Methane vs. CO2 in global warming

This letter was published in The Economist in August 2016...food for thought for evolving science.

The effects of methane

When you stated that methane is “25 times as potent” a cause of global warming as carbon dioxide, you perpetuated the myth that there is a single conversion factor that translates the climate effect of methane into what would be caused by an “equivalent” amount of carbon dioxide (“Tunnel vision”, July 23rd). The number you quoted is based on a measure called “global warming potential”. This measure exaggerates the importance of methane because it fails to properly reflect the importance of the short (12 year) lifetime of methane in the atmosphere compared with carbon dioxide, which continues to transform the climate for centuries.

A simple financial analogy is useful. If you opened a bank account for storing your methane emissions, it would be as if the account paid a negative interest rate of -8.3% annually (a concept which may become all too familiar in the real world of banking before long). The balance in the account represents the warming effect of the methane emitted.

If you deposited $1,000-worth of methane today, in 50 years your account would be worth only $16. A big pulse of methane released today would have virtually no effect on the temperature around the time we hope global warming will be peaking. If you were to deposit a steady $100 of methane a year your account would be valued at $1,205 in a few decades but would then stop growing. The only way to increase the amount of warming from methane is to increase the annual emissions rate. Not so with carbon dioxide, which acts more like a bank account with a zero interest rate (rather like a real bank account these days). A fixed emission-rate of carbon dioxide accumulates in the atmosphere, leading to warming that grows without bounds over time.

In fact, if warming causes the land ecosystems to start releasing rather than storing carbon, it would be as if your bank account had a positive interest rate. Not a bad thing for a real bank account, but bad news for climate if it is carbon dioxide you are banking.

RAYMOND PIERREHUMBERT
Professor of Physics
University of Oxford