Understanding the Science of Methane Emissions [Making Sense of Methane]
Although carbon dioxide (CO2) makes up the majority of greenhouse gas (GHG) pollutants, non-CO2 GHGs such as methane, nitrous oxide, and fluorinated gases also contribute to climate change. The US Environmental Protection Agency writes that, on a per-ton basis, non-CO2 GHGs have more significant climate change effects than carbon dioxide and many, including methane, the principal component of natural gas, have greater short-term impacts.
The two key factors in determining the effects of a GHG are its warming effect and the length of time that it remains in the atmosphere. Carbon dioxide, the largest of GHG sources, is the least potent of GHGs, but it remains in the atmosphere for thousands of years. Methane, on the other hand, has a stronger warming effect than CO2, but its lifetime in the atmosphere is only about 12 years.
Looking more specifically at methane, or CH4, it is primarily emitted from industry, agriculture, and waste management activities. Globally, these human activities (anthropogenic), account for over 60% of total CH4 emissions. The remaining 40% of CH4 emissions come from natural sources, the largest of which is wetlands. Smaller natural methane emitting sources include termites, oceans, sediments, volcanos, and wildfires.
Given different gases have different warming effects and different lifetimes in the atmosphere, the global warming potential (GWP) is being used as the standard measure to compare gases on a consistent basis. GWP measures the amount of heat trapped in the atmosphere by a particular gas over a given timeframe, expressed as an equivalent amount of CO2.
The science and policy communities have historically looked to the UN Intergovernmental Panel on Climate Change (IPCC) assessment reports as the authoritative basis for GWP values. The IPCC calculates the GWP based on a 100-year and 20-year lifetime to provide alternative bases for analyzing emission impacts. Depending on the lifetime of the individual gas, the 20-year GWP can be higher or lower than the 100-year GWP.
Both of these values are correct, but they reflect a different snapshot of the warming effect of the subject gases.
In its Fifth Assessment Report (finalized in 2014), the IPCC estimated that the GWP of fossil methane – natural gas – is 30 times that of CO2 over a 100-year period. The IPCC also estimated that the GWP of fossil methane is 85 times greater than CO2 over a 20-year period. What this says is that methane traps more heat and has a more powerful short-term impact on climate change than CO2.
According to the IPCC, “the most appropriate metric and time horizon will depend on which aspects of climate change are considered most important to a particular application. No single metric can accurately compare all consequences of different emissions, and all have limitations and uncertainties.”
The IPCC also states that the global temperature change potential (GTP) metric is better suited to target-based policies, while the GWP metric is not directly related to a temperature limit such as the 2° C target. Despite this statement in the IPCC’s current assessment (the Fifth Assessment Report), more work appears to be needed to substantiate and establish GTP as the most appropriate metric to be used.
Given the IPCC is currently in its sixth assessment cycle and is expected to finalize the Sixth Assessment Report in 2022 – in time for the first global stock-take under the Paris Agreement – perhaps we will be hearing more of GTP in the years ahead.
In addition to the GWP and GTP, in 2014 Jessika Trancik, associate professor in the Institute for Data, Systems and Society (IDSS) at the Massachusetts Institute of Technology, and colleagues, proposed an instantaneous climate impact (ICI), which compares gases in an expected radiative forcing stabilization year, and a cumulative climate impact (CCI), which compares gases on their time-integrated radiative forcing up to a stabilisation year.
Published by Nature Publishing Group, Trancik, in her article titled Climate Impacts of Energy Technologies Depend on Emissions Timing, writes “standard practice for evaluating technologies, which uses the GWP to compare the integrated radiative forcing of emitted gases over a fixed time horizon, does not acknowledge the importance of a changing background climate relative to climate change mitigation targets.”
According to Trancick, “the GWP mis-values the impact of CH4-emitting technologies as mid-century approaches,” and proposes a new class of metrics to evaluate technologies based on their time of use.
The increased interest in methane emissions from government and the natural gas industry has resulted in numerous studies. Many of these studies are very technical, rely on significant assumptions or embedded uncertainties that are often contradictory and, alarmingly, are making their way into the hands of policymakers, advocacy groups, and media voices.
Methane measurements vary from study to study and include the following approaches: “Bottom Up” (direct on-site measurement); “Top Down” (ambient air measurement); and Life-Cycle Analysis.
The methodologies used for calculating emissions are diverse and include activity emission factors, engineering estimates and direct measurements.
Going beyond onshore natural gas extraction methane measurements, even more data is required globally for offshore extraction, coal bed methane extraction, liquids unloading, well completions with reduced emission completions (RECs), transmission and distribution pipelines, and methane emission measurement from all LNG stages.