The latest census data about New Orleans confirm what has long been known, that the post-Katrina city is whiter – and therefore richer – but, above all, much smaller than before the storm. The population as recorded by the previous census in 2000 was more than 484,000.

Last year's count showed a decline of almost 30 per cent to 344,000, meaning that New Orleans no longer ranks among the 50 largest US cities.

The sad fact, however, is that this most idiosyncratic and beloved of American cities was in decline long before the hurricane hit in August 2005. Katrina simply turned a cruel spotlight on problems that already existed, albeit invisible to the countless visitors attracted by the revelries of Mardi Gras and New Orleans's wonderful cultural heritage. It is likely that the census figures slightly understate the true population, because of people living in officially abandoned homes. It is also true that some of those who left will return as soon as housing is available.

But others will not come back. They have found greater opportunity in better-run southern cities where they have been forced to resettle, places such as Atlanta, Houston and Dallas. They are the ones who were already exasperated by the city's bad public schools, inadequate social services and malfunctioning criminal justice system – shortcomings that weighed particularly heavily on the poorer black population. Small wonder, then, that the exodus has been most pronounced among black families with young children. The decline in population will have adverse consequences for the state of Louisiana. It is losing one of its seven congressional districts (although that loss might have occurred even without Katrina), meaning that its clout in Washington will diminish. A smaller city will be entitled to less aid from the federal government, while it might also lose privileges under state laws, which pointedly confer certain benefits on cities of more than 400,000 people, in other words, not New Orleans.

But ultimately numbers alone do not tell the story. Homes must be rebuilt – but so must the city's education, police and legal systems, and there are encouraging signs this is starting to happen. Post-Katrina New Orleans is already a different city. It must also be a better city.