Microbes break down organic material that contains carbon. Without much oxygen, this process produces methane. This methane is transferred to our atmosphere through three different processes.
The first source is bubbling which mostly happens near shores or in shallow water. It happens due to the water pressure not being high enough to keep the methane dissolved.
Those bubbles reach the surface, at which point the methane escapes into the atmosphere.
Methane is absorbed under pressure by water and eventually transferred at the surface to the atmosphere through diffusion.
Similar to the reaction when opening a carbonated drink, degassing takes place after the water is discharged from the turbines and is the most constant source of methane emission. It is responsible for about half of methane emission from reservoirs.
Carbon dioxide has been well mapped. However, sources of methane are only starting to be fully understood. Below are some explanations to understand this area further.
Methane has a global warming potential (GWP) 34 times greater than carbon dioxide. But why?
Different greenhouse gases (GHGs) last in the atmosphere for different lengths of time, and they also absorb different amounts of heat. The GWP of a GHG indicates the amount of warming a gas causes over a given time. We use the conservative GWP of 34 for a 100-year time horizon relative to CO2. We could also have opted for the 20-year time horizon GWP of 84. Both are suggested by the IPCC Fifth Assessment Report (AR5), 2014
For simplicity and to allow comparison we calculate methane emissions as carbon dioxide equivalent or CO2e.
We use carbon dioxide equivalent (CO2e) as our unit of ‘currency’ for all greenhouse gases including methane. This factors in the global warming potential (GWP) and length of time the gas remains in the atmosphere so that we can compare like with like.
Anthropogenic methane creates the equivalent of 13 billion tonnes of CO2e every year.
COP26 recognised that the rapid reduction of methane was one of the most effective ways to reduce near-term global warming. Over 100 governments are now forming new policies to curb methane emissions.
There is increasing evidence supporting the need to reduce methane emissions. President Biden has proposed the price per tonne of methane emissions to be $1,500 because “these emissions cause more warming, health impacts, and economic damage in the short term.”
We estimate the potential market for carbon offsets from reservoirs to be in excess of £20bn.
Anthropogenic methane from water accounts for the equivalent of 3 billion tonnes of CO2e each year. The main bodies include reservoirs, waste water and rice cultivation.
While there are multiple sources of methane from water, methane released worldwide from reservoirs accounts for the equivalent of one billion tonnes of CO2e every year or 6% of total anthropogenic emissions.
Anthropogenic methane from water treatment and waste water accounts for more than the equivalent of one billion tonnes of CO2e every year.
Anthropogenic methane from rice cultivation accounts for the equivalent of more than one billion tonnes of CO2e every year.
Parlons Bentata and Rueda Vallejo, 2021, The case for methane and other GHG emissions capture at hydropower dams
McKinsey, 2021, Curbing methane emissions