The media have recently been covering big changes in a city I know well: San Francisco. The Silicon Valley tech boom has attracted large numbers of highly paid newcomers. Rents have spiralled out of reach for existing residents, some of whom have responded by mounting protests. Traditional businesses and cultural activities are being driven out by high-end brands.
People are moving into less desirable neighbourhoods, and this is driving poorer families even further out.
Gentrification seems to be taking off in the world's most prosperous urban centres. In New York and London, for example, people are seeking to move further out, to areas like Brooklyn and the docklands where rents are lower, again driving out local residents and businesses.
In Hong Kong, we associate this trend with the huge influx of mainland shoppers, which has caused a major expansion of designer and luxury stores. Rising rents have caused the closure of much-loved outlets catering to local residents. However, the impact may prove temporary.
In the longer term, our gentrification may well be more like that in San Francisco and other cities. We can expect continued inflows of bankers, for example, from the mainland, Asia and the rest of the world. Other professionals will probably come as new high-value activities develop, like creative industries, or indeed technology. More districts like Kennedy Town will become trendy and less affordable, and more dilapidated blocks in areas like Sham Shui Po will be targeted for redevelopment.
This is not new. The reason cities have skyscrapers in the centre is because land prices go up there as the economy grows. There is bound to be spillover into once-poorer areas nearby over the years.
But global trends are adding to the effect. Globalisation has facilitated greater mobility of people and their fortunes. Chinese, Russian, Middle Eastern and other Asian wealth has grown, and the new rich want to diversify their holdings. Some of it might go into art and yachts, but a lot goes into real estate. Hong Kong, like Vancouver and California, has seen a lot of mainland Chinese wealth going into property.
At the same time, many cities have clearly lagged behind in expanding their housing stock. We think of this as a Hong Kong problem, but home building has not kept up with population growth in many centres on the east and west coasts of the US, the south of England and other areas. High liquidity and low returns in other investments have further pushed housing prices up.
Most of all, globalisation goes with a widening gap between rich and poor, and the way people with certain education and skills are accumulating a greater share of wealth. If the better-off cluster in particular cities, it is easy to see how gentrification of poorer neighbourhoods can follow.
As a successful city, our population and physical area are likely to grow along with the economy. Our definition of "downtown" will expand, and something will have to give. It is hard to see how we can preserve whole ageing neighbourhoods in the urban area, or keep every cheap noodle place. Indeed, as in San Francisco, some older residents are probably looking forward to a windfall when they sell their homes in newly fashionable neighbourhoods.
This all leaves a very serious question in Hong Kong, with its limited space: where will the less well-off go? We need to earmark sufficient space for public and subsidised homes and facilities.
People who say these things should have priority for space over high-end malls or luxury investment property probably have a good point. But people who automatically oppose all development everywhere need to be more realistic.
Bernard Chan is a member of the Executive Council. He chairs the Advisory Committee on Revitalisation of Historic Buildings