Thread by @PFriedling

A bit more quantitative analysis of the difference between zero emissions and the constant concentration as in Hansen's warming in the pipeline manuscript.
Read this thread from @hausfath first.
Then come back to my tweet for some quantification
1/n

https://twitter.com/hausfath/status/1660694123320578048

Hansen assumes present day forcing, i.e. concentration of greenhouse gases (GHGs) forever (let's say 10,000s of years).
That is very different from assuming zero emissions.
Let's start with the easy ones, the non-CO₂ GHGs.
2/n

Take methane (CH₄), it has a residence time of about 10 years in the atmosphere. Meaning that if anthropogenic emission stopped, the CH₄ concentration would rapidly decrease, by a factor of e (2.72) every 10 years.
3/n

It would be back to pre-industrial concentrations (~700 ppb as opposed to ~1900ppb now) in less than a century
To keep concentrations at current level *forever* we would need to emit the same amount of CH₄ as today *forever*
4/n

That's a lot of CH₄ to emit every year, around 300-400 TgCH4 per year, forever.
See essd.copernicus.org/articles/12/15…
5/n

Same for N2O. Residence time is around 100 years. Zero human emissions would lead to a decline of concentration, back to preindustrial in less than one thousand years.
6/n

To keep N2O concentration at current level *forever*, we would need to emit about 7 TgN every year, *forever*
See nature.com/articles/s4301…
7/n

Same for all other non-CO₂ GHGs (O3, HCFs, etc).
See IPCC AR6 WG1 Chapter 7, radiative forcing section.
8/n

CO₂ is more complicated, as it has more than one single residence time. Many papers showed this complex multiple time scale removal time involving many land/ocean/sediments processes.
See for example acp.copernicus.org/articles/13/27…
or annualreviews.org/doi/10.1146/an…
9/n

If emission cease, atmospheric CO₂ declines "fast" initially but then more and more slowly.
After 100 years, about 40% of the initial CO₂ input is still in the atmosphere.
After 1000 years, it's about 20% still in the atmosphere
10/n

And after 10,000 years, ocean sediments continue to take some of the carbon excess, but there is still about 5-10% of the initial CO₂ in the atmosphere.
11/n

So, knowing that, how much CO₂ would we need to emit to still have the present-day concentration in 10,000 years? Simple back of the envelope calculation:
12/n

Pre-industrial CO₂ was 280ppm, present day CO₂ is 420 ppm. An excess of 140 ppm. That's about 300 billion tons of C (GtC).
To still have 420 ppm in the atmosphere in 10,000 years, we would need to have emitted an amount of CO₂ for which ~7.5% (5 to 10%) = 300GtC.
13/n

That's ~4000 billion tons C.
Since the start of the industrial revolution, we have emitted ~700 billion tones of C.
To have a present-day (420 ppm) CO₂ concentration of CO₂ in 10,000 years (*forever*) we would need to emit ~6 times more CO₂ in the future.
14/n

So in summary. The warming in the pipeline with present-day GHGs concentration implicitly assumes a huge amount of GHG emissions *forever* in the future.
15/n

One can rightly say that zeroing all emissions over night is not realistic. Fair point.
But keeping all non-CO₂ emissions at current level and emitting another 3-4 billions of tons of C seems equally unrealistic !
16/n

Hope it helped !