Can the Tonga Eruption Account for the 2023-2024 Warming Event?

On January 15, 2022, the volcano Hunga Tonga–Hunga Ha’apai (hereafter Tonga) erupted. Normally, volcanic eruptions such as this send sulfate aerosols into the atmosphere that cause a short-term cooling effect in the troposphere (and warming effect on the stratosphere) for a couple years.  However, the Tonga eruption was somewhat unique in that it erupted in shall water and therefore injected 146 MtH2O along with 0.42 MtSO2 into the stratosphere.[1] The injection of water vapor into the stratosphere created the potential that this particular volcano might not cause the short-term tropospheric cooling normally expected. Ever since the anomalous warming began in 2023 (roughly coinciding with the onset of El Nino conditions), a lot of misinformation has circulated on social media that the Tonga eruption is responsible for the warming spike the globe is currently experiencing. This misinformation appears to come from 1) a misreading of one early study[1] that examined only the impact of water vapor (WV) injected to the stratosphere and 2) ignorance of several studies [2][3][4] that examined the impact of both water vapor and sulfate aerosols. The overall conclusion from the available evidence is that the impact of the Tonga eruption has been essentially negligible. Let's look briefly at the conclusions from each study.

• Jenkins et al [1] did not evaluate the full impact of Tonga, but considered only the impact of WV in the stratosphere. The paper concluded that Tonga could increase the global average surface temperature by a few hundredths of a degree over the next 5 years or so. The impact of this would be "increasing the chance of at least one of the next 5 years exceeding 1.5 °C by 7%." However, Jenkins also points out that this is a short-term effect and not terribly relevant to the global warming outlook on decadal timescales.
• Zhang et al [2] evaluated both sulfate aerosol water vapor and concluded that the cooling effect of aerosols would be slightly larger than water vapor. GMST should "decrease by about 0.0315–0.1118°C in the next 1–2 years. The Tonga eruption will slightly slow down the global warming in a short period of time, but it will not change the global warming trend in the long term."
• Zhu et al. [3] likewise evaluated both aerosols and water vaper and calculated "the global radiative forcing of HHTH is on the order of −0.1 to −0.2 W m−2." To put these values in terms of temperature, if sensitivity is 0.8 °C/W/m^2 (ECS = 3 °C), this is about a 0.08 °C to 0.16 °C cooling influence.
• Shoeberl et al [4] also concluded "Hunga produced a global decrease in radiative of less than ∼0.25 W/m2 over the 2 yrs period," and also showed that "by the end of 2023, most of the Hunga induced radiative forcing changes have disappeared."

It should be clear from the above evidence that while the Hunga eruption was unique in that it injected ~140 MtH2O into the stratosphere, the cooling effect of the 0.42 MtSO2 aerosols at least canceled out the warming influence of water vapor, resulting in at most a negligible effect on GMST, and quite possibly a slight cooling influence. This is despite passionate objections and superficial analysis on blogs and social media.

The above graph shows the progression of global temperatures with altitude from ERA5 reanalysis data from 1940 to October 2024. If you look carefully at the stratospheric temperatures, you can see the effects of the El Chichon eruption in 1982 and the Pinatubo eruption in 1991. They appear as vertical red lines in stratospheric temperatures, since aerosol pollution reflects solar radiation back towards space. You can also see slight cooling in the troposphere, since less solar energy is absorbed at the surface. If you look at 2022, though, you don't see any impact of Tonga in the stratosphere, and so there's no reason to expect it had a significant effect on the troposphere. The stratosphere remains cool and the troposphere continues to warm. This appears to be the uniqueness of the Tonga eruption; the water vapor injected into the stratosphere had the effect of nearly negating the typical cooling influence of this kind of eruption, but it was not responsible for any fraction of the warming spike of 2023-2024.

References:

[1] Jenkins, S., Smith, C., Allen, M. et al. Tonga eruption increases chance of temporary surface temperature anomaly above 1.5 °C. Nat. Clim. Chang. 13, 127–129 (2023). https://doi.org/10.1038/s41558-022-01568-2
https://www.nature.com/articles/s41558-022-01568-2

[2] Zhang, H., Wang, F., Li, J. et al. Potential Impact of Tonga Volcano Eruption on Global Mean Surface Air Temperature. J Meteorol Res 36, 1–5 (2022). https://doi.org/10.1007/s13351-022-2013-6
https://link.springer.com/article/10.1007/s13351-022-2013-6

[3] Zhu, Y., Bardeen, C.G., Tilmes, S. et al. Perturbations in stratospheric aerosol evolution due to the water-rich plume of the 2022 Hunga-Tonga eruption. Commun Earth Environ 3, 248 (2022). https://doi.org/10.1038/s43247-022-00580-w

https://www.nature.com/articles/s43247-022-00580-w

[4] Schoeberl, M. R., Wang, Y., Taha, G., Zawada, D. J., Ueyama, R., & Dessler, A. (2024). Evolution of the climate forcing during the two years after the Hunga Tonga-Hunga Ha'apai eruption. Journal of Geophysical Research: Atmospheres, 129, e2024JD041296. https://doi.org/10.1029/2024JD041296

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024JD041296