Hamburg is a sleepy place most days. When the G20 isn't in town, the port city churns with the comings and goings of large, steel barges in its spacious harbor. To the millions of visitors who arrive in the city each year, for business or pleasure, Hamburg appears as a sort of peaceful locus of global enterprise and capitalism.

But the northern German city isn't so placid today. As world leaders meet for the G20 summit to discuss the world's economic well-being, police and violent protesters are squaring off for a second day straight. The target of demonstrators in Hamburg are familiar: any and all symbols of authority and capitalism — storefronts and banks, Porsches and police vehicles.

The drive toward bigger and better buildings should not expend a city's historical integrity.

Still, Hamburg — like past global conference host cities Seattle, Genoa and Toronto — is bound to recover from the momentary burst of destruction. There is, however, an example of a more insidious threat to cities some 900 kilometers to the southwest. The historic city center of Vienna has been added to UNESCO's endangered world heritage sites in light of a planned high-rise development project just south of Stadtpark in the Austrian capital. Developers are set to break ground on the 20-story building project in 2019, and UNESCO officials contend that the development of a high-rise will fundamentally alter the district's character.

César Pelli, the architect behind the Petronas Towers in Kuala Lumpur and the World Financial Center in New York, recently voiced similar concern for the conservation of historical landmarks in an essay he penned for the Argentine daily Clarín. In it, he argued that the drive toward bigger and better buildings should not expend a city's historical integrity. Indeed, dated architecture and outmoded style are often what give cities their timeless charm.

An estimated 54% of the world's population now lives in urban areas, according to the UN. But that proportion will jump even higher, to 66% by 2050, with projections showing an additional 2.5 billion people living in urban areas by then. The future of cities will determine the future of the human race. They are places of innovation and paradox, conflict and coming together. In Hamburg, anarchists lash out with destructive force in the name of preserving their own personal liberty. In Vienna, developers design state-of-the-art buildings that risk destroying what is most precious of the past. Cities can heal, but history affords no second chances.

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