If you’ve been out and about in Manhattan over the past six weeks and you have eyes and ears, you know something’s happening — something worrisome.

The urban streetscape is ­degrading.

Take a walk down Broadway on the Upper West Side from the 100s to the 70s, as I did Sunday, and you’ll see it everywhere. It seems every barren storefront with a rental sign in the window has ­become impromptu outdoor housing for a homeless person.

There are many such storefronts — ironic signs of prosperity, not recession. Rents have risen so high that small businesses often can’t afford to continue and landlords will keep a storefront unoccupied for a very long time to secure a wealthy customer willing to take a very lengthy lease (i.e., a bank).

The number of people living on the street in the neighborhood, or at least taking up daytime residence to beg for change, has skyrocketed from a mere handful to several dozen or more.

And many of the faces on the street are a type new to New York City. They’re often startlingly young and white and look like nothing so much as the hippies who used to populate the Upper West Side in the late 1960s and early ’70s. They would have fit right in at the Occupy Wall Street encampment two years ago or at a G20 protest.

Along with them are the classic street people — deinstitutionalized schizophrenics whose tragic unsupervised lives are a moral stain on this city and this country. Many live in the neighborhood’s halfway houses and shelters and are out and about now that the weather has gotten warmer.

And while most are living within the law and trying to make new lives, among their number are some very discomfiting people — panhandlers from the neighborhood’s bad old days, who are just this side of aggressive.

The change in atmosphere over the holiday weekend was startling. And it wasn’t just the constant demands for money or the filthy bedding in the storefront doorways.

In a shop in the low 90s on Broadway where I was buying my kids ice cream, a young man came in and demanded free sample ­after free sample from a clearly uncomfortable teen girl behind the counter, bought nothing, then planted himself in the store, started making phone calls, and wouldn’t leave.

Only the fact that people kept coming in and out, and that someone was working in the back who could call for help if necessary, prevented me from alerting the police. But what would I have told them? He’d done nothing wrong, yet he’d crossed some civilized boundary.

Walking along the sidewalk on Broadway between 86th and 87th, a bearded and unkempt middle-aged man muttering to himself suddenly threw a bottle into the street that shattered into a thousand pieces as cars were driving by.

On the same corner a month or two ago, a younger man bumped into me, looked me over, spat out something anti-Semitic — and when I kept walking said, “Just like you Jews not to fight for yourselves.”

These three anecdotes are all examples of what made living in pre-Giuliani New York City so problematic. It wasn’t crime per se that made you uncomfortable (even at the height of its troubles, New York had a lower per-capita crime rate than other US cities). No, the problem was a general feeling of menace — the sense that violence could break out around you at any moment.

My wife and I have three kids, ages 5, 8 and 11. Having grown up on the Upper West Side pre-Etan Patz during the 1960s and ’70s — when crime was so much worse, it almost beggars description, and yet when kids roamed far more freely through the city than they do now — I’ve argued for giving our older children a greater degree of independence. To go to the store. To walk to a friend’s apartment.

My wife has never been comfortable with that, but has gone along to some degree. Yet now, the evidence of our eyes and ears makes it clear our neighborhood is simply more menacing than it was a year or two ago, and that civil society is decaying.

I’m not offering an explanation for why this has happened. I’m only describing a change in mood.

And if I were Bill de Blasio looking ahead to 2017, I’d take this very seriously. He won election in 2013 in part because the argument that he would return the city to the bad old days didn’t resonate with voters.

But if they feel two years from now as though the city is a worse place to live than it was when he took office, the 73 percent of the city’s eligible voters who didn’t vote for de Blasio in 2013 will have no difficulty sending the moving truck to Gracie Mansion and shipping him right back to Park Slope.