Reducing Global Catastrophic Risks from Unseen Climate Extremes

Climate extremes can cause devastation. The most severe events have killed millions and, through complex cascades, resulted in profound additional impacts, for example the collapse of East Pakistan following the 0.5 million deaths from the 1970 Bhola Cyclone. As the climate warms to a level not experienced in human history, the potential for unprecedented extremes is also growing, either from familiar hazards like heatwaves reaching unfamiliar intensities, or the arrival of events that are new in character -- for example the first time a powerful tropical cyclone ravages the highly-developed shorelines of the Persian Gulf. Such novel threats are 'unseen': they sit outside all human experience, and their potential impacts are, therefore, highly uncertain. Upward extrapolation of the consequences from the most similar events in history may provide a very conservative estimate of what to expect, as the element of surprise could cause even greater impacts on a society that is unprepared. The potential for novel climate extremes to manifest as Global Catastrophic Risks (GCRs) -- with world-wide impacts, killing at least 10 million people or causing at least $10 trillion of damages -- therefore deserves urgent attention. The most concerning unseen extreme on the horizon is the emergence of heatwaves so severe that, for the first time in human history, the atmosphere becomes a heat source rather than a sink over large, well-populated regions. Such conditions mean that regardless of fitness, levels of hydration, or access to fans, prolonged exposure (in the order of hours) would be deadly. We do know that these events will occur given sufficient global warming, and that they may become widespread as early as this century; but precisely where is at risk and how soon the threat may emerge is not well understood. The potential societal impacts of such an event are almost completely unknown. For example, is unprecedented mortality inevitable if the atmosphere becomes a heat sink? If so, might it trigger mass migration due to place abandonment? Or could the most at-risk communities become resilient to such extreme heat? The Fellowship will address these critical research gaps in our understanding of unseen heatwaves. Through an ambitious program of physical science it will provide new insight into how much warming is required for regions to experience heat beyond human tolerance. Focussing on one of the hottest cities in the world (Jacobabad, Pakistan), it will grapple with the complexity of how heat at regional scales manifests within people's homes, finally joining up future climate model projections with the conditions that people are likely to experience within their community. Through an equally ambitious program of social science research, the Fellowship will also tackle the question of how resilient communities may be to these future heatwaves, and the extent to which that could be strengthened through adaptation. At the same time, the interdisciplinary Fellowship team will also zoom out to explore what other unseen climate extremes most threaten our future, and which -- perhaps with GCR potential -- should be prioritised for urgent risk reduction efforts. This challenge of identifying novel threats is particularly difficult because it requires that we overcome the cognitive bias of history to identify the much wider set of extreme events that could have occurred, whilst also accounting for a changing climate. It will be overcome by the Fellowship using 'top-down' (hazard-centred) techniques, whereby historical events and climate model simulations are used together to map out the domain of physically-plausible threats. Bottom-up (vulnerability-centred) approaches will in turn inspire and narrow the search of this very large phase space. These perspectives, once refined, will constitute a new framework to systematically identify, and reduce risk, from the most dangerous unseen climate extremes.