Surviving writings describing the veiling of the solar radiation during and after AD 536 largely originates from the Mediterranean sources once created by court historians and chroniclers. In these writings the sun was observed blue-colored, without brightness, spring without mildness, summer without heat10. The overriding reason for these anomalies was the mystery cloud, a persistent dry fog that darkened the sky, the cloud that was observed by contemporaries over wide areas across the Palearctic all the way from the British Isles to China11,38,39. Palaeoclimate literature has for long attributed the cloud to volcanic aerosol emissions6,7,8. Moreover, the anomalous climates during the event have been largely described in terms of cold summers8 such conditions having probably lasted as a protracted, at least decadal event much over the Northern Hemisphere12. An essential point is that the existing palaeoclimatic inferences have thus far been extracted from tree-ring width/density chronologies that are proxies for past summer temperatures. However, the temperature effects remain subordinate to the primary diagnosis, the opaque skies and the vastly reduced sunlight under them11. As a consequence, the survey of the climate processes during the event has remained, at best, one-sided, and somewhat biased towards its temperature characteristics which, albeit playing an important role, may actually represent secondary effects. By contrast, the δ13C data we have presented facilitate the first quantification of sunlight conditions from year to year during the dust veil episode and make it possible to reconstruct the markedly varying solar radiation from subfossil tree rings. Importantly, our results provide several aspects of volcanic summers independent of temperature effects and contribute to our understanding of the event as a multifaceted climate crisis during which the adverse effects of cold temperatures may have been reinforced by strong reductions in irradiance with the hardship of rapid climate change on human societies.

Away from key regions of written evidence our results still demonstrate the dust veil to have resulted in low levels of irradiance not only in AD 536 but subsequent to the second (AD 540) (ref.2) eruption with reductions in irradiance nearly three standard deviations below the mean, and a continuation of low irradiance over the coming two decades. This evidence overlap with indications of cold summer temperatures persisting from AD 536 to 545 (ref.12), probably until the AD 570 s (ref.14), and suggests that the dust veil may have acted as a radiative forcing agent for the decadal cooling. We also note that previous literature has depicted coinciding δ13C anomalies in Siberian tree rings which may be an indication of extended spatial scale of the event, albeit the actual reasons behind the anomalies in Siberian data remain to be reconstructed40. Historical accounts of the dust veil concentrate on an 18-month period from March of AD 536 until June of AD 537 (refs10,11). The δ13C proxy data suggests an abrupt reduction in irradiance in AD 536, with even a more prolonged anomaly after the second eruption. We assume these patterns to reflect the potential spatiotemporal regimes in aerosol forcing through the anomalous AD 536–545 decade that are likely to differentiate the two eruption signals2. While the subarctic regions provide no documentation from that time the existing archaeological evidence from sites adjacent to ours do indicate crop failures, demographic catastrophe41, disruption of settlement, and population displacement42. Moreover, climate modelling demonstrates cold summer temperatures having decreased the agricultural production over the same areas, at least over the AD 536–545 decade13. In areas central to the mystery cloud observations, however, the plant products are less limited by temperature19. Clearly, some factor other than low temperature should have played a role in generating the severe damages to crop yields described in the written Mediterranean sources during the same years. Such damages are increasingly documented in the eastern Roman Empire around AD 536, recorded as the highest peak in the number of famines for that region and period, in the context of the Roman and post-Roman world from 100 B.C. until AD 800 (ref.15). In Mediterranean-type environments, the sum of solar radiation increases the growing season length and the production of crop dry matter depends primarily on the amount of solar radiation43. Our results add to this stream of research by quantifying low irradiance during the years topical to the human consequences thus providing regulatory mechanism to crop reductions with direct links from volcanic dust veil to food crises over wide geographical scales.

Yet another factor possibly causing the crop failures and famines was the drought described in the written sources, according to which the winter (conceivably that of AD 536/537), or possibly later part of the year, was rendered dry and without storms10. Our δ13C data remain sensitive to summer climate27 and cannot comment on any wintertime anomalies. Moreover, the coming of drought would not be consistent with hypothetical post-eruption hydroclimate summertime responses, expected to mimic the configuration of North Atlantic pressure fields during the negative phase of the East Atlantic (EA) teleconnection pattern44, and to result in wet conditions around the Mediterranean realm44,45,46. The EA pattern is the leading mode of climate variability over the North Atlantic and surrounding continents representing a north-south dipole of pressure centers47. The EA is negatively correlated with instrumental precipitation/soil moisture across the Mediterranean, the respective correlations in the region of our δ13C evidence remaining slightly positive but non-significant during the summer months44,48 (see Fig. S5 for correlations with could cover). Assuming that the mid-sixth century eruptions were followed by similar, negative EA phase (there are currently no palaeoclimate EA reconstructions available for the first millennium), the presented correlations44,48 would agree with our palaeoecological model, suggesting a negligible hydroclimatic response over the tree-ring sites during the event years.

The years of strongest sunlight deficiency coincides remarkably with the years of Justinianic plague (AD 541–544), the first historically recorded outbreak of true plague that ravaged the Mediterranean world9,49,50. After the first epidemic that broke out in Constantinople, the eastern Roman Empire, the pandemic spread widely to the entire Mediterranean and central Europe, as north as Finland51, killing tens of millions until the mid-8th century52. Volcanic forcing triggering the climatic cooling arguably provoked the first impulse to the plague in AD 541 (ref.9). The plague was possibly introduced to Constantinople by rats and fleas infected by Yersinia pestis bacteria travelling on board from Egypt. Moreover, it is hypothesised that the exceptionally cool weather of AD 541 was beneficial to rat survival and flea reproduction. As a consequence, the climatic event appears having played a role in creating an unusual opportunity for animal vectors to cross the Mediterranean52. The recent historical evidence for coinciding high number of famines over the same region suggests that the nutritional background may have contributed to the explosion of the plague15. The interval of strongest sunlight deficiency in our reconstruction, AD 541–544, coincides strikingly with the years of the Justinianic plague and help to explain the environmental drivers of the chronic food shortages. In addition to undernutrition, we hypothesize that the death toll may have been raised by adverse effects from deficiency symptoms of vitamin D. Photosynthesis of the vitamin D in human skin occurs only when a certain threshold of incident solar radiation is exceeded and can halt completely even at the equator under a very thick overcast cloud53. Topically, adequate vitamin D status is important for overall health and well-being with positive effects on the immune system, for example, in the case of bacterial infections54. Atmospheric aerosols attenuate incoming solar ultraviolet radiation at the Earth’s surface53 and the volcanic dust veil such as that indicated from our reconstruction between AD 541 and 544 could thus have contributed to human consequences by environmentally modifying the chemical reactions directly in humans. Although no climatic proxy may be adequate to validate such effects on vitamin status, it seems plausible that the photosynthesis of the vitamin D may have been reduced in human skin during the irradiance anomaly. Translating into suboptimal vitamin D status, the low levels of irradiance under the volcanic dust veil may have hypothetically predisposed the contemporaries to bacterial infection by Yersinia pestis at the same time the cold climates assisted the bacteria to land the Roman Empire.

Collectively, our data confirm abrupt changes to the growth seasons (i.e., summers) following the large volcanic eruptions in AD 536 and 541–544 in the form of cooling and, more importantly, strong reduction in incoming solar radiation. During and after these events, the cooling was likely driven by the dust veil and photosynthetic products were limited to such an extent that they likely affected food security and human immune system. These findings add to our knowledge of volcanic aerosol forcing and emphasize their importance with respect to the temperature-related scenarios frequently described in the literature. Understanding the multifaceted environmental impacts of ancient explosive eruptions requires the use of proxy data that are sensitive to variable irradiance levels and which faithfully track this vital component of a productive ecosystem. Our results underscore the pressing need for a database of tree-ring isotope chronologies and archaeological/historical records in order to investigate the relationship between irradiation and human society. Only these data can describe the spatial and temporal variation in volcanic aerosol emission forcing from one event to another and allow them to be compared to accounts from the regional to continental scale. Nevertheless, we endorse the need of combining the data of climate forcings with thorough analyses of political and economic structures15,16,49,50 of which determinants have almost certainly contributed to the event. Such a comparison would reveal the relative impact(s) of climatic forcing on agriculture, human health, urbanisation and movement during the first millennium – a period considered to contain the main ‘hinges’ of human history55.