Thematic Articles

Cities are responsible for more than 70% of global anthropogenic greenhouse gas emissions from buildings, transport, energy, industry, and waste-related sources. Improved urban-scale emission estimates are essential for understanding local trends and providing guidance for mitigation strategies. Current research in cities around the world is focused on establishing more robust methods for quantifying and modeling urban-scale emissions of the most abundant anthropogenic greenhouse gases: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O).

Cities are typically evaluated by metrics involving transportation, energy, and economics, but increasingly, environmental quality and human health are becoming important indicators of safe and habitable cities. Population density and industrialization history have resulted in urban contaminant legacies that can impact the health of urban populations. Integrating environmental assessment with human exposure and health studies is in its infancy, but combined geospatial and geotemporal studies have the capacity to explain and predict the health of urban environments. Studies integrating metal geochemistry with human health impacts reveal the complicated layering of environment, exposure, uptake, and human health in cities, and they call for more effort towards the integration of Earth and health science data.

Over the last two decades, long-term ecological research in the United States has expanded to urban sites. Cities, despite the dominance of built structures, utilize unexpected amounts of human-generated nutrients. Additionally, cities can both intensify and weaken local impacts of processes such as climate. Challenges remain at these sites, as property-scale biogeochemical forcings from individuals and institutions must be accounted for throughout this research. Meeting the challenge is crucial as prediction of biogeochemical processes is fundamental to the development of sustainable strategies for managing human inputs to cities and surrounding areas.

Urban environments significantly alter physical and chemical hydrogeologic settings. The physical alteration of the landscape can change recharge, groundwater flow dynamics, and local water balances. Microbial contamination of water sources due to wastewater is an everpresent threat, but contamination by metals and industrial compounds is a long-term concern in cities with industrial economies. The hydrogeologic setting and the age and wealth of a city are important factors influencing the magnitude of the impact on and the recovery of a hydrogeologic system from urban activities. Urban environments can have unique influences on water geochemistry, making delineation of site-specific urban geochemical markers necessary to quantify the extent of urban effects on water quality.

Modern cities are affected by multiple sources of contamination and pollution, the effects of which overlap in space and time. Toxic metal contamination, organic pollution, smog, acid rain, and greenhouse gas accumulation are the most widespread legacies of an often uncontrolled growth that has deeply changed the geochemical character of the urban environment over the last four millennia. Even though progress has changed human habits and positively influenced the quality of city life, the past is frequently a hidden source of environmental problems with the potential to affect the health of current and future urban residents.

In a very short period of time, the majority of the human population has become urban, and by 2050 two out of every three people in the world will live in cities. Urban areas are extremely important socially, economically, and culturally, but they also have a profound impact on the environment. In that context, this issue of Elements considers the geochemical significance of 21st-century cities and some of the unprecedented challenges they face.