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Abstract:

As climate changes – potentially to a warmer state than any time during the evolution of humans – heat extremes threatening human health, global ecosystem and socio-economic fabric of our society are occurring at increasing frequency and intensity in most parts of the world. This study examines changes in global land area and population exposed to both tails of temperature distribution in changing climate since heat and cold exposure is directly associated with a range of health impacts and affects thermal comfort and occupational capacity. We first utilise the latest ECMWF atmospheric reanalysis, ERA5, to examine changes over the satellite era (since 1979), and then we explore the equivalent changes in CMIP6 archive of historical runs and future projections. Besides daily maximum and minimum of dry-bulb surface air temperature (SAT), we also consider daily extremes of the universal thermal climate index (UTCI) that includes the influence of humidity, wind and radiation encapsulating the synergetic heat exchanges between the environment and the human body. Our analysis dissects changes in spatial and temporal exposure to both heat waves and cold waves and presents metrics contrasting changes in the opposite extremes of SAT and UTCI distributions. We assess the significance of the observed, modelled and projected changes and relate them to external drivers of climate change.

Abstract:

A potential glacial lake outburst flood from Lake Palcacocha (Cordillera Blanca, Peru) threatens Huaraz, a city of 120,000 people. In 1941, an outburst flood destroyed one-third of the city and caused at least 1,800 fatalities. Since pre-industrial times, Lake Palcacocha has expanded due to the retreat of Palcaraju glacier. Here we used observations and numerical models to evaluate the anthropogenic contribution to the glacier’s retreat and glacial lake outburst flood hazard. We found that the magnitude of human-induced warming equals between 85 and 105% (5–95% confidence interval) of the observed 1 °C warming since 1880 in this region. We conclude that it is virtually certain (>99% probability) that the retreat of Palcaraju glacier to the present day cannot be explained by natural variability alone, and that the retreat by 1941 represented an early impact of anthropogenic greenhouse gas emissions. Our central estimate is that the overall retreat is entirely attributable to the observed temperature trend, and that the resulting change in the geometry of the lake and valley has substantially increased the outburst flood hazard.