The climate impact of the Hunga-Tonga volcanic eruption

No, it's not responsible for the blistering temperatures we're experiencing

Volcanoes play a key role in the Earth’s climate. On geologic time scales, they are a key regulator of the carbon cycle that regulates atmospheric carbon dioxide. On shorter time scales, eruptions can also have profound, temporary impacts.

In January 2022, Hunga Tonga–Hunga Haʻapai (hereafter, HT) erupted in one of the most dramatic geologic events in recorded history. The eruption of HT sent sulfur and water vapor deep into the stratosphere. The spectacular nature of this event has led many climate deniers to proclaim that this is why it’s so hot this summer. Let’s dig into that claim.

Fixed it

Sulfur Gases: The Coolers of the Climate

The most well-known effect of volcanic eruptions is to cool the climate. This comes from the injection of sulfur gases into the stratosphere. Once in the stratosphere, the sulfur combines with water vapor to form little droplets referred to as volcanic aerosols. These tiny particles reflect incoming sunlight back to space, acting like a shade over the Earth's surface, leading to a cooling effect.

For most eruptions, this is the dominant effect. The 1991 eruption of Mt. Pinatubo, for example, released enough sulfur to cool the climate by about 0.5C for a few years after the eruption. A more dramatic example of the cooling effect of volcanic eruptions occurred in 1816, often referred to as the “Year Without a Summer”. The eruption of Mt. Tambora in Indonesia the previous year had released an enormous quantity of sulfur into the stratosphere, leading to a drastic decrease in global temperatures. The Northern Hemisphere experienced unseasonable frosts, snowfalls, and prolonged cold during summer, resulting in widespread crop failures, skyrocketing food prices, and severe famine. This unusual and grim weather influenced the arts, most notably by confining Mary Shelley indoors, where she began penning her iconic novel, Frankenstein.

HT did inject sulfur gases into the stratosphere (0.4 MtSO2), but far less than Mt. Pinatubo did (~20 MtSO2). Thus, we expect it to produce a relatively small cooling effect.

The unusual case of the HT volcano: A huge amount of water vapor

The eruption of the HT volcano was unusual in that it also injected a massive amount of water vapor into the stratosphere. Water vapor is a greenhouse gas, so this injection of water will tend to warm the climate.

In this video, I go over the impact of HT on stratospheric aerosols and water vapor.

The aerosol/water vapor animation was made by Dr. Mark Schoeberl using OMPS aerosol extinction measurements and MLS water vapor measurements. Watch the original video here.

The climate impact of HT

The overall impact of HT will therefore be the net difference between the cooling effect of aerosols and the warming effect of water vapor. I am presently working with a group led by Dr. Mark Schoeberl on a publication estimating these terms, which is presently in peer review, so I won’t comment on our results other than to say they’re generally consistent with previous work.

Here is a summary of what others have found:

• Jenkins et al.: They just calculated the warming impact of water vapor and concluded that it would increase the global average surface temperature by a few hundredths of a degree.

• Zhang et al.: They included both aerosol cooling and water vapor warming and concluded: Surface temperature “will decrease by about 0.0315–0.1118°C in the next 1–2 years”.

• Zhu et al.: They concluded that the net effect of the volcano would be to cool, with total radiative forcing of around -0.2 W/m^2.

Thus, previous work has concluded that the climate impact of HT is small, and two out of three suggest its main effect is to cool the climate system.

Nerd alert: a back-of-the-envelope calculation

You’re probably wondering why the huge amount of water injected into the stratosphere isn’t warming the climate much. The reason is where the water went: most of the water was sent really high into the stratosphere, above 25 km. At that height, water has a minimal effect on the climate.

You can see that in this figure from a study by Solomon et al. It shows how a 1-ppm increase in water vapor at different heights affects the climate (the metric of influence here is what’s known as radiative forcing).

Effect of stratospheric water vapor changes on radiative forcing based on detailed line-by-line calculations. Instantaneous longwave (LW) and instantaneous shortwave (SW) radiative forcing, along with the adjusted net total forcing versus altitude at 35°N obtained for a uniform change of 1 ppmv in 1-km layers. From Solomon et al. 2010.

If we assume that HT increased water vapor from 25-30 km by 5 ppm, the Solomon et al. results tell us that this will generate a radiative forcing of ~0.1 W/m^2. This will cause some climate change, and we can come up with a simple estimate of that using a climate sensitivity value of 0.75C/(W/m^2), which corresponds to 3C warming for doubled CO2. This means that the climate will warm 0.75C in response to a 1 W/m^2 radiative forcing.

Thus, the HT radiative forcing of ~0.1 W/m^2 will generate a warming of ~0.075C. But this is an overestimate — it is the amount of warming that would eventually occur if you maintained this forcing for the millennia required to reach equilibrium. The HT radiative perturbation will last no more than a decade before the air in the stratosphere is completely replaced, so we will get maybe half of the equilibrium warming over the next few years.

This explains why scientists estimate just a few hundredths of a degree of warming from HT’s water vapor. Aerosols will offset this warming perhaps even driving net cooling. In any case, the global climate impact of the eruption will be small.

The impact of HT is something that we understand reasonably well and everything we know suggests that it will have a very small impact on the global climate — in fact, it’s as likely to be cooling the climate as it is to be warming it. If you’re sweating right now, don’t blame HT. Blame fossil fuels.