However, the Hunga Tonga–Hunga Haʻapai volcano was under the ocean’s surface when it erupted, causing it to inject millions of tonnes of water vapour into the atmosphere. Water vapour is a strong greenhouse gas, so this causes warming. See here for a more detailed explanation.
Not yearlong heating though. Added water vapor stays in the atmosphere for a measly 9 days. Not the 5 years claimed by NPR. I don’t know where that graph came from but it seems absurd. Not the 5 years claimed by NPR. Not sure how that graph was made but it seems absurd.
Edit: Asking the source of the graph, this seems like a reasonable explanation:
Yes. In the lower atmosphere, the process of rain/snow causes water vapor to cycle every few weeks.
In the upper atmosphere, above the tropopause, there is no more precipitation, and the tiny fraction of water vapor that does reach that high will linger for years.
Well, one possibility is (1) that the article got it terribly wrong. But to my ignorant eye there are at least two others. (2) Perhaps water vapour in the stratosphere stays around for much longer than water vapour in the troposphere. (And most water vapour is in the troposphere, so any sort of average figure will be dominated by that.) (3) Your link says that an average molecule of water stays in the atmosphere for 9 days, but that isn’t the same as saying that a change in the amount of water will only persist for that long; maybe there is a constant exchange of water molecules that leaves amounts roughly unchanged, so that if you put 2.3 metric fucktons of extra water into the atmosphere then a month later there will still be 2.3 metric fucktons of excess water but the specific water molecules will be different.
Perhaps someone who knows some actual climatology can tell us how plausible 1,2,3 are.
Here’s the paper I think everyone claiming years is referencing: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022GL099381. That in turn references https://agupubs.onlinelibrary.wiley.com/doi/10.1029/97GL01289 for which I can see only the abstract, which says “tau=1.3 years”. If tau has the usual meaning (time to decay by a factor of e) then 5 years would be roughly the time for a 20x decay; but there may be more details that make the “5-10 years” figure less misleading than that makes it sound (e.g., upper versus lower stratosphere—the water vapour from the recent eruption went a long way up).
I asked the source of the graph, and he said it was so long because it doesn’t rain in the troposphere. This seems a believable explanation. (though also, why doesn’t it rain?)
Not sure if there’s some other reason, but in the stratosphere you don’t afaik* get big convective updrafts like there are in the troposphere, which I presume is due to the rate at which temperature declines with altitude getting smaller than the rate at which a rising air body will cool due to expansion.
*Actually I think that this property is basically what defines the stratosphere vs the troposphere?
Yeah, thanks for highlighting this. I started writing about it but realised I was out of my depth (even further out of my depth than for the rest of the post!) so I scrapped it.
Thanks for clarifying with Robert Rohde!
I reached roughly the conclusion you did. When water vapour is injected into the troposphere (the lowest level of the atmosphere) it is quickly rained out, as you point out. However, the power of the Hunga-Tonga explosion meant that the water vapour was injected much higher, into the stratosphere (what the diagram calls the ‘upper atmosphere’). For some reason, water vapour in the stratosphere doesn’t move back down and get rained out as easily so it sits there. Which is why ‘upper atmosphere’ water vapour levels are still elevated almost two years after the explosion.
Not yearlong heating though. Added water vapor stays in the atmosphere for a measly 9 days. Not the 5 years claimed by NPR. I don’t know where that graph came from but it seems absurd. Not the 5 years claimed by NPR. Not sure how that graph was made but it seems absurd.
Edit: Asking the source of the graph, this seems like a reasonable explanation:
From Dr. Robert Rohde on Twitter.
Well, one possibility is (1) that the article got it terribly wrong. But to my ignorant eye there are at least two others. (2) Perhaps water vapour in the stratosphere stays around for much longer than water vapour in the troposphere. (And most water vapour is in the troposphere, so any sort of average figure will be dominated by that.) (3) Your link says that an average molecule of water stays in the atmosphere for 9 days, but that isn’t the same as saying that a change in the amount of water will only persist for that long; maybe there is a constant exchange of water molecules that leaves amounts roughly unchanged, so that if you put 2.3 metric fucktons of extra water into the atmosphere then a month later there will still be 2.3 metric fucktons of excess water but the specific water molecules will be different.
Perhaps someone who knows some actual climatology can tell us how plausible 1,2,3 are.
Here’s the paper I think everyone claiming years is referencing: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022GL099381. That in turn references https://agupubs.onlinelibrary.wiley.com/doi/10.1029/97GL01289 for which I can see only the abstract, which says “tau=1.3 years”. If tau has the usual meaning (time to decay by a factor of e) then 5 years would be roughly the time for a 20x decay; but there may be more details that make the “5-10 years” figure less misleading than that makes it sound (e.g., upper versus lower stratosphere—the water vapour from the recent eruption went a long way up).
I asked the source of the graph, and he said it was so long because it doesn’t rain in the troposphere. This seems a believable explanation. (though also, why doesn’t it rain?)
Not sure if there’s some other reason, but in the stratosphere you don’t afaik* get big convective updrafts like there are in the troposphere, which I presume is due to the rate at which temperature declines with altitude getting smaller than the rate at which a rising air body will cool due to expansion.
*Actually I think that this property is basically what defines the stratosphere vs the troposphere?
Yeah, thanks for highlighting this. I started writing about it but realised I was out of my depth (even further out of my depth than for the rest of the post!) so I scrapped it.
Thanks for clarifying with Robert Rohde!
I reached roughly the conclusion you did. When water vapour is injected into the troposphere (the lowest level of the atmosphere) it is quickly rained out, as you point out. However, the power of the Hunga-Tonga explosion meant that the water vapour was injected much higher, into the stratosphere (what the diagram calls the ‘upper atmosphere’). For some reason, water vapour in the stratosphere doesn’t move back down and get rained out as easily so it sits there. Which is why ‘upper atmosphere’ water vapour levels are still elevated almost two years after the explosion.
Though also there may be complications from it being higher up than water vapor usually goes.