Urbanization continues to be a transformative process globally, affecting ecosystem integrity and the health and well being of people around the world. Although cities tend to be centers for both the production and consumption of goods and services that degrade natural environments, there is also evidence that urban ecosystems can play a positive role in sustainability efforts. Despite the fact that most of the urbanization is now occurring in the developing countries of the Global South, much of what we know about urban ecosystems has been developed from studying cities in the United States and across Europe. We propose a conceptual framework to broaden the development of urban ecological research and its application to sustainability. Our framework describes four key contemporary urban features that should be accounted for in any attempt to build a unified theory of cities that contributes to urban sustainability efforts. We evaluated a range of examples from cities around the world, highlighting how urban areas are complex, connected, diffuse and diverse and what these interconnected features mean for the study of urban ecosystems and sustainability.

Human modification and management of urban landscapes drastically alters vegetation and soils, thereby altering carbon (C) storage and rates of net primary productivity (NPP). Complex social and ecological processes drive vegetation cover in cities, leading to heterogeneity in C dynamics depending on regional climate, land use, and land cover. Recent work has demonstrated homogenization in ecological processes within human‐dominated landscapes (the urban convergence hypothesis) in soils and biotic communities. However, a lack of information on vegetation in arid land cities has hindered an understanding of potential C storage andNPPconvergence across a diversity of ecosystem types. We estimated C storage andNPPof trees and shrubs for six different land‐use types in the arid metropolis of Phoenix, Arizona,USA, and compared those results to native desert ecosystems, as well as other urban and natural systems around the world. Results from Phoenix do not support the convergence hypothesis. In particular, C storage in urban trees and shrubs was 42% of that found in desert vegetation, whileNPPwas only 20% of the totalNPPestimated for comparable natural ecosystems. Furthermore, the overall estimates of C storage andNPPassociated with urban trees in theCAPecosystem were much lower (8–63%) than the other cities included in this analysis. We also found that C storage (175.25–388.94 g/m²) andNPP(8.07–15.99 g·m⁻²·yr⁻¹) were dominated by trees in the urban residential land uses, while in the desert, shrubs were the primary source for pools (183.65 g/m²) and fluxes (6.51 g·m⁻²·yr⁻¹). These results indicate a trade‐off between shrubs and trees in arid ecosystems, with shrubs playing a major role in overall C storage andNPPin deserts and trees serving as the dominant C pool in cities. Our research supports current literature that calls for the development of spatially explicit and standardized methods for analyzing C dynamics associated with vegetation in urbanizing areas.