Volcano & Paleoclimate - Volcano Climate Impact

Drawing on examples, compare and contrast the impacts of volcanic eruptions on climate depending on the timescale considered.

Fiona Fang, Trinity Hall, 11/2023

Anthropogenic global warming has elicited widespread attention towards the causes, processes and consequences of climate changes. The study of volcanoes, one of the most significant natural variabilities influencing climate, could not only aid in discerning natural and anthropogenic signals amidst climate change but also deepen our understanding of the complicated climate system. This essay employs examples spanning different time scales to analyze volcanoes’ impacts on climate. It first discusses the impacts of explosive eruptions, with a focal point on the control by sulfate aerosols on temperature, precipitation, monsoons, and the North Atlantic Oscillation (NAO), as well as sea ice feedback which might linked to longer climatic impacts such as little ice age and stadial periods. The essay then focuses on the role of volcanism on a further longer time scale (10^5 ya) by assessing Large Igneous Provinces (LIPs). These substantial volume (> 0.1 Mkm3, frequently exceeding 1 Mkm3), predominantly mafic (~ultramafic) magmatic events could influence climate and biodiversity through mechanisms encompassing global cooling, global warming, and oceanic anoxia (Ernst and Youbi, 2017). Lastly, this essay illustrates a more intricate relationship between volcanism and climate by considering other factors that may impact volcanoes’ climatic effects and the reciprocal influence of climatic conditions on volcanic activity.

As for explosive volcanoes, different volcanic ejecta can lead to distinct climatic effects. Firstly, volcanic ash, characterized by light color due to high silicon content, reflects sunlight and leads to cooling. Moreover, by burying vegetation and diminishing evapotranspiration, the ash reduces the moisture availability in the air and thus diminishes precipitation rates (Schmidt, Fristad and Elkins-Tanton, 2015). Secondly, the halogen compounds emitted by volcanoes, such as chlorine and bromine, react with the ozone (O3), exacerbating ozone depletion (Timmreck, 2012). For instance, (Solomon et al., 2016) pointed out the occurrence of a record October ozone hole in Antarctica following the 2015 Calbuco eruption. The ejecta with most notable climate impact is SO2, which undergoes oxidation in the atmosphere and forms long-lived sulfate aerosols (as shown in the following chemial equations).

SO2 + OH → HOSO2 HOSO2 + O2 → SO3 + HO2 SO3 + H2O → H2SO4

The first impact of these aerosols is that they can exacerbate ozone depletion by serving as surfaces for heterogeneous chemical reactions (Robock, 2015). More importantly, these aerosols are capable to scatter part of solar radiation back into space and cool the earth surface (Robock, 2000). When volcanic plume is confined below the tropopause (10-20 km altitude), the impact is local and short-term, as the aerosols act as nuclei to form droplets in clouds and are then washout by precipitation, as observed in in the cases of Kilauea and Etna eruptions (Robock, 2015). However, when the plume reaches the stratosphere, which is much drier than the troposphere and thereby precluding washout, the formed aerosols can be kept for a longer time, leading to radiative effect up to 2 or 3 years for large volcanic eruptions (Swingedouw et al., 2017). This type of eruptions that inject SO2 into stratosphere is known as ‘Pilinian eruptions’. A notable example is the Pinatubo eruption in 1991. By emitting 20 million tons SO2 into the stratosphere, it reduced the solar radiation reaching Earth’s surface by 20-30% and resulted in a global cooling of 0.5°C (Hoppe, 1992).

Among all kinds of volcanic ejecta, the amount of sulfur emitted is the most pivotal one in influencing global climate. Illustratively, the Eyjafjallajökull eruption in 2010, despite significantly disrupted air traffic for weeks with substantial ashfall, emitted so little SO2 that it had no impact on climate (Robock, 2015). Simultaneously, the geographical location of eruption assumes significance in the scale of its impact on climate, given that the aerosol loading into the stratosphere is in general not uniform spatially. For extra-tropical eruptions, the aerosol formation, transport and sedimentation (and therefore the climate impact) are largely confined to proximate regions, extending up to the hemispheric scale for exceptionally large events such as the Laki eruption in 1783. In contrast, tropical eruptions, exemplified by Samalas in 1257 and Mt. Pinatubo in 1991, facilitate the trans-hemispheric transportation of aerosols through the Brewer-Dobson circulation in the stratosphere (Swingedouw et al., 2017). Succinctly, eruptions possessing greatest potential to influence climate are generally characterized by large, tropical, sulfur-rich. Both El Chichon in 1982 and Mt. Pinatubo in 1991, the two eruptions exerting the most notable radiative forcing since the 1980s, exemplify this category, as depicted in Figure 1.

Figure 1. The volcanic forcing, latitude, mass of SO2 emitted and date of eruptions from 1980 to 2016 (Schmidt et al., 2018)

Once penetrated into the stratosphere, these sulphate aerosols can trigger further effects global climate patterns by modifying atmospheric and oceanic dynamics. First of all, to maintain small upper tropospheric temperature gradients within the tropics, the branch of the Hadley cell in the cooler hemisphere strengthens, transporting heat from the warmer hemisphere to the cooler one. Concurrently, moisture, which is concentrated in the lower troposphere, is transported in the opposite direction, causing a displacement of the Inter Tropical Convergence Zone (ITCZ) away from the cooler hemisphere (Iles and Hegerl, 2014). For instance, following the 1991 eruption of Mount Pinatubo (which is a Northern Hemisphere eruption), ITCZ moved southward, reducing the moisture transport from the ocean and weakening the South Asian Summer Monsoon (SASM) (Zhuo et al., 2021). Specifically, between October 1991 and September 1992, there was a substantial decrease in precipitation over land and a record decrease in runoff and river discharge into the ocean (Trenberth and Dai, 2007). Furthermore, tropical eruptions can impact polar vortex and the North Atlantic oscillation (NAO). Whilst cooling the earth surface by scattering solar radiation, sulfur aerosols also heat the stratosphere as they absorb both solar (shortwave) and terrestrial (longwave) radiation (Robock, 2015). The stratosphere heating effect of equatorial eruptions is more pronounced in the tropics than in the high latitudes, intensifying the pole-to-equator temperature gradient (EPTG) and resulting in a stronger polar vortex in North Hemisphere (NH) winter. As a consequence, the NAO, which reflects the surface sea-level pressure difference between the Subtropical (Azores) High and the Subpolar Low, experiences enhancement. According to the Greenland ice cores and stimulation by (Sjolte et al., 2018), an average of winters with positive NAO occurred following major tropical volcanic eruptions from 1241 to 1970 CE, which means warmer winters in northwest Europe and north America and colder winters in Greenland and east coast of Canada.

Last but not least, explosive eruptions influence oceanic circulation and sea ice as well. According to (Slawinska and Robock, 2018), at least in the Last Millennium Ensemble, volcanic eruptions instigate a decadal-scale Atlantic Multidecadal Overturning Circulation (AMOC) positive response, and a centennial-scale amplification of the NH sea ice extent. The research by (Gagné et al., 2017) on the volcanic activities since the 20th century reveals a consistent pattern wherein each activity is accompanied by a decade of increased Arctic sea extent of up to half a million square kilometers. This feedback assumes a pivotal role in perpetuating the long-term climate effects of volcanic eruptions. A compelling example for reference is the Little Ice Age (LIA), a period from 1300 to around 1850 CE characterized by a drop of 2°C in global temperature. As Miller et al. (2012) illustrated, the initiation of LIA can be attributed to a 50-year-long episode with four large sulfur-rich explosive eruptions (each with global sulfate loading >60 Tg), whilst the persistence of cold summers is best explained by consequent sea-ice or ocean feedbacks during a hemispheric summer insolation minimum. Similarly, it is argued that the long-lasting and spatially synchronized cooling following a cluster of large volcanic eruptions in 536, 540 and 547 AD, known as the Late Antique Little Ice Age, was likely sustained by ocean and sea-ice feedbacks (Büntgen et al., 2016). A more impactful instance is the Toba eruption 74,000 years ago (VEI 8), which is considered as a pivotal factor triggering a millennium of cool climate preceding Dansgaard-Oeschger event 19. This influence is attributed to induced perennial snow cover and expanding sea-ice extent at northern latitudes (Rampino and Self, 1992; Oppenheimer, 2002) .

On a longer time scale, the formation of Large Igneous Provinces (LIPs) can induce more profound climate impacts. Similar to explosive volcanic eruptions, LIPs can cause global cooling due to the emission of SO2, especially in instances of more sulfur-rich eruptions like Silicic Large Igneous Provinces (SLIPs) and equatorial eruptions (Ernst and Youbi, 2017). However, in the case of LIPs, the cooling effect of sulfate aerosols merely marks the initiation of a more intricate story. The substantial amount of CO2 emitted by LIPs stays a far longer time in the atmosphere compared to SO2, and its accumulation can lead to the greenhouse effect and trigger positive feedbacks such as gas hydrate dissociation and wildfires. For example, the CO2 released by the Siberian LIP precipitated a global warming of 8-11°C, which was further exacerbated by the positive feedback of methane release from permafrost and shelf sediment hydrate, calminating in a catastrophic global warming of > 34°C and ultimately instigating the end-Permian mass extinction (Brand et al., 2016). Notably, as indicated in Table 1, four out of the five major extinctions since the Phanerozoic (indicated in bold) are associated with global warming induced by LIP formation. Additionally, some minor extinctions can also be attributed to LIPs. However, with the accumulation of CO2 and rise in temperature, silicate weathering also intensifies, which would consume CO2 and offset global warming. Given that chemical weathering rates double with a 10°C temperature increase, this effect is more pronounced under warm environmental background (e.g. Hothouses and greenhouses) and after tropical eruptions (Ruddiman, 2001). According to the climate mode proposed by Goddéris et al. (2003), the weathering of a 6 million km2 equatorial basaltic province is adequate to result in a snowball glaciation. Last but not least, as illustrated in Table 1, besides temperature changes, marine anoxia also serves as a potent kill mechanism of LIPs in causing biological extinctions. This can be explained by the slow down of ocean circulation under the reduced Equator-to-Pole Temperature Gradient (EPTG) during global warming. Meanwhile, the solubility of O2 also diminishes when temperature rises (Wignall, 2001). Additionally, the intensified chemical weathering also results in heightened levels of nutrients and organic matter flush into lakes and oceans, contributing to eutrophication and anoxia (Xu et al., 2022).

Table 1. Phanerozoic Mass Extinctions Since the Early Cambrian (Bond and Sun, 2021)

It should be noted that the climate is influenced by more than just volcanic activity over a certain period. Short-term volcanic effects are notably influenced by the El Niño-Southern Oscillation (ENSO), another short-term natural variability. For instance, (Lehner and Fasullo, 2017) suggest that an eruption of comparable magnitude to the 1963 Mount Agung eruption occurring during a La Niña would result in a more substantial cooling effect (0.3°C) than if it were to occur during an El Niño (0.1°C). Additionally, some scholars argue grand solar minima as a compelling factor in explaining the trigger of Little Ice Age (for example Mörner, 2015). These natural variabilities make it harder to summarize the impact of volcanoes themselves on climate through historical data and observational records. The climate effects of LIPs may also be subject to other influential factors. For example, the amount and rate of CO2 emission from the Deccan Traps are an order of magnitude lower than the other LIPs linked to mass extinction events. This discrepancy may be ascribed to the significant environmental impact of the contemporaneous Chicxulub asteroid event (Schulte et al., 2010). When it comes to contemporary volcanic eruption, it is important to take anthropogenic influences on climate into account. For instance, despite the substantial impact of the 1991 eruption of Mt Pinatubo in Luzon, Philippines, which released 20 Tg of SO2, reduced direct sunlight by 25-30%, and caused a peak reduction in temperature of 0.5°C (Schmidt, Fristad and Elkins-Tanton, 2015), the short-term cooling effect was surpassed by anthropogenic warming within two years (Oppenheimer, 2011). Finally, the intricacy of the relationship between volcanoes and climate also lies in the bidirectional nature of their influence. For example, while volcanic activity may contribute to global warming, extensive glacier melting can also exacerbate volcanic activity. Specifically, when ice cover is thick, volcanic eruptions occur less frequently; an ice layer of 1-3 km thickness can delay the eruption of ice-covered volcanoes by several years to decade (Lucas et al., 2022). Conversely, when ice cover melts, the reduction in overburden pressure and increased mantle melting facilitate the ascent of magma through the crust, as illustrated in Figure 2. For instance, volcanic activity in Iceland during the early post-glacial period (∼12.5–10 ka) increased by a factor of up to 50 (Maclennan et al., 2002). Similarly, changes in sea level, tides, and precipitation can impact shalloweragma chambers and induce more volcanic eruptions (Satow et al., 2021).

Figure 2. The possible mechanism by which Glacial-Interglacial cycle impacts volcanic activities (Conway, 2023)

In summary, volcanic activities constitute a crucial factor influencing Earth’s climate. Explosive volcanic eruptions, through the emission of volcanic ash, halogens, and SO2, induce cooling, acid rain, and ozone depletion, exerting widespread impacts on global climate variability. These effects typically persist for 1-2 years, but sustained eruptions, with feedback mechanisms such as sea ice extension, can lead to impacts lasting up to centuries. On a longer time scale, LIPs can invoke kill mechanisms that contribute to mass extinctions through global cooling, global warming, and oceanic anoxia effects. The scale, location, timing, and variations in environmental conditions of volcanic activities all influence the ultimate climate effects, whilst the complexity is strengthened in the context of anthropogenic global warming. Investigating the relationship between volcanoes and climate aids in preparing for potential volcanic events in the future. Simultaneously, it prompts reflection on the potential climatic consequences of contemporary man-made global warming. For instance, the fact that LIP warming is substantially intensified by gas hydrates dissociation serves as a warning to monitor positive feedbacks and tipping points in global warming. Additionally, the ozone-depleting and acid rain effects of SO2 prompt geoengineers to question the viability of injecting it into the stratosphere as a means of achieving global cooling.

Supervisor feedback: Fiona, this is a fantastic essay demonstrating a really good understanding of the topic. You have done an impressive amount of reading for this essay and you have clearly worked very hard on this. You structured the essay very well and I was pleased to see you discussing different lines of evidence/facts/figures from various different papers – this shows you have a good understanding of academia. The only area to improve would be trying to be more concise in your writing. There’s some repetition in places but you also need to rein yourself in with how much you include. The essay title is quite broad so you need to focus it in on your interests or what you find the most interesting. That being said, this essay will be an excellent tool for you revise from. Well done and keep up the good work.

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