The Global Warming Potential of Methane
While atmospheric methane (CH4) traps more thermal energy than carbon dioxide (CO2), its Global Warming Potential (GWP) is variously reported as either ~30 times or ~80 times that of CO2. Which of those values is correct?
They both are.
The complication is that methane persists in the atmosphere for only around 10 years before being broken-down, whereas carbon dioxide persists for centuries (up to ~1,000 years). To compare the GWP of these gases, a time-scale needs to be selected. If the average GWP is calculated over a 100 year time-scale, then the figure of ~30 is reached, while using a 20-year time-scale results in the ~80 figure. Other numbers can be reached by selecting other time-scales, and the selection is mostly arbitrary. Corrections to the GWP calculation have been proposed to take this into account, however it remains to be seen whether they are adopted. The Intergovernmental Panel on Climate Change (IPCC) uses a value of 28 for their methane modelling (i.e. over the longer-term).
There are many natural and human-induced sources of methane in the atmosphere, however for various reasons cows get the brunt of the blame. In reality, methane from cows is not necessarily contributing to climate change, especially if the cows are raised regeneratively. This is because methane from cows can act as a flow gas, rather than as a stock gas.
Flow vs. Stock gases
Flow gases are usually short-lived gases that are part of a cycle (or flow) that maintains their concentration in the atmosphere at a steady level. Methane from the biogenic carbon cycle (that cows are a part of) is primarily a flow gas. Its breakdown product is CO2, however this simply replaces the CO2 that the plant absorbed during photosynthesis and the cow ate, leaving the cycle in equilibrium as long as cow numbers remain constant. Globally, cow numbers have been constant over the last decade, at ~1 billion. During that same period, methane concentration in the atmosphere increased substantially (there are signs the increase may be accelerating). Therefore this increase in atmospheric methane most likely came from stock methane.
Stock gases are those that accumulate (stockpile) in the atmosphere. Carbon dioxide from fossil fuel use is the most abundant stock gas in the atmosphere. Its accumulation and long life steadily increases atmospheric heating. Carbon dioxide emitted today adds to emissions from previous centuries and will remain in the atmosphere for centuries to come.
Methane can also behave as a stock gas, albeit over a shorter time-scale than carbon dioxide. This shorter time-scale means we could make progress against climate change over the shorter-term by targeting stock methane, buying time as we wrestle with reducing carbon dioxide emissions.
Primary sources of stock methane include fossil fuel production, gas pipeline and related leaks and organic landfill. As global heating proceeds, there will be additional methane release from expanding tropical wetlands and melting permafrost that will largely be out of our control.
While it may seem there could still be benefit in reducing flow methane from cows, it is likely to be a futile exercise in comparison, and could lead to unexpected consequences. For example, avoiding nutrient-dense red meat could have adverse effects on public health depending on what it is replaced with. The healthcare system is a significant source of greenhouse gases. Reducing cow numbers in western countries is also likely to be futile. The country with the largest number of cows (by far) is India. The second is Brazil. Between them they account for nearly half the world’s cows.
Meanwhile, confounding a flow gas with a stock gas distracts from real climate change action.
