Health impacts in Australia in a four degree world

Two plus two equals four, though not when considering impacts in a warming world. Much of the recent modelling of the impacts (physical, ecological and biological) of human-induced warming has assumed global temperature increases this century of the order of 2°C. But what of the impacts at around 4°C? The magnitude of risks to human health in a world that is an average of 4°C warmer than in pre-industrial times (circa ad 1750) will not be simply twice the risks due to 2°C warming. Biological organisms and ecological systems are not simple mechanistic Newtonian devices. When living entities are exposed to increasing external stressors, the increase in impact is generally not linear - indeed, a different type of impact can emerge once a critical threshold of stress is passed. As the average temperature rises in Australia, there will be a similar upwardturning escalation of climatic impact on many aspects of human health and wellbeing. Assorted disorders, diseases, distress and deaths will occur at much higher rates. The non-linear risk increase, the likely exceeding of some thresholds and uncertainty about future congurations of other social and environmental inuences on health outcomes preclude a comprehensive projection of climatic impacts on health and survival in an Australia where the global average temperature has risen by 4°C or more (a Four Degree World). Guesses and highly qualied speculations are possible, but we must recognize that we are moving even further ‘into the unknown’ when contemplating a future Four Degree World and the resultant, potentially very damaging, changes in environmental and social conditions. To start at the simpler end of the health-impact spectrum, we can be condent that the projected increase by one or more orders-of-magnitude in the frequency of very hot episodes in a 14°C Australia (see Braganza et al., 2013, Chapter 3 in this volume) will have serious health consequences - especially in a population that may already be carrying into late adulthood the detrimental health-eroding legacy of having been overweight and obese in childhood more than a halfcentury earlier. Similarly, 14°C warming will greatly increase the other great weather extremes for which the young and homesick Dorothea Mackellar pined in her iconic poem ‘My Country’, written in London a century ago. Deaths, injuries, poverty and mental trauma will, as ever, result from various disasters of ood, drought and re. Projection of other future health risks in a 14°C climate then gets tougher. A Four Degree World could seriously erode the essential foundations of human population health - food yields, water supplies and the constraints on infectious disease rates, population displacement, conict and warfare. But will various infectious diseases, especially those spread by ‘vector’ mosquitoes, become resurgent and extend their geographic range in Australia? (Or might some part of Australia become too hot and perhaps too dry to support such vectors and the lifecycle of particular infectious agents?) An even more complex question is: if climatic extremes coincide with very great increases in numbers of displaced persons from the adjoining Asian and Pacic region, perhaps unavoidably housed in emergency, poorly resourced settlements, might Australia be faced with unusual epidemics of cholera and rampant childhood diarrhoeal diseases? Further, might our healthcare system and public health activity become overwhelmed and inadequate to the mounting task of risk reduction, damage repair and recovery? These are complex multi-faceted situations in which many concurrent inuences other than altered climatic conditions affect human biology, psychology and health. Projecting the rate at which ice might melt at higher temperatures, or how coral reefs are likely to respond to steadily increasing temperatures, is an inherently simpler task than projecting human health outcomes within the ‘noisy’ setting of unknowable trends in other risk factors, including variegated cultural practices, consumer behaviours and other external exposures. Clearly, this chapter cannot offer a fully itemised schedule of estimated future health impacts. The risks to human health and wellbeing from changes in climatic conditions are wide-ranging, diverse and differ greatly in character and complexity of causation. For most of those risks climate change will be a multiplier or amplier of risk, not its initiator. Only some risks are amenable to formal modelling, particularly when considered within such a qualitatively different future Australia and future world. For small changes in climate, a simpler extrapolation of impact levels in recent past experience to the postulated future is reasonable. But for larger climatic changes and all that will ow from them, this conventional approach is not warranted. We should instead pay more attention to the range of human impacts of a diverse but plausible range of future conditions and scenarios. Accordingly, we should be readier to make policy decisions on a precautionary basis that recognises the extremes of adverse human consequences that could result. A simple entry point for understanding these limitations to future high-exposure risk modelling is to review the experience of Europe in the hot summer of 2003.