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How best to measure the climate impact of methane

Deciding exactly how powerful methane is as a greenhouse gas relative to carbon dioxide is a complicated business

Correct data is vital to inform the energy transition and climate action. This includes using the right metrics to measure the climate impact of all greenhouse gases especially those like methane whose impact compared to carbon dioxide changes over time. Environmental groups warn actions based on incorrect data could do more harm than good

LATEST SCIENCE
New research published in science journal Nature indicates that estimates of methane emissions from oil and gas production are still being underestimated and should be revised upwards by as much as 40%

MEASURING METHANE
The main metric used to decide the climate impact of a greenhouse gas is known as its global warming potential (GWP) compared to carbon dioxide, but measurements of GWP are being made over different timeframes so can yield apparently conflicting results. Environmental groups want emissions reporting systems to be updated to reflect this

KEY QUOTE
The reality is that climate change is not simple and by oversimplifying to the point of expressing only long-term impacts does a huge disservice to climate action.

We could be making climate change worse in the near-term without realising it or we could miss crucial opportunities to slow the rate of warming Previous estimates of methane emissions from natural sources have been far too high, concludes scientists in an article published in science journal Nature in February 2020, leading them to deduce that emissions from human activities, namely from fossil fuel production, are much more significant than previously believed. Recent inventories have placed methane emissions from natural sources at around 40 million to 60 million tonnes each year. But the study indicates that at the very most, natural sources might contribute about five million tonnes a year and that this figure could be as low as 1.5 million tonnes annually on average. Estimates of methane emissions from oil and gas production should therefore be revised upwards by as much as 40%, suggest the authors. If they are correct, it would mean that more concerted action to reduce methane emissions could do more to combat climate change than previously thought. But deciding exactly how powerful methane is as a greenhouse gas (GHG) relative to CO2 is a complicated business.
The main metric used to assess the climate impact of a GHG is known as its global warming potential (GWP) and is based on three factors compared to CO2: how much of it is emitted, how efficient it is at trapping energy and how long it lasts in the atmosphere. “To compare pollutants on the same scale, we need to incorporate all three factors,” explains Ilissa Ocko, climate scientist with the Environmental Defense Fund, a US NGO. “GWP is the most popular metric due mostly to its simplicity and longevity in the climate community. It compares a non-CO2 pollutant’s warming impact over a designated time period to that from CO2.”

IMPORTANCE OF TIME Inclusion of the time factor is vitally important since GHGs have different lifespans and their GWP changes based on how long they last in the atmosphere. Many have a very high GWP when they are first released, but it diminishes over time if they don’t last very long. Methane is 120 times more potent than carbon dioxide kilogram for kilogram when initially emitted, some 80 times more potent over the first 20 years after it is emitted, with the factor declining to around 25 times more potent over a 100 year timeframe because the CO2 is still there, while the methane has pretty much disappeared. Using an instantaneous GWP for a non-CO2 pollutant means we eliminate the time horizon component,” says Ocko. “This is not a good idea because it does not capture the long-term impacts of CO2 on the climate.” To illustrate her point she gives the example of black carbon. “If you emit the same amount of black carbon and CO2 into the atmosphere, the black carbon will have an instantaneous warming impact a million times greater than CO2 and make it look like black carbon is nearly all of the global warming problem and we could solve climate change by eliminating it,” says Ocko. But black carbon only lasts for a few days in the atmosphere, while CO2 can last for centuries — 20% of what we emit today will still be in the atmosphere in 1000 years. “If we use instantaneous GWP or even a seven, ten or 20 year GWP, we are downplaying the role of CO2 in the long term,” she adds. “If we only use a 100 year GWP, however, we completely overlook the strong near-term impacts of pollutants like methane.”

CONSISTENCY AND COMPREHENSIVENESS For Ocko and other environmental campaigners, the solution would be to always report GWPs for two timeframes — 20 and 100 years — to adequately convey impacts over all timescales. This change would not be too much of a leap for the climate community, policy makers or the general public, she believes. “Reporting a two-valued GWP does not require much of a learning curve and would bring an immense improvement in emissions reporting,” says Ocko. “There are many familiar metrics that have two values,” she adds, citing blood pressure (systolic/diastolic), fuel mileage (city/highway) and temperature (high/low) as examples. “Each pair is incomplete without the other,” says Ocko. “Our objective is to get to a point where GWP is perceived as the same — one time horizon is incomplete without the other.” As a general rule, “consistency and comprehensiveness” should be the guiding principles for explaining the climate impact of all GHG, says Ocko. “Consistency in the reporting structure across all gases, as opposed to selecting the time horizon that fits your desired outcome, so all GHGs are treated the same. Comprehensive in that both near and long-term impacts are articulated and a complete picture provided.” She adds: “The reality is that climate change is not simple and by oversimplifying to the point of expressing only long-term impacts does a huge disservice to climate action. We could be making climate change worse in the near-term without realising it or we could miss crucial opportunities to slow the rate of warming.” The Environmental Defense Fund is working with international organisations, such as the Intergovernmental Panel on Climate Change (IPCC), industry, other NGOS, and scientists on the issue and developing an online tool “so people can see how time horizons plays a role in our understanding of emissions reporting”, says Ocko.

SCIENTIFIC AND SOCIETAL IMPACTS Piers Forster, climate physics professor at the University of Leeds, UK, who has worked on many IPCC reports and is contributing to the organisation’s upcoming Sixth Assessment Report on climate change (AR6), acknowledges the importance of choosing the right emissions metrics, but insists the choice “cannot be decided by science alone” and underlines the importance of “subjective societal choices”. These include: “What impacts do we care most about and how do we quantify these? Do we care more about the temperature impact on ourselves over the next 20 years or the higher sea-level that will impact our grandchildren?” For these reasons, Forster says the IPCC “does not recommend the use of GWP over other metrics” and uses a variety of timeframes for given purposes. “Horizons of up to 500 years were given in previous IPCC reports except in the most recent one, the Fifth Assessment Report (AR5), where we presented GWPs for 20 and 100 years,” he says. “Other metric choices such as the Global Temperature change Potential (GTP), which relates to the temperature change on different time horizons were also given in AR5.” Indeed, AR5 states that: “The choice of emission metric and time horizon depends on type of application and policy context; hence, no single metric is optimal for all policy goals.” Forster insists: “All time frames are relevant for climate change; we need to reduce emissions to avoid impacts now and in the future. A GWP of 100 would be equivalent to around a 30 year GTP and so relevant to affecting temperatures around mid-century, where most policy concentrates, but policies often also consider longer term effects on future generations.” The use of a GWP based on a 100 year time horizon “for most climate policies” is a political choice by the governments within the UNFCCC process, he adds.

TEXT Philippa Nuttall Jones IMAGE Jaymantri