Few aspects of human history are as mindboggling as the sudden disintegration of advanced societies. It is tempting to seek common patterns or even draw some lessons for modern times from the many ancient cases of societal disintegration. In PNAS, Downey et al. (1) report that universal warning signals of reduced resilience systematically preceded the collapse of Stone Age societies. Might similar indicators of fragility be relevant in modern times? Of course, the nature of human societies has changed entirely. However, there are at times striking parallels between stories of collapse even if they happened in entirely different periods. Consider the abrupt abandonment of the iconic alcove sites in Mesa Verde by the ancestral Puebloan people: the greatest “vanishing act” in prehistoric America (2). Archaeological evidence now reveals that before Pueblo peoples massively migrated in the mid-to-late 1200s, there had actually been a slow build-up of tension (3, 4). Over a century of drought, violence, and political turmoil drove increasing numbers of people into the Mesa Verde region, which was relatively productive for farming, straining carrying capacity as well as cultural traditions and resulting in destabilizing conflicts. Portions of the northern Southwest began to empty out in the first decades of the 13th century and by the mid-1200s even the favored central Mesa Verde region was starting to lose population, well in advance of the “Great Drought” beginning in the late 1270s that seems to have given the final blow. Now, Syria is the scene of a sudden massive exodus, and some aspects of the complex situation do seem to echo the Pueblo story. The Fertile Crescent has likely been experiencing the worst drought in 900 y, making subsistence farming in the countryside extremely challenging and driving millions in Syria to the cities, where tensions increased …
Concern has been spreading across scientific disciplines that the pervasive human transformation of Earth's natural systems is an urgent threat to human health. The simultaneous emergence of “GeoHealth” and “Planetary Health” signals recognition that developing a new relationship between humanity and our natural systems is becoming an urgent global health priority—if we are to prevent a backsliding from the past century's great public health gains. Achieving meaningful progress will require collaboration across a broad swath of scientific disciplines as well as with policy makers, natural resource managers, members of faith communities, and movement builders around the world in order to build a rigorous evidence base of scientific understanding as the foundation for more robust policy and resource management decisions that incorporate both environmental and human health outcomes.
The aggregate human impact on the environment now exceeds the limits of absorption or regeneration of various major biophysical systems, at global and regional levels. The resultant global environmental changes include altered atmospheric composition, widespread land degradation, depletion of fisheries, freshwater shortages, and biodiversity losses. The drive for further social and economic development, plus an unavoidable substantial increase in population size by 2050--especially in less developed countries--will tend to augment these large-scale environmental problems. Disturbances of the Earth's life-support systems (the source of climatic stability, food, freshwater, and robust ecosystems) will affect disproportionately the resource-poor and geographically vulnerable populations in many tropical countries. Ecological disturbances will alter the pattern of various pests and pathogens in plants, livestock and humans. Overall, these large-scale environmental changes are likely to increase the range and seasonality of various (especially vector-borne) infectious diseases, food insecurity, of water stress, and of population displacement with its various adverse health consequences.
Climate change poses threats to human health, safety, and survival via weather extremes and climatic impacts on food yields, fresh water, infectious diseases, conflict, and displacement. Paradoxically, these risks to health are neither widely nor fully recognized. Historical experiences of diverse societies experiencing climatic changes, spanning multicentury to single-year duration, provide insights into population health vulnerability—even though most climatic changes were considerably less than those anticipated this century and beyond. Historical experience indicates the following. (i) Long-term climate changes have often destabilized civilizations, typically via food shortages, consequent hunger, disease, and unrest. (ii) Medium-term climatic adversity has frequently caused similar health, social, and sometimes political consequences. (iii) Infectious disease epidemics have often occurred in association with briefer episodes of temperature shifts, food shortages, impoverishment, and social disruption. (iv) Societies have often learnt to cope (despite hardship for some groups) with recurring shorterterm (decadal to multiyear) regional climatic cycles (e.g., El Niño Southern Oscillation)—except when extreme phases occur. (v) The drought–famine–starvation nexus has been the main, recurring, serious threat to health. Warming this century is not only likely to greatly exceed the Holocene’s natural multidecadal temperature fluctuations but to occur faster. Along with greater climatic variability, models project an increased geographic range and severity of droughts. Modern societies, although larger, better resourced, and more interconnected than past societies, are less flexible, more infrastructure-dependent, densely populated, and hence are vulnerable. Adverse historical climate-related health experiences underscore the case for abating human-induced climate change.