Examining the role of solar activity, climate, and the socio-historical context in high all-cause mortality (northern Portugal, 1700–1880)
The far-reaching impact of the Sun on Earth's climate and on people's health and well-being is a poorly understood and non-consensual scientific issue, with empirical literature stressing the need to expand the knowledge of such relationships. Here, the interplay between solar activity (SA) and climate, and its likely cascading effects on all-cause mortality, were examined at several time scales. To this end, the parish records of Braga (1700–1880) and Torre de Moncorvo (1700–1850), in two different geographical locations of northern Portugal (Iberia, SW Europe), were used. Crude mortality rate (CMR) and winter–summer ratio (W/S) values were computed to characterize mortality patterns/trends and couple them with potential relevant drivers: total solar irradiance (TSI) as a proxy of SA, the North Atlantic Oscillation (NAO), and key historical events. What emerged, albeit incomplete, was a complex picture of death deeply embedded in people’s physical and socioeconomic environments, at a time when ubiquitous poverty (and co-morbid malnutrition) was the most inveterate cause of ill health. After identifying the positive mortality episodes in both municipalities, their incidence was found to be higher in periods of weakened SA (normal/grand minima). Standard inference statistics were used to estimate the significance of the observations. The highest CMR peaks matched not only with wars but also with known wide-ranging mortality crises, which seem to have been triggered by major agricultural production shortfalls, followed by substantial increases in food prices, driven, in turn, by climate deterioration, including extreme weather occurrences. The outcome was social unrest, famines, and outbreaks of infectious diseases, heightening the death toll. The influence of prominent solar/climate variations was investigated using wavelet transform coherence analysis (WTC). The results showed (multi)decadal oscillations in both (TSI and NAO) somehow regulating mortality. But the WTC analysis also estimated SA signals in low-frequency mortality dynamics disguised by time-varying determinants, where distinct players of space weather might have been implicated.
Several data sources and methodologies were combined to explore the role of the physical and socioeconomic environments in explaining mortality patterns/trends (and peaks) in two pre-industrial Portuguese municipalities (1700–1880). The results of the analyses together with the currently available historical testimonies suggested: (i) (multi)decadal oscillations in solar/climate-related parameters influencing death rates; and (ii) shorter-term detrimental changes in regional climate harmfully impacting agricultural and human (social and biological) systems, which led to increased mortality. Importantly, high all-cause mortality was found to be preferentially associated with periods of weakened SA (quasi-11-year minima “half-cycles” plus grand minima), implying lower levels of radiant energy emitted at all wavelengths, less solar magnetic shielding, and more GCR reaching the Earth. On the face of this, greater frequency of anomalously high mortality over (grand) solar minimum conditions, in Braga and Moncorvo, might have been the aftermath of the cumulative impact of low TSI/high GCR-particle on individuals and their health condition. Two probable causal pathways for such solar influence have been described here. The results of the WTC analysis also highlighted the existence of convoluted relationships between SA, climate, and human mortality (nonlinearity and nonstationarity), concerning the oscillatory modes (periods/frequencies) detected in the analysed data. Additional time series (populational and environmental) and further explorations are needed to support both the accuracy of our inferences/suggestions and their underlying assumptions, leading to a better understanding of these challenging scientific questions. From a historical perspective, major restrictions stand in the way. The under-registration of infant/child deaths (introducing biases in death estimates or in stratified analyses), misleading diagnoses of diseases (preventing the establishment of a parallel with those from more recent times), the advent of vaccination (eradicating many prevalent infectious diseases), and increases in life expectancy (with the emergence of new illnesses associated with ageing and genetic damage) are just a few. To conclude, as the essence of the main subject explored here continues the same, it should not be disregarded by contemporary analysts/policy-makers. Some of their goals might be to make surveillance/monitoring of drivers available (or improve it); formulate and implement strategies to selectively address the problems posed by the Sun–climate interaction on populations – agrarian and non-agrarian livelihoods – and mitigate its demographic consequences (targeting the most vulnerable); and raising (local/global) awareness of these lesser-known natural hazards, particularly among environmental/public health professionals.