change, 2022, unfccc.int/process/transparency-and-reporting/greenhouse-gas-data/greenhouse-gas-data-unfccc/global-warming-potentials. Accessed 9 Dec. 2024.
“Understanding Global Warming Potentials.” United States Environmental Protection Agency, 8 Aug. 2024, www.epa.gov/ghgemissions/understanding-global-warming-potentials. Accessed 9 Dec. 2024.