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Extratropical Explosive Volcanic Eruptions – Climate Forcing is Disproportionally Strong

It is common for volcanic eruptions located outside the tropics to be thought to be less effective as drivers of surface cooling on a scale that is smaller than tropical eruptions. Recent minor extratropical eruptions have, however, produced a measureable climate impact, and it is suggested by proxy results that the Northern Hemisphere cold period that was the most extreme was initiated by an extratropical eruption 1,483 years ago. In this paper Toohey et al. show by the use of stratospheric sulphur injections derived from ice cores and summer temperature reconstructions from tree rings, that in proportion to their estimated stratospheric sulphur injection, extratropical explosive eruptions since 750 CE have produced stronger hemispheric cooling than have tropical eruptions. It is demonstrated by stratospheric aerosol simulations that for eruptions that have a sulphur injection magnitude and height that are equal to that of the Pinatubo eruption of 1991, time integrated radiative forcing anomalies over the extratropical Northern Hemisphere are up to 80% greater than tropical eruptions, because decreases in the lifetime of aerosols are overwhelmed by the enhanced radiative impact that is associated with the relative confinement of aerosol to a single hemisphere. According to Toohey et al. the model results are consistent with the temperature reconstructions, and elucidate the way in which the radiative forcing that is produced by extratropical eruptions is strongly dependent on the season in which the eruption occurs and the height of sulphur injection in the stratosphere.

Climate is impacted by major volcanic eruptions trough the injection of sulphur into the stratosphere, which increases the abundance of sulphate aerosol in the stratosphere and its capacity to scatter incoming solar radiation and thereby cool the surface of the Earth. The impact on climate of any eruption depends on the properties such as stratospheric aerosol enhancement, and is tied principally to the amount of sulphur that is injected, though also to the lifetime of the aerosol in the atmosphere, as well as the spread and size distribution of the aerosol.

Extratropical eruptions are commonly believed to have a weaker impact on the climate than eruptions in the tropics (Kirtman et al., 2013; Myhre et al., 2013). According to Toohey et al. the hypothesis is based on the idea that aerosols produced by tropical eruptions spread globally, and that due to a longer transport path from the tropics to removal across the middle and higher latitude tropopause (Schneider et al., 2009), has a longer lifetime in the stratosphere. It has been assumed in prior volcanic forcing reconstructions (Gao, Robock & Ammann, 2008; Ammann et al., 2003) that stratospheric aerosols from extratropical eruptions have shorter lifetimes in the stratosphere and are therefore implicit in model studies that support the idea weaker climate forcing from extratropical eruptions (Schneider et al., 2009).

A number of confounding factors complicates the results of comparing the aerosol clouds and impacts on climate that result from tropical versus extratropical eruptions that are based on observations and proxy records. Stratospheric injections of sulphur from the strongest extratropical eruptions during the satellite era were an order of magnitude weaker than the largest tropical eruptions (Carn et al., 2016). On longer timescales, ice cores record sulphate from major eruptions in tropics as well as extratropics (Sigl et al., 2015), though uncertainty is added to the estimates of radioactive forcing from ice cores (Schmidt et al., 2012), by limited knowledge of the height of the injection of volcanic sulphur from those eruptions.

Interest has recently increased in extratropical eruptions as a result, in part, of a series of minor extratropical volcanic eruptions that produced significant radiative forcing on the climate, which counteracted a portion of warming by greenhouse gas (Santer et al., 2014; Solomon et al., 2011). Also, it has been suggested by model results that the hemisphericaly asymmetric radiative forcing from extratropical eruptions has distinct impacts on precipitation in the tropics (Haywood et al., 2013; Colose et al., 2016) and circulation of the atmosphere and the ocean (Pausata et al., 2015; Stevenson et al., 2017). Large extratropical volcanic events with associated large-scale cooling (Sigl et al., 2015), which include extreme cold conditions in the Northern Hemisphere that was initiated by an extratropical eruption around 536 CE (Sigl et al., 2015; Toohey et al., 2016; Büntgen et al., 2016), have been identified by reconstructions of volcanic activity over the last 2,500 years from ice cores. A re-examination of radiative forcing and impacts on the climate of extratropical eruptions has motivated these findings.

Implications of extratropical eruptions for the past and the future

It has been assumed by previous reconstructions of volcanic forcing that there were  short lifetime in the stratosphere and weak radiative forcing of aerosol from extratropical eruptions. In modelling experiments by Toohey et al. a shorter aerosol lifetime for tropical injections, but for a constant height of injection in the stratospheric overworld, the effect is as little as 10%. It was demonstrated by the simulations of Toohey et al. that the lifetime of atmospheric aerosol from injections by extratropical eruptions is connected strongly to the height of injection in the stratosphere. The assumption of a short lifetime for extratropical eruptions that was used in previous studies (Schneider et al., 2009), was suggested by Toohey et al. to probably be tied to an implicit assumption of lower height of injection. Volcanic plume models suggest, however, the volcanic plumes have a weak dependence on the latitude of the eruption (Glaze & Baloga, 1996; Sparks, 19986), and maximum heights of the plume for the 1912 Katmai eruption (58^oN) that were estimated that were based on the estimated mass eruption rates and dispersal of tephra reach 28 km (Hildreth & Fierstein, 2012), which is comparable to that of Pinatubo. Previous reconstructions of volcanic forcing, e.g. those of (Gao, Robock & Ammann, 2008; Crowley & Unterman, 2013) used simulations of the last millennium as part of the 5^th phase of the Coupled Model Intercomparison Project (Schmidt et al., 2011), appear, according to Toohey et al., to underestimate the impact on climate of extratropical eruptions (Supplementary Fig. 7). Toohey et al. suggest that a more accurate reconstruction of the magnitude and timing of extratropical eruptions in the past, and the optical properties of the aerosol of the associated stratospheric aerosol, may increase the proportion of variability of temperature that is attributable to external forcing.

The strong sensitivity of radiative forcing that was simulated to the season and height of injection of extratropical eruptions – which impacts on the stratospheric lifetime, effective radius of the aerosol and phasing between SAOD and incoming solar radiation (Kravitz & Robock, 2011) – is consistent with the scatter in the relationship between cooling that is derived from tree rings and VSSI for extratropical eruptions. Therefore, reconstructions of volcanic forcing could benefit from information of the season and height of the injection. In some cases, information on the season of an eruption has been obtained from high resolution analysis of ice cores (Cole-Dai et al., 2009), historical records (Stoffel et al., 2015; Guillet et al., 2017) and geochemical analysis of volcanic tephra in ice cores (Jensen et al., 2014; Sun et al., 2014; Oppenheimer et al., 2018). It has been suggested that the isotopic composition of ice core sulphate is indicative of the height that has been reached by the sulphate aerosol (Baroni et al., 2008; Savarino et al., 2003; Lanciki et al., 2012), though this approach has been criticised on the basis that isotopic fractionation of sulphur is related to height of the aerosol with respect to the peak in the vertical profile of the concentration of extratropical ozone and not with respect to the tropopause (Schmidt et al., 2012). The results of Toohey et al. therefore support the utility of sulphur isotope analysis for providing valuable information on volcanic radiative forcing.

In the extratropics, over recent centuries volcanic eruptions with large VSSI have been less frequent. Many of the largest sulphate signals observed in ice cores that originated from extratropical eruptions that originate from eruptions that were at least effusive, e.g. Laki (1783/1784 CE) (Thordarson & Larsen, 2007). There have been other cases in which extratropical eruptions that erupted a large mass, such as Changbaishan (946 CE) (Oppenheimer et al., 2017) appear to have had a small VSSI. A clear example of an extratropical explosive eruption with a strong VSSI is that of the ~536 CE eruption, which produced solar dimming that lasted more than a year (Stothers & Mystery, 1984), which resulted in some of the coldest temperatures in the Northern Hemisphere in the CE. Toohey et al. suggest 2,000 years is a short sample in geological terms, and it is suggested by the volcanic record of the Holocene (Global Volcanism Program, 201328), which is admittedly incomplete (Watt et al., 2013), that major eruptions of volcanic explosive index (VEI) of ≥ 5, are as common in the Northern Hemisphere extratropics (NHET) as in the tropics. Extratropical volcanic eruptions that injected sulphur into the stratosphere overworld have occurred in earlier times, and they will in the future. It is suggested by the results of Toohey et al. that the latitude of the extratropical eruptions acts to focus the radiative impacts within the Northern Hemisphere, which strengthens the hemispheric impact on climate, rather than reducing the radiative forcing and impact on the climate of such major eruptions.

Sources & Further reading

Toohey, M., et al. (2019). "Disproportionately strong climate forcing from extratropical explosive volcanic eruptions." Nature Geoscience 12(2): 100-107.