The Science of Hunga Tonga Global Warming - Part 2

OK, let’s get right into it shall we. We’ll start with this, published in Nature in January this year:

On 15 January 2022, the Hunga Tonga–Hunga Ha’apai (HTHH) eruption injected 146 MtH2O and 0.42 MtSO2 into the stratosphere. This large water vapour perturbation means that HTHH will probably increase the net radiative forcing, unusual for a large volcanic eruption, increasing the chance of the global surface temperature anomaly temporarily exceeding 1.5 °C over the coming decade.

The eruption of Hunga Tonga–Hunga Ha’apai (HTHH) on 15 January 2022 was one of the most well-observed in human history. Ranked with a Volcanic Explosivity Index of 5 (ref. 3), this was the most explosive eruption since Pinatubo in 1991, producing perturbations in surface pressure that reverberated around the globe for days after the climactic eruption event itself. Although this received less attention, the eruption was also notable because of the composition of its stratospheric perturbation—an estimated 0.42 Mt SO sulfur dioxide injection and 146 MtH2O water vapour injection. The HTHH eruption resulted in the largest stratospheric water vapour perturbation observed in the satellite era (a 10–15% increase in the water vapour content of the stratosphere), with a modest accompanying SO injection (approximately one-fiftieth the size of the Pinatubo eruption).

Most large volcanic eruptions are notable for their negative perturbation on global surface temperatures, since they emit large quantities of SO2, an aerosol particulate which scatters incoming solar radiation. However, it is possible that over a multiyear period HTHH will cause a temporary increase in global surface temperatures due to this large water vapour increase and lack of a large counterbalancing sulfate aerosol perturbation. Some groups have separately calculated the radiative impact of the SO injection, ignoring the impact of the large water vapour perturbation, while others have included the water vapour but focus on the negative radiative perturbation caused by an increased rate of hydrolysis of SO2 to H2SO4 and not the impact of the water vapour itself. Estimates of the combined radiative perturbation resulting from the HTHH eruption are dominated by the water vapour contribution, resulting in a positive net radiative forcing perturbation despite the increased rate of SO hydrolysis and meaning that the multiyear climate response to HTHH is determined by the evolution of the stratospheric water vapour perturbation. If a large fraction of the injected stratospheric water vapour plume remains over several years, the HTHH eruption could measurably, albeit temporarily, change the likelihood of the global mean surface temperature (GMST) anomaly exceeding 1.5 °C. This is not identical to 1.5 °C exceedance in the context of the Paris Agreement, which relies on GMST averaged over a multidecade interval, isolating the long-term trend. Despite this, the first year which exceeds 1.5 °C will garner substantial media attention, even if a portion of this results from HTHH.

Couldn’t be much clearer than that could it? Rather than affecting global temperature by ‘a few hundredths of a degree’, radiative forcing from the Hunga Tonga stratospheric water vapour perturbation could be significant enough in the next few years to bump up global mean surface temperatures temporarily beyond the 1.5C [OMG - run for the hills!] limit. The authors admit that the press will go nuts with the news and probably blame it on man-made global warming, not Hunga Tonga. They already are. This is a peer reviewed paper published in nature, by scientists who are not known for their sceptic views on global warming - Prof Myles Allen is in fact a fully paid up climate alarmist with hundreds of climate change papers to his name, yet here he is, telling us that Hunga Tonga might increase global temperature beyond 1.5C very soon. Why is his paper being largely ignored?

Another Nature paper, published in November 2022:

The underwater Hunga Tonga-Hunga Ha-apai volcano erupted in the early hours of 15th January 2022, and injected volcanic gases and aerosols to over 50 km altitude. Here we synthesise satellite, ground-based, in situ and radiosonde observations of the eruption to investigate the strength of the stratospheric aerosol and water vapour perturbations in the initial weeks after the eruption and we quantify the net radiative impact across the two species using offline radiative transfer modelling. We find that the Hunga Tonga-Hunga Ha-apai eruption produced the largest global perturbation of stratospheric aerosols since the Pinatubo eruption in 1991 and the largest perturbation of stratospheric water vapour observed in the satellite era. Immediately after the eruption, water vapour radiative cooling dominated the local stratospheric heating/cooling rates, while at the top-of-the-atmosphere and surface, volcanic aerosol cooling dominated the radiative forcing. However, after two weeks, due to dispersion/dilution, water vapour heating started to dominate the top-of-the-atmosphere radiative forcing, leading to a net warming of the climate system.

What? No, you must be mistaken guys. Hunga Tonga was no more than a tiny blip. Having said that however, there is some comfort for those who maintain that HT will have only a slight effect upon surface temperatures later in the article, where the authors say:

By contrast, the LW and SW water vapour RF switches from negative to positive values (due to its descent in altitude) and dominates at TOA for the aged plume. At this dispersion stage, the aerosol plume is relatively homogeneous in the approximate latitude range 10°N-30°S, so this RF estimate, though not an hemispheric average, is representative of this full latitude band. Thus, contrary to what was observed for all the stratospheric volcanic eruptions of the last 30 years, the HT plume might produce a slightly positive TOA RF, with a subsequent small warming effect of the climate system. The surface RF of the aged plume remains dominated by aerosols and reaches negative values as large as almost −2 Wm⁻², which is quite large if compared with recent events.

However, the authors’ ‘aged plume’ is not so aged obviously that the aerosols have dispersed completely. They are still producing −2 Wm⁻² cooling at the surface. They’re not now; the aerosols have completely dispersed, but the water vapour cloud has not. It will probably remain up there for the next 2 to 3 years, dispersing only slowly. So you have to ask, what is this ‘small warming effect’ doing to global mean surface temperatures now, and just how ‘small’ is it? I’ll be looking at the possible quantification of the water vapour radiative forcing in the next post, but for now I will leave you with some measurements of post eruption water vapour concentrations as stated by the authors of this same paper and they are very significant compared to what is normal at this height:

The large HT water vapour emissions are likely due to both volcanic caldera intrusion of seawater and mechanical interaction with seawater, over the underwater eruptive crater, during the eruption. As a result, record-breaking water vapour content is found in the stratosphere, within the plume, with radio-soundings (see Methods). Water vapour concentrations exceed 0.5–0.8 molecules cm⁻³ (up to over 1500 ppmv) for the fresh plume over Australia on January 19–20th, 0.4 molecules cm⁻³ (~500 ppmv) for the dispersed plume at Saint Helena Island on January 25th, and 0.1 molecules cm⁻³ (~100 ppmv) after a full circumnavigation of the Earth, back over Australia on 8th February (Fig. 4a).

The water vapour concentration may be a clue to the net radiative forcing, as we shall see.