By Andrew Dessler

When Hunga Tonga-Hunga Ha’apai (HT) erupted on Jan. 15, 2022, it was one of the most extreme volcanic events since the eruption of Krakatoa in 1883. The eruption released energy equivalent to hundreds of atomic bombs. It generated shock waves that traveled around the world multiple times and produced a sound that was heard thousands of miles away. It injected a plume higher into the stratosphere than any other volcano of the satellite era, and it produced the most intense lightning activity ever recorded.

The GOES-17 satellite captured images of an umbrella cloud generated by the underwater eruption of the Hunga Tonga-Hunga Ha’apai volcano on Jan. 15, 2022. Crescent-shaped bow shock waves and numerous lighting strikes are also visible.
from LiveScience

The peak of HT sat 150 meters underwater, so when it erupted, it pushed an enormous amount of water vapor into the atmosphere: 146 megatons¹, equal to about 40 billion gallons of liquid water. Water vapor is a greenhouse gas, so one might wonder if this eruption could be warming the Earth and possibly responsible for the incredible warmth of 2023 and 2024.

This argument relies on the fact that 40 billion gallons seems like a large number and, on human scales, it is. It’s equal to a million backyard swimming pools. But the atmosphere is a vast place that holds a lot of water vapor — about 10 million megatons. So adding 146 megatons increased water in the atmosphere by a factor of 1.0000146 (alternatively, it was an increase of 0.00146%).

What about humans?

Let’s compare HT to humanity. On Jan. 15, 2022, the same day that the HT volcano injected 146 megatons of water into the atmosphere, humans dumped about 100 megatons of CO2 into the atmosphere.

On the next day, Jan. 16, HT was silent, but humans dumped another 100 megatons of CO2 into the atmosphere. On Jan. 17, humans again dumped 100 megatons of CO2 into the atmosphere. Ditto for Jan. 18, 19, 20, …

Put this way, HT looks a whole lot less impressive. So the real question is: What is the relative climate impact of HT’s 146 megatons of water vapor vs. humanity’s 100 megatons per day of CO2?

You can’t simply compare kilograms of water vapor to kilograms of carbon dioxide to determine which is having a bigger climate impact. You need to do calculations that account for the detailed absorption of each gas as well as where the gas is injected into the atmosphere.

The positive radiative forcing² from HT’s water vapor is around 0.1 W/m2. Human emissions of carbon dioxide are, of course, also warming the climate and it takes 1,000 days of human CO2 emissions to generate the same radiative forcing.

But HT also injected cooling aerosols into the stratosphere, which offset its warming. Our best estimate is therefore that HT has not warmed the global-average climate at all. Humans, of course, continue to warm the climate.


What’s the real lesson here?

HT’s eruption was one of the most violent geophysical events in modern human history. Yet its effect on the global-average climate was basically zero.

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On the other hand, humans have dumped around a 2.5 million megatons of CO2 into the atmosphere since the mid-19th century, and this has warmed the climate by 1.2C. This puts HT’s 146 megatons of water vapor into the proper context and is a sharp reminder of who’s really in charge of climate change.

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