"Volcanic eruptions rarely inject much water into the stratosphere [See NASA Article Click Here]. In the 18 years that NASA has been taking measurements, only two other eruptions – the 2008 Kasatochi event in Alaska and the 2015 Calbuco eruption in Chile – sent appreciable amounts of water vapor to such high altitudes."
""The amount of water vapor injected into the stratosphere after the eruption of Hunga Tonga-Hunga Ha’apai (HTHH) was unprecedented, and it is therefore unclear what it might mean for surface climate. We use chemistry climate model simulations to assess the long-term surface impacts of stratospheric water vapor (SWV) anomalies similar to those caused by HTHH, but neglect the relatively minor aerosol loading from the eruption"". Footnote 1
""The simulations show that the SWV anomalies lead to strong and persistent warming of Northern Hemisphere landmasses in boreal winter, and austral winter cooling over Australia, years after eruption, demonstrating that large SWV forcing can have surface impacts on a decadal timescale. We also emphasize that the surface response to SWV anomalies is more complex than simple warming due to greenhouse forcing and is influenced by factors such as regional circulation patterns and cloud feedbacks"". Footnote 2
""This extra water vapor could influence atmospheric chemistry, boosting certain chemical reactions that could temporarily worsen depletion of the ozone layer. It could also influence surface temperatures."" NASA
Massive volcanic eruptions like Krakatoa and Mount Pinatubo typically cool Earth’s surface by ejecting gases, dust, and ash that reflect sunlight back into space.
In contrast, the Tonga volcano didn’t inject large amounts of aerosols into the stratosphere, and the huge amounts of water vapor from the eruption may have a small, temporary warming effect, since water vapor traps heat.
""The effect would dissipate when the extra water vapor cycles out of the stratosphere and would not be enough to noticeably exacerbate climate change effects."" NASA
The MLS instrument was well situated to detect this water vapor plume because it observes natural microwave signals emitted from Earth’s atmosphere. Measuring these signals enables MLS to “see” through obstacles like ash clouds that can blind other instruments measuring water vapor in the stratosphere. “MLS was the only instrument with dense enough coverage to capture the water vapor plume as it happened, and the only one that wasn’t affected by the ash that the volcano released,” said Millán.
The MLS instrument was designed and built by JPL, which is managed for NASA by Caltech in Pasadena. NASA’s Goddard Space Flight Center manages the Aura mission.Jane J. Lee / Andrew Wang
Jet Propulsion Laboratory, Pasadena, Calif.NASA
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""With all the attention given to humans’ climate-warming carbon dioxide (CO2) emissions, you might be surprised to learn that CO2 is not the most important greenhouse gas affecting the Earth’s temperature. That distinction belongs to water."" MIT Study 3rd Nov 2023 Prof Emanuel - A climate Change Proponent
""We can thank water vapor for about half of the “greenhouse effect” keeping heat from the sun inside our atmosphere". Footnote 3
“It’s the most important greenhouse gas in our climate system, because of its relatively high concentrations,” says Kerry Emanuel, professor emeritus of atmospheric science at MIT.
“It can vary from almost nothing to as much as 3% of a volume of air.”
Compare that to CO2, which today makes up about 420 parts per million of our atmosphere—0.04%—and you can see immediately why water vapor is such a linchpin of our climate system.
So why do we never hear climate scientists raising the alarm about our “water emissions”?
It’s not because humans don’t put water into the atmosphere. Even the exhaust coming from a coal power plant—the classic example of a climate-warming greenhouse gas emission—contains almost as much water vapor as CO2Footnote 4.
"It’s why that exhaust forms a visible cloud. But water vapor differs in one crucial way from other greenhouse gases like CO2, methane {CH4}, and nitrous oxide. Those greenhouse gases are always gases (at least when they’re in our atmosphere). Water isn’t. It can turn from a gas to a liquid at temperatures and pressures very common in our atmosphere, and so it frequently does. When it’s colder it falls from the air as rain or snow; when it’s hotter it evaporates and rises up as a gas again.“
"This process is so rapid that, on average, a molecule of water resides in the atmosphere for only about two weeks,” says Emanuel.
"This means extra water we put into the atmosphere simply doesn’t stick around long enough to alter the climate; you don’t have to worry about warming the Earth every time you boil a kettle. And there’s really no amount of water vapor we could emit that would change this."
“If we were to magically double the amount of water vapor in the atmosphere, in roughly two weeks the excess water would rain and snow back into oceans, ice sheets, rivers, lakes, and groundwater,” Emanuel says.
Nonetheless, water vapor is an important part of the climate change story—just in a slightly roundabout way.
At any given temperature, this is a theoretical upper limit to the amount of water vapor the air can hold. The warmer the air, the higher that upper limit. And while the air rarely holds as much water as it could—thanks to rain and snow—Emanuel says that over the long term, rising temperatures steadily raise the average amount of water vapor in the atmosphere at any given time.
And of course, temperatures today are rising, thanks to humans’ emissions of longer-lasting greenhouse gases like CO2. {BUT, this statement is yet to be proven in a scientific or in empirical terms}
Water vapor amplifies that effect. {??}
“If the temperature rises, the amount of water vapor rises with it,” says Emanuel.
“But since water vapor is itself a greenhouse gas, rising water vapor causes yet higher temperatures. We refer to this process as a positive feedback, and it is thought to be the most important positive feedback in the climate system.”
In short, it’s true that water vapor is in some sense the “biggest” greenhouse gas involved in climate change, but it’s not in the driver’s seat. CO2 is still the main culprit of the global warming we’re experiencing today. Water vapor is just one of the features of our climate that our CO2 emissions are pushing out of balance—well beyond the stable levels humanity has enjoyed for thousands of years.
27 May 2024: Abstract of the Study by Jucker, Lucas and Dutta: Displayed acceptance dates for articles published prior to 2023 are approximate to within a week. If needed, exact acceptance dates can be obtained by emailing
What did we find out?
The large ozone hole from August to December 2023 was at least in part due to Hunga Tonga. Our simulations predicted that ozone hole almost two years in advance.
Notably, this was the only year we would expect any influence of the volcanic eruption on the ozone hole. By then, the water vapour had just enough time to reach the polar stratosphere over Antarctica, and during any later years there will not be enough water vapour left to enlarge the ozone hole.
As the ozone hole lasted until late December, with it came a positive phase of the Southern Annular Mode during the summer of 2024. For Australia this meant a higher chance of a wet summer, which was exactly opposite what most people expected with the declared El Niño. Again, our model predicted this two years ahead.
In terms of global mean temperatures, which are a measure of how much climate change we are experiencing, the impact of Hunga Tonga is very small, only about 0.015 degrees Celsius. (This was independently confirmed by another study.) This means that the incredibly high temperatures we have measured for about a year now cannot be attributed to the Hunga Tonga eruption.
Disruption for the rest of the decade
But there are some surprising, lasting impacts in some regions of the planet.
For the northern half of Australia, our model predicts colder and wetter than usual winters up to about 2029. For North America, it predicts warmer than usual winters, while for Scandinavia, it again predicts colder than usual winters.
The volcano seems to change the way some waves travel through the atmosphere. And atmospheric waves are responsible for highs and lows, which directly influence our weather.
It is important here to clarify that this is only one study, and one particular way of investigating what impact the Hunga Tonga eruption might have on our weather and climate. Like any other climate model, ours is not perfect.
We also didn't include any other effects, such as the El Niño–La Niña cycle. But we hope that our study will stir scientific interest to try and understand what such a large amount of water vapor in the stratosphere might mean for our climate.
Whether it is to confirm or contradict our findings, that remains to be seen – we welcome either outcome. The Conversation Martin Jucker, Lecturer in Atmospheric Dynamics, UNSW Sydney