The crisis, in short: Offices anchor almost everything that makes downtown work. Roads and public transit systems, for example, were often designed to funnel workers into and out of central business districts. It costs a lot to build and operate transit systems, but very little to add one additional passenger, so those systems do well when there is a lot of commuting. As the number of passengers declines, the per-passenger cost increases, and so does the environmental footprint — a bus with three passengers is worse than one person driving a car. Those costs can be reduced somewhat by running fewer buses and trains, but infrequent service depresses ridership still further.

The economy of the central business district is also designed to operate at high capacity. A typical restaurant in Manhattan’s Midtown of course served tourists, as well as suburbanites coming into the city for a special occasion. But much of their business came from people in the surrounding offices: power breakfasts, working lunches, happy hours, retirement dinners. If too many of those people stay home, those restaurants’ balance sheets tip from black to red…

It’s probably not an exaggeration to say that this is the most important problem facing cities in the next decade. Empty streets and stores mean fewer jobs, lower taxes, higher costs and a risk that things will decay further.