Global rise in urbanites: The heat is on

Background/Question/Methods . Human societies in both developed and developing countries across the globe are being transformed by an unprecedented rise in urbanization. The most rapid period of urban growth in human history will occur over the next 20 years. As of 2009, more than 50% of the world’s population live in cities. China, for example, home to 400 urban agglomerations, has the world’s largest population (over half a billion people) that is concentrated in cities. While the drivers of urbanization are usually expressed in economic terms, the long-term trade-offs are complex and uncertain. In this talk we focus on a important topic for current and future urban dwellers: the world’s climate is changing and potential increases in global aridity could have serious socioeconomic and environmental consequences for urban areas. We present the latest UN data on populations and urbanization trends and compare these to climate observations and model simulations for 1948-2100. We find that global aridity has expanded in the last 60 years and will continue to expand in the 21st century. (Aridity is defined here as the ratio of precipitation to potential evapotranspiration.) In our scenarios we compare aridity in urban regions under Current (1981-2010) conditions to Imminent-Future (2011-2040), Near-Future (2041-2070) and Far-Future (2071-2100) projections. Results/Conclusions .  As of 2014, of nearly 1700 global cities with a population of over 300K (ca. 2.2 billion total people), 55%, 71% and 72% cities become drier in the Imminent-Future, Near-Future and Far-Future scenarios, respectively (compared to 14%, 18% and 18% becoming wetter). These drier cities contain 70% of the world’s urban population (based on 2014 estimates). We compare spatial variations in water stress, the ratio of water withdrawal (for irrigation, livestock, domestic and industrial use, etc.) to water availability (average water availability over 30-year period) for these urban areas to address the aforementioned trade-offs between economic gains of urbanization to potential environmental, political and social consequences. Yes, the heat is on.