Thread by @hausfath

The recent IPCC report had a big focus on methane (CH4) – and rightly so. We should work to cut methane emissions quickly, but not at the expense of cutting CO2.

Methane is temporary, while CO2 is forever.

A quick thread: 1/13

Methane is a strong greenhouse gas – over 100x more effective at trapping heat than CO2 while its in the atmosphere. Its responsible for around 28% of positive radiative forcing (and historical warming). 2/

However, methane has a short atmospheric lifetime. Most of the methane we emit this year will be gone from the atmosphere in around a decade. Methane interacts with hydroxyl radicals (OH) in the atmosphere, and ultimately breaks down into (mostly) CO2 and H2O. 3/

This has an important implication: the amount of methane in the atmosphere depends largely on our rate of emissions. To put it a different way, if we emit 1 ton of methane per year for a decade or so, the amount of methane in the atmosphere will increase by around 10 tons. 4/

If we keep emitting 1 ton of methane, the amount in the atmosphere will stay around 10 tons. If we stop emitting methane, the amount in the atmosphere will quickly decrease. Its determined by the rate of our emissions over a 10-year or so average (e.g. the lifetime of methane) 5/

CO2 is very different. While around 60% of our CO2 emissions is taken up by the oceans and biosphere, much of the remainder sticks around for tens of thousands of years. CO2 does not break down in the atmosphere; rather it accumulates. 6/

This means the amount of CO2 in the atmosphere is based not on the rate of our emissions – as is the case for methane – but rather on our cumulative emissions. The difference is quite clear when we compare what happens when we emit 1 ton of CO2 and 1 ton of methane each year: 7/

This has some very important implications for our climate. First, if we increase methane emissions we warm the planet. If we keep methane emissions flat the planet does not warm (much*).

*CH4 oxidizes into CO2, but the amount of total CO2 coming from CH4 is pretty minor. 8/

For CO2, on the other hand, if we keep emissions flat the planet continues to warm. If we reduce emissions the planet continues to warm. As long as CO2 emissions are (much) above zero, the planet continues to warm. 9/

Second, if we cut methane emissions the planet cools quickly, as atmospheric methane falls. If we cut CO2 emissions, the planet warms more slowly. We cannot really cool the planet by cutting CO2, unless we actively suck more CO2 out of the atmosphere than we are emitting. 10/

This makes methane a much more powerful lever to affect short term warming. It also means if we cut methane today or a decade from now the long-term climate effects are largely the same (provided we at least stop methane emissions from increasing). 11/

However, if we wait to cut CO2 – say, because we prioritize cutting methane today – we will be stuck with additional warming from CO2 accumulating in the atmosphere that is much harder to deal with in the future. 12/

We can both reduce methane and CO2 emissions in the near term. But we need to be careful in cases when there are tradeoffs. CO2 is ultimately the main climate villain; methane is just its sidekick. The longer we wait to cut CO2 emissions the more warming we will lock in. 13/

For more, see my thread on the subject last year:

https://twitter.com/hausfath/status/1230573108949446657

Or this excellent work by @ClimateBook on the issue over the years.