Built-up area and population density: Two Essential Societal Variables to address climate hazard impact

Highlights: • This paper introduces two essential societal variables (ESV) – global built-up area and global population density. • The two ESVs are the building blocks for quantifying the human societal system as they measure the human presence on Earth. • The two ESVs complement the essential climate variables in modelling climate impact on societies. • The two ESVs are already used in disaster early warning systems, within disaster risk models, in system of indicators and in crisis management. • The two ESVs are also tested for use in system of indicators used to measure progress towards the 2030 Development Agenda. Scientists use Essential Climate Variables to understand and model the Earth's climate. Complementary to the Climate Variables this paper introduces global built-up area and population density, referred to as Essential Societal Variables, that can be used to model human activities and the impact of climate induced hazards on society. Climate impact scenarios inform policy makers on current and future risk and on the cost for mitigation and adaptation measures. The global built-up area and global population densities are generated from Earth observation image archives and from national population census data in the framework of the Global Human Settlement Layer (GHSL) project. The layers are produced with fine granularity for four epochs: 1975, 1990, 2000 and 2015, and will be updated on a regular basis with open satellite imagery. The paper discusses the relevance of global built-up area and population density for a number of policy areas, in particular to understand regional and global urbanization processes and for use in operational crisis management and risk assessment. The paper also provides examples of global statistics on exposure to natural hazards based on the two ESVs and their use in policy making. Finally, the paper discusses the potential of using population and built-up area for developing indicators to monitor the progress in Agenda 2030 including the Sustainable Development Goals (SDGs).