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On April 20, 2023, SpaceX attempted an orbital flight of the Starship rocket. The test ended when the Flight Termination System blew up the rocket after it started tumbling out of control.

When the rocket blew up, some fire was visible from the explosion, but large white clouds were prominent, which I assume is unburnt propellant.

As far as I know pure oxygen is completely harmless diluted in the atmosphere, but methane is much stronger than CO2 as a greenhouse gas.

Judging from the SpaceX stream overlay (around 49:00 on youtube) the Superheavy booster had about 14% left in the tank before the abort.

How much of the remaining methane in the tanks actually ignited during the abort, and how much was left unburnt and released in the atmosphere?

White plumes screenshot from SpaceX stream: White plume visible on SpaceX stream

White plumes screenshot from Everyday Astronaut's Slow Mo: White plume visible on Everyday Astronaut stream

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Wikipedia's information is a little ambiguous:

The booster's tanks were reported as holding 3,600 t (7,900,000 lb) of propellant, consisting of 2,800 t (6,200,000 lb) of liquid oxygen and 800 t (1,800,000 lb) of liquid methane. However, current booster prototypes can only hold 3,400 t (7,500,000 lb) of propellant.

Depending on how current "current" was when that was written, the booster carries no more than around 800 tons of methane fully loaded; 14% would be 112 tons. Add another 30% or so for Starship and we're up to 145 tons.

The total amount of methane in the atmosphere is about 5,000,000,000 tons, so this is an increase of about 0.0000029%.

large white clouds were prominent, which I assume is unburnt propellant.

Some of it was unburned propellant, but some would be water condensing out of combusted methane; I'm unsure of the proportions. Even if none of the methane had burned, it would take the destruction of another 344,000 Super Heavy boosters under similar conditions to increase atmospheric methane by 1%.

How much of the remaining methane in the tanks actually ignited during the abort

This, I really don't know. I believe that flight termination systems are generally designed to help the fuel and oxidizer mix, so that they burn (relatively harmlessly) in the air instead of falling unburnt, but this is more critical for rockets using toxic hypergolic propellants than for methane & liquid oxygen. My wild, unsupported guess is that half of the remaining methane may have burned.

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    $\begingroup$ So about 750 years of worth of a single individual emissions (world average), $\endgroup$
    – Antzi
    Commented May 24, 2023 at 5:18
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    $\begingroup$ And this is a worst-case assumption based on all methane escaping unburnt. Which, judging by the two giant fireballs, clearly wasn't the case. $\endgroup$
    – Jörg W Mittag
    Commented May 24, 2023 at 7:38
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    $\begingroup$ for comparison May 22, 2023 Pipeline Technology Journal: Two Fields In Turkmenistan Leak More Greenhouse Gases Than The UK: Leaks Could Be Easily Fixed estimates 2.6+1.8=4.4 million tons per year (2022) for just these two newly-discovered leaks. So the max here is 0.0002 "newly-discovered annual Turkmenistan leak units". It is true that burning it and converting it to CO2+H2O might be greener, but... $\endgroup$
    – uhoh
    Commented May 24, 2023 at 9:04
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    $\begingroup$ Only takes ~12 years for methane in the atmosphere to turn into CO2. $\endgroup$
    – Michael
    Commented May 24, 2023 at 14:27
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    $\begingroup$ @Antzi So about an extra 3 seconds of total-world-population emissions, yeah. $\endgroup$
    – Russell Borogove
    Commented May 24, 2023 at 16:07

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