Frederick Steiner is dean and Paley professor at the University of Pennsylvania School of Design, and coeditor, with George F. Thompson and Armando Carbonell, of "Nature and Cities: The Ecological Imperative in Urban Design and Planning" (Lincoln Institute of Land Policy, 2016)

Mayor Kenney recently described American cities, including his own, as "the bastion of protection for minorities, LGBT people, for immigrants," and in doing so, vowed to keep it that way. University of Pennsylvania president Amy Gutmann was similarly explicit in articulating Penn's commitment to undocumented students in an email to the university community, insisting that "we are a richer campus for our inclusion and diversity."

As dean of the School of Design at Penn, where I also teach in the Departments of City Planning and Landscape Architecture, I believe now is the time to expand this sanctuary city concept; to make all our cities refuges for learning, for health and safety, for tolerance and inclusion, and environmental quality.

From coast to coast, major metropolises, along with smaller cities such as Ann Arbor, Madison, and Eugene, are home to our nation's largest and most important universities. With science under siege, the role of universities to advance knowledge is more important than ever. Beyond science and technology, universities foster the arts and humanities, which are essential for our culture.

Large cities also house important health science centers. With the Affordable Care Act under attack, these hospitals will need to amplify their public health missions, while continuing to pioneer new medical science. Cities need to become safer for all people - a significant challenge, but an essential one.

Safety can be enhanced, in part, by making cities more tolerant. Cities already represent our nation's diverse ethnic and religious populations. Affordable housing, transportation, and food are fundamental elements of just and inclusive cities.

Cities must also address the most significant environmental issues of our time: climate change, energy and water use, and biodiversity.

Nearly half the greenhouse-gas production and energy use in the United States comes from urban buildings, with an additional 30 percent of greenhouse gases and energy consumption resulting from transportation systems. Urban places are at the heart and the solution to the problem. Redesigning the built environment can reduce energy use and greenhouse-gas production.

At the same time, the energy-water nexus cannot be ignored. Conventional energy production and hydrologic fracking require considerable amounts of water. Energy conservation conserves water, too. In some places, global climate change produces drought, while in others a surplus of water.

Now is the time to reconceive of cities as giant sponges - places to both capture and cleanse water. The creation of these so-called ecological infrastructures can not only help manage water but also provide refuges for both native plants and animals. Now is the time to reconceive cities and suburbs as wildlife habitats.

In the Middle Ages, human civilization was concentrated in walled cities. As commerce and invention flourished, these enclaves gave way to the Renaissance. Embracing the concept of sanctuary cities suggests not a retreat but an exposition - an amplification of how and where the majority of Americans already live, for the sake of knowledge and health, safety and fairness, and perhaps most of all, environmental quality.