## Introduction

Throughout Earth’s history volcanic eruptions have perturbed the climate and caused regional and global environmental disasters, notably by creating a sunlight reflecting aerosol layer in the stratosphere that can persist for many years. In 1991 the Mount Pinatubo eruption in the Philippines released nearly 20 Mt of sulfur dioxide (SO2) into the stratosphere which oxidized into sulfate aerosols, which then caused a global cooling of the oceans of about 0.3 °C and prolonged an El Nino event1,2,3,4. In 1815 the Tambora eruption in Indonesia injected three times more SO2 into the stratosphere, which produced 0.7 °C global cooling, with profound environmental impacts, including the “year without a summer” in 18165,6,7. Following the Tambora eruption, anomalous cooling and reduced precipitation provoked crop failure, famine, and the outbreak of diseases such as cholera in North America, Europe and Asia. The Toba supervolcano eruption at 74 ka has been the largest natural disaster known in the past 2.5 million years8. It injected up to 100 times more SO2 into the stratosphere than Mt Pinatubo, and climate model simulations suggest a global cooling of 3.5–9 °C, and up to 25% reduction in precipitation9,10,11,12. Due to the scale and magnitude of such volcanic winter effects, several supereruptions have had a distinct footprint in planetary history, and need to be accounted for in theories of human evolution, notably of Homo sapiens in the past 200 ka.

The volcanic winter concept was consolidated after it was implicated as an important evolutionary factor, often linked to the Toba eruption. A common theory of the origin and dispersal of modern humans suggests that Homo sapiens spread from a single tropical African source, though experienced dramatic reduction in effective population size down to 1000–10,000 individuals13,14. According to DNA analysis, a population bottleneck happened around 50–100 ka15,16, during the peak of the glacial stage (δ18O 5a-4), after climate transitioned from a relatively warm to a cold state, i.e., during glaciation. Based on the coincidence of these two factors with the supervolcano eruption, the Toba catastrophe theory was put forward to explain the human genetic bottleneck as well as the onset of the glacial period13,14.

Recently, the Toba catastrophe hypothesis has become disputed and the paleoanthropological support has dwindled notably from progress in genomics, improved climate modeling, and new paleoenvironmental core samples from eastern Africa. With the advancement of whole-genome sequencing, the possible bottlenecks are now dated around 50 ka15,17 and 130–150 ka18 and could also be explained via the founder effect (small size of the founding population)19 in addition to a population reduction. While these studies and the lack of consensus between genomics interpretations do not exclude the bottleneck around the Toba event horizon, none of the wide range of dates pinpoints the bottleneck exactly to the Toba eruption at 74 ka.

The petrological estimates of the sulfur emissions from the Toba eruption is another line of argumentation that has questioned the catastrophe theory. Analyses based on the melt inclusion technique suggested that the Toba eruption was anomalously sulfur-poor (as low as 3–6× Pinatubo), while being extremely halogen-rich at the same time (as high as 300× the average Central American Volcanic Arc eruption, considered to be particularly halogen-rich)20,21,22. This low, controversial estimate of the sulfur emissions, which implies a non-catastrophic magnitude of the volcanic winter, is not Toba specific but is a part of an emerging “sulfur paradox”. For almost all volcanic eruptions during the satellite era, the petrologic indicators and sulfur yield in magma predict SO2 emissions at least 1 order of magnitude smaller than measured using remote sensing methods23,24. The discrepancy between petrologic and remote sensing estimates is probably due to the presence of an exsolved vapor phase in the magma before eruption23,25,26 and petrologic estimates therefore underestimate the total erupted sulfur amount. While the "missing" sulfur source remains to be better understood and quantified to resolve this paradox, a sulfur-rich supereruption around the Toba event horizon (up to 100× Pinatubo) is evident from multiple climate proxies: the analyses of ice cores from both hemispheres27,28 and the integral ocean cooling from studies of coral reefs29,30.

Paleoclimatic reconstructions from Lake Malawi sediments in tropical Africa and the simulated magnitude of the volcanic winter climate perturbations are the least conflicting among arguments that discredit the Toba catastrophe theory. Climate model simulations do not corroborate the initiation of glaciation10,31, but substantiate the global extent of strong climate cooling. The recent global climate simulations with improved representation of stratospheric chemistry and aerosol microphysics mechanisms have shown that the volcanic winter effects are significantly less extreme than assumed originally11,12,32. The new simulations suggest that the global mean cooling could peak at 3.5 °C (rather than 15 °C, assumed previously) and that the sulfate aerosol optical depth (which causes the radiative forcing of climate) returns to background levels within 4–5 years (rather than 20 years). This magnitude of the simulated surface cooling represents the lower limit of many model studies, which is in line with the recent analyzes of the lake Malawi sediments. The sediment core analysis, although with a resolution of 9 years per record, substantiate reduced precipitation and consequent vegetation perturbation after the Toba eruption, but do not confirm a cooling event that could have been catastrophic for humans19,33.

Here we present a complementary, solar ultraviolet (UV) radiation volcanic catastrophe theory, which highlights that the volcanic impact is represented by a spectrum of environmental stresses rather then by climate cooling and drying only. We show that the Toba volcanic plume caused massive ozone (O3) loss and produced hostile UV radiation conditions at the Earth’s surface, particularly in the tropics. To account for uncertainties, we cover the range of sulfur emissions (1–100× Pinatubo) to illustrate that these environmental and evolutionary stress factors are general and do not strongly depend upon the magnitude of the model simulated or sediment-analyzed volcanic winter effects.

## Results

In view of the fundamental life-protecting role of stratospheric ozone on our planet, several environmental disaster options and catastrophic ozone loss mechanisms have been considered. For example, in a nuclear winter scenario smoke and soil particles from explosions and fires are expected to reach the stratosphere, leading to local heating from the absorption of sunlight (by black carbon) and cooling the Earth’s surface34,35. The stratospheric heating allows more water vapor to cross the tropical tropopause, which accelerates catalytic ozone loss by hydrogen and nitrogen containing species35. Catalytic ozone loss by anthropogenic halogen (chlorine and bromine) compounds has been identified as the cause of stratospheric ozone depletion in the past half century, resulting in the recurrent exceptionally depleted ozone conditions over Antarctica during austral spring, when column O3 drops below 220 Dobson Units (DU), reaching minima below 125 DU36. The catastrophic ozone loss over Antarctica results from the heterogeneous chemical interactions between halogens and polar stratospheric clouds that form in this extremely cold part of the stratosphere. Such chemistry also takes place on sulfate particles, and large volcanic eruptions that inject both sulfur and halogen species are expected to contribute to ozone loss21,37,38. The halogen loading from volcano eruptions can vary greatly depending on the composition of the magma. For example, Mt Pinatubo did not release much halogen containing compounds into the stratosphere, whereas large explosive eruptions in Central America, such as that of Los Chocoyos at about 81 ka, may have led to exceptionally depleted ozone conditions, characterized by a column abundance less than 220 DU21,22.

Here, we consider an additional volcanic and radiation attenuation mechanism that influences ozone and surface UV radiation. Chemical ozone production is highest in the lower tropical stratosphere, from the photolysis of oxygen by solar photons with a wavelength λ < 242 nm, because at low latitudes shortwave solar radiation is particularly intense. During volcanically quiescent conditions, absorption by O3 and molecular Rayleigh scattering control radiative transfer at these wavelengths. But massive tropical volcanic eruptions such as that of Toba give rise to large amounts of SO2 and sulfate aerosols in the stratosphere, which strongly enhance the UV extinction optical depth. The SO2 cloud strongly absorbs UV radiation, and weakly absorbing sulfate additionally scatters the photons back to space, which decreases the radiation flux needed to form ozone within and below the volcanic plume39,40. We computed the Toba effects on solar photon fluxes with a detailed radiative-transfer model, showing shutdown of the O2 photolysis and a strong reduction of O3 within the volcanic plume39 (“Methods”). Since the spectral absorption of SO2 overlaps with that of O3, both the photochemical sources and sinks of ozone are perturbed under these conditions, which moreover delays the conversion of SO2 into sulfate, sustaining the plume lifetime but reducing the climate impact in the tropics12,41,42.

To quantify the combined radiative, chemical, and circulation impacts of the Toba eruption, we applied the NASA-GISS ModelE global interactive atmospheric chemistry-climate model, assuming an SO2 injection into the stratosphere of about 100 times that of Mt Pinatubo (“Methods”). Our simulations indicate massive ozone depletion that peaks at about 20% globally in half a year, and recovery to near-background levels in three years. This anomaly was most devastating at low latitudes where column ozone declined by more than a factor of two, from >250 to <125 DU (Fig. 1). In the Methods section we present sensitivity calculations of SO2 injections that are up to orders of magnitude smaller, showing that the impact is not strongly dependent on the size of the eruption. With a 20-times Pinatubo simulation, we still find a tropical ozone depletion that is more severe and much larger than the current one over Antarctica, with a duration of nearly a year, and a column O3 minimum below 175 DU for several months. Subsequent to the Toba perturbation of the tropical stratosphere, advection by the large-scale overturning (Brewer–Dobson) circulation redistributed the plume toward the extra-tropics, which reduced UV radiation penetration to the surface at those latitudes, but enhanced it in the tropics (Fig. 2). Figure 2 shows that the aerosol optical depth (AOD, being the sunlight extinction) maximized outside the tropics, especially in the Northern Hemisphere where the Brewer-Dobson circulation is strongest. Therefore, volcanic winter conditions were most severe poleward of 20° latitude, consistent with the Lake Malawi sedimentary data analyses that indicate only limited cooling in the tropics19,33.

Conditions 6–12 months after the eruption roughly represent the apogee of the Toba environmental impacts, i.e., in terms of UV exposure and cooling at the surface. At this stage of the volcanic plume evolution the SO2 was mostly converted into sulfate, and the aerosol optical depth at middle latitudes reached a maximum of >5 (Fig. 2, Fig. 3 and “Methods”). This means that only a small fraction of the sunlight could penetrate the plume for about a year during the already cold glacial climate. Furthermore, tropical stratospheric ozone had collapsed. Our calculations indicate that near the equator O3 declined to 125 DU, which translates into a 140% increase of the maximum daily UV index (UVI) from a background value of about 12 to about 28 (“Methods”). A UV index larger than 10 is already considered "extreme” by the World Health Organization (WHO) in terms of the need for protection, and causes sunburn damage in about 15 min. Nevertheless, the surface UVI increase due to ozone loss was partially mitigated by the volcanic plume itself. During the initial stage of the eruption, 2000 Mt of emitted SO2 (comparable to the global O3 inventory of 3200 Mt) or 350 DU in tropics, almost entirely blocked UV radiation. Afterward, during the peak AOD stage of the volcanic plume evolution, SO2 was converted into the less efficiently UV-absorbing sulfate aerosols, leading to a partial offset of the ozone depletion and the associated UVI increase. This explains the non-linear behavior as a function of plume size, and the moderate dependence upon the assumed amount of SO2 emitted by the Toba eruption.

The high tropical UV index was perpetuated for about a year, because the gradual weakening of the ozone depletion was counteracted by waning aerosol shielding. At middle latitudes the ozone loading was reduced but did not drop below 275 DU, being close to the climatological value at the equator, while the volcanic plume inhibited UV radiation transmission. It thus appears that the Toba climate and UV impacts were distributed meridionally and acted complementary. The tropics were most strongly affected by highly detrimental UV levels, in part related to the low solar zenith angles, while volcanic winter (cooling and drying), with an intensity much beyond that of Tambora in 1816, caused extreme weather conditions at extra-tropical latitudes.

## Discussion

The UV index increase (i.e., several times above the maximum on the WHO scale) over a period of about a year is likely to have had critical consequences for humanity, although biomedical studies of such conditions are not available to test the specific outcomes. The range of possible UV impacts is extensive, with environmental, ecological, health-hazardous and societal consequences43. In the short term, health risks include eye damage (photokeratitis and photoconjunctivitis) and erythema. Hindered vision (painful inflammatory reactions) and sunburns combined with the decay of precipitation and shortfall of food availability must have significantly worsened survival challenges. In the longer term, the increased carcinogenesis (cataract and skin cancer), immune system suppression, and general DNA damage44 likely added to a potential population decline. Overall, the magnitude of UV-induced health-hazardous effects following the Toba supereruption were more severe and much longer lasting than those during the recent exceptionally depleted ozone conditions, located over populated land (contrary to Antarctica), including most of Africa, and several times larger than during the aftermath of a massive hypothetical nuclear conflict34,35.

Although it remains difficult to quantitatively determine the environmental impacts of a supervolcano eruption at 74 ka, it is unlikely that we have underrated the role of a tropical stratospheric ozone depletion. The physical mechanism of radiation extinction and O3 formation is straightforward, and even with a much-reduced amount of stratospheric aerosol our results are robust. Furthermore, we have not included the chemical impact of halogen compounds, which catalytically destroy ozone, as there is no reliable information on their emissions from Toba. While the indications that the Toba eruption was halogen-rich are weak, some fraction of these compounds will have been co-emitted and will have aggravated ozone loss22, hence our results represent a lower limit of the UV increase. For example, we can compare the Toba eruption with that of Krakatau in Indonesia, with Toba injecting orders of magnitude more SO2 into the stratosphere, while chlorine emissions from Krakatau may have caused about 7% of column ozone depletion22. Considering the scale of these impacts, it seems likely that halogen-induced chemistry from Toba has added substantially to the depth of the tropical ozone decline.

Our proposed stratospheric ozone and UV calamity extends the concept of the volcanic winter effects and offers a way to reconcile the seemingly conflicting data about classical volcanic winter conditions, which, for example, have been shown to be insufficiently severe in Africa to bring humanity to the verge of extinction19,33. The radiation attenuation mechanism from volcanic sulfur emissions, inherent to any volcanic eruption, presents a plausible ozone-UV scenario. We note the role of volcanic halogens, which enhance the ozone loss21,45,46 in addition to the radiation mechanism proposed here.

In a broader view, the ozone-UV scenario augments the classical volcanic winter theory. For example, our findings are in line with and complement the emerging evolutionary role of stratospheric ozone across the much wider time scales extending to the end-Permian crisis47,48. We conclude that limited cooling and drying plus severe biological UV damage in the tropics, combined with harsh volcanic winter conditions in the extra-tropics, could have provided the prerequisites for a population bottleneck. Finally, we recommend that the impact of sunlight extinction by thick stratospheric aerosol plumes on ozone formation are accounted for in future studies of the environmental consequences of explosive volcanoes, nuclear conflicts, and solar radiation management (geoengineering).

## Methods

### Model setup and experiments

We analyzed the volcanic plume evolution and impacts on atmospheric chemistry using the Goddard Institute for Space Studies (GISS) Earth system model (ModelE) simulations. Below we highlight the details specific to this study, while ref. 49 provides a complete description of ModelE and ref. 12 describes the model setup and simulations.

To simulate the volcanic plume evolution, ModelE includes fully interactive chemistry related to ozone50 and sulfate51,52. The eruption of Toba is modeled by the instantaneous injection of 2000 Mt of SO2 (about 100× Pinatubo53) at the beginning of the year. Emitted SO2 is distributed with a mixing ratio of about 11 ppmv in the 20S–20N equatorial belt between 10 and 50 hPa.

### Photochemistry and effects of SO2 and microphysics

Actinic fluxes and photolysis rates were calculated in the presence of cloud and aerosol layers, using the accurate and efficient Fast-J2 scheme54,55. However the original scheme was not designed to capture the effects of volcanic plumes on stratospheric ozone chemistry, and several essential improvements were implemented. Here the full multiple-scattering code was used to compute the radiation field above wavelengths of 200 nm (previously 291 nm, important for O2 photolysis), which replaces the default pseudo-absorption simplification55. Figure 4 shows the radiative effects of the volcanic plume on O2 and O3 photolysis, computed with the reference model (Tropospheric Ultraviolet and Visible radiation model56). SO2 strongly absorbs in the UV part of the spectrum (similar to O3) and inhibits photolytic ozone loss12. Along with aerosols, SO2 was coupled to the photochemistry, and its radiative effects at UV wavelengths were taken into account.

We also incorporated the microphysics of the aerosols and their effects on the spectral optical properties on ozone photochemistry. Coagulation and growth of the sulfate aerosol droplets decreases the extinction efficiency per unit mass for shorter wavelengths. In the visible spectrum, this effect reduces the efficiency of radiative cooling by volcanic aerosols and modulates climate impact. Similarly, we coupled the dependence of aerosol extinction in the UV on the size distribution into the Fast-J2 code. The microphysics, particle growth, and UV scattering efficiency control the volcanic impact on stratospheric ozone. Figure 5 shows that the ozone loss non-linearly reduces with the size of the eruption, indicating that the depth of the tropical ozone depletion does not strongly depend on assumptions about the eruption magnitude.

### UV index

We define the UV index (UVI) as

$${\rm{UVI}}=\frac{11}{17}\frac{1}{25\ {{\rm{mWm}}}^{-2}}\mathop{\int}\nolimits_{286.5\ {\rm{nm}}}^{400\ {\rm{nm}}}I(\lambda )w(\lambda )d\lambda$$
(1)

where I(λ) is the spectral actinic flux and w(λ) is the erythemal weighting function57.

In addition to ozone, optical properties of the SO2 and sulfate aerosols should be taken into account to capture the perturbations of the surface UV radiation. The concentrations of these radiative agents and their effects on UV radiation evolve in time. During the initial stage of volcanic plume evolution, the still unperturbed ozone column and maximum SO2 concentrations almost entirely block the UV radiation, preventing it from reaching the surface. The suppression the O2 photolysis is strongest, but O3 depletion is not instantaneous and equilibration takes time. As the volcanic plume evolves, the surface UVI near the equator increases due to depletion of the ozone (Fig. 1) and SO2 columns (Fig. 3, note the log scale) as it is oxidized into SO4. Approximately 9 months after the eruption, SO2 is almost entirely oxidized (10 DU compared to the initial 350 DU), and O3 column depletion peaks around 40-45% (DU). The effective radius of sulfate aerosols also maximizes after 9 months, reducing the efficiency to scatter the UV radiation. The enhanced SO4 loading partially offsets the increased UVI due to ozone depletion. Together, perturbed O3, SO2, and SO4 combine to produce the largest overall increase in UVI near the equator after about 9 months. During the final stage of the volcanic plume evolution, the surface UVI returns to the background level as SO4 is removed from the atmosphere and O3 recovers.