A decade of heat waves in two European cities: impacts and perspectives

Climate models for the 21st century suggest that the year-to-year variability of summer temperatures might experience a significant increase in the incidence, intensity and duration of heat waves, particularly at mid-latitudes. Over the last few decades, persistent anticyclonic anomalies have generated numerous heat waves. These are especially deadly in large cities given the heat island effects and physiological vulnerability of the aging population. This study examines the impact of heat waves in two European cities, Paris and London, over the last decade, particularly in the years 2003, 2006, 2010, and 2013. The work addresses in turn: 1) the regional key processes and feedbacks that contribute to extreme heat events and allow us to detect their predictability; 2) the spatial and temporal variability of urban surface temperature and related risks of heat-stress and mortality; and 3) the changes and perspectives in summertime urban climate monitoring, public health surveillance, and warning systems. The methodology is based on the analysis of land use and climate data from in-situ and satellite sensors, and public health statistics. The results highlight: 1) the role of land-atmosphere interactions in the occurrence and intensity of heat waves, in particular, rainfall deficit and high net radiation, which contribute to early green-up and drying of the soil by evapotranspiration; 2) the spatial and temporal variation of urban surface temperature and the contribution of heat islands to the heat wave process, progressively raising the minimum nocturnal temperature, which is linked to heat-stress and mortality; 3) the influence of nighttime temperatures and impact of heat exposure and duration upon the death of elderly people; 4) the relevance of radiant surface temperature in assessing heat related stress; and 5) the cooling impact of urban parks. Over the last decade, new cooling techniques were proposed, some of which were applied in Paris during July 2013. The heat warning systems of Paris and London were adapted according to feedback analyses, especially those of the 2003 heat wave. Observation systems and data assimilation into numerical models and media devices were implemented. These developments allow a better anticipation of the social and environmental impacts of extreme heat events in cities, introducing new perspectives for climate change adaptation and mitigation strategies.