Special issue: Urban Planet

This Special Issue of Science, "Urban Planet," includes two Reviews, six Perspectives, and a series of new stories that feature the wide range of challenges from and impacts of an increasingly urbanized world.

First, in a Review, Daniel Kammen and Deborah Sunter highlight ways in which cities, which host 54% of the world population but contribute up to 70% of anthropogenic greenhouse gas emissions, can better harness renewable energy. The authors summarize research from cities around the world, demonstrating that, although biomass sources are likely not a practical option, there is much untapped energy through solar, wind, geothermal and even waste sources. For example, if London were to optimize the sides of its buildings for solar panels, this could increase the solar irradiation up to 45%. In China, specially designed buildings funnel wind at 1.5 to 2.5 times the speed of ambient wind, allowing turbines to generate 15 times more energy. Along with building design, transportation is a key means for cities to become more energy-efficient. For example, installation of photovoltaics in parking lots in Frauenfeld, Switzerland, could supply 15 to 40% of the future electric vehicle energy demand in that city. The authors note that there are economic benefits to adopting renewable energy designs, as well as retrofitting existing ones, but behavioral and policy challenges remain.

A Review by Tove Larsen et al. delves into urban water management (UWM), which is becoming increasingly strained due to factors such as aging infrastructures and emerging contaminants. Throughout the 20th century, central water systems have been the standard, but the authors note that more innovative approaches are needed, especially as infrastructure is established in fast growing urban areas in Asia and Africa. They propose several key means to improve UWM, including increased water productivity, distributed or on-site treatment of wastewater, source separation of human waste, and institutional and organizational reforms.

A Perspective by Karen Seto and Navin Ramankutty discusses how increasing urbanization will affect the food supply chain, as well as drive changes in diet. The urban lifestyle is associated with a higher proportion of animal proteins, post-consumer waste, fewer home-cooked meals, and more food packaging. While many studies have focused on the agricultural, or supply, side of this supply chain, the authors emphasize that much more research is need to understand the impact of increasing urbanization on the demand side.

In a Perspective, Frank Kelly and Tong Zhu discuss efforts in London and Beijing to reduce air pollution from transportation. In both cities, creative attempts to reduce the number of cars on the road initially showed some success, but the results were either not sufficiently significant or long-lasting. The authors say that more efficient transport of increased numbers of people around a city can only be achieved by expanded mass transit systems.

In a Perspective, Mark McDonnell and Ian MacGregor-Fors discuss insights gained from urban ecology. Although urbanization can often drive species away, some species have adapted to the city life; for example studies reveal that great tits (Parus major) will modify their song to communicate in noisy locations, and that plants living in highly fragmented urban environments may shift to localized seed dispersal strategies. Urban ecology, the authors note, is increasingly being accounted for during urban planning, helping humans and wildlife to live together in cities.

A Perspective by Terry Hartig and Peter Kahn Jr. discusses how natural features in urban areas can provide psychological benefits. It can be difficult to determine differences between the psychological effects of urban and rural environments, but studies have clearly shown the psychological benefits of nature experiences. Incorporating natural features with urban design will not only facilitate better mental health, but can help shape attitudes toward the environment, the authors say.

Some scientists estimate that 60% of the urban areas that will exist by 2050 are yet to be built. To accommodate such growth, Anu Ramaswami et al. highlight eight areas in which policymakers and others need to take definitive action to ensure that these urban areas are built sustainably. These include recognizing that diverse strategies are needed for different types of cities.

In a Perspective, J. Vernon Henderson et al. argue that more efficient land use would help to improve the functionality of many cities in the developing world. They illustrate their point by modeling slums near the center of Nairobi, Kenya, suggesting that pockets of slums throughout the city remain undeveloped largely due to institutional corruption.

A package of news stories covers a range of urban topics. Rats, perhaps the most notorious of city-dwelling critters, are the focus of one story, which delves into the challenges of stopping these persistent rodents from spreading leptospirosis to humans. A second story looks at how the Chinese government is focusing on making its recent explosion of urbanization more sustainable. A third story turns to Vancouver, Canada, a city embarking on the ambitious quest to become the greenest urban area on the planet. Lastly, another story walks readers through how humans evolved into the city life.

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At embargo lift time, readers can explore a related interactive suite at the following link http://scim.ag/1Tb0WdZ. Its components will include an interactive map showing trends in urbanization across countries, an interactive slider displaying the way cities encroach on biodiverse regions over time, and a heat map illustrating the challenge that cities face in achieving energy efficiency.

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