NORFOLK, Va. — Huge vertical rulers are sprouting beside low spots in the streets of Norfolk, so people can judge if the tidal floods that increasingly inundate their roads are too deep to drive through.

About 500 miles down the Atlantic Coast, the only road to Tybee Island, Ga., is disappearing beneath the sea several times a year, cutting the town off from the mainland.

And another 500 miles on, in Fort Lauderdale, Fla., increased tidal flooding is forcing the city to spend millions fixing battered roads and drains — and, at times, to send out giant vacuum trucks to suck saltwater off the streets.

For decades, as the global warming created by human emissions caused land ice to melt and ocean water to expand, scientists warned that the accelerating rise of the sea would eventually imperil the United States’ coastline.

Now, those warnings are no longer theoretical: The inundation of the coast has begun. The sea has crept up to the point that a high tide and a brisk wind are all it takes to send water pouring into streets and homes.

Federal scientists have documented a sharp jump in this nuisance flooding — often called “sunny-day flooding’’ — along both the East Coast and the Gulf Coast in recent years.

The sea is now so near the brim in many places that they believe the problem is likely to worsen quickly. Shifts in the Pacific Ocean mean that the West Coast, partly spared over the past two decades, may be hit hard, too.

These tidal floods are often just a foot or two deep, but they can stop traffic, swamp basements, damage cars, kill lawns and forests, and poison wells with salt. Moreover, the high seas interfere with the drainage of stormwater.

In coastal regions, that compounds the damage from the increasingly heavy rains plaguing the country, like those that recently caused extensive flooding in Louisiana. Scientists say these rains are also a consequence of human greenhouse emissions.

“Once impacts become noticeable, they’re going to be upon you quickly,’’ said William V. Sweet, a scientist with the National Oceanic and Atmospheric Administration in Silver Spring, Md., who is among the leaders in research on coastal inundation. “It’s not a hundred years off — it’s now.’’

Local governments, under pressure from annoyed citizens, are beginning to act. Elections are being won on promises to invest money to protect against flooding. Miami Beach is leading the way, increasing local fees to finance a $400 million plan that includes raising streets, installing pumps and elevating sea walls.

In many of the worst-hit cities, mayors of both parties are sounding an alarm.

“I’m a Republican, but I also realize, by any objective analysis, the sea level is rising,’’ said Jason Buelterman, the mayor of tiny Tybee Island, one of the first Georgia communities to adopt a detailed climate plan.

But the local leaders say they cannot tackle this problem alone. They are pleading with state and federal governments for guidance and help, including billions to pay for flood walls, pumps and road improvements that would buy them time.

Yet Congress has largely ignored these pleas, and has even tried to block plans by the military to head off future problems at the numerous bases imperiled by a rising sea. A Republican representative from Colorado, Ken Buck, recently called one military proposal part of a “radical climate change agenda.’’

The gridlock in Washington means the United States lacks not only a broad national policy on sea-level rise, it has something close to the opposite: The federal government spends billions of taxpayer dollars in ways that add to the risks, by subsidizing local governments and homeowners who build in imperiled locations along the coast.

As the problem worsens, experts are warning that national security is on the line. Naval bases, in particular, are threatened; they can hardly be moved away from the ocean, yet much of their land is at risk of disappearing within this century.

“It’s as if the country was being attacked along every border, simultaneously,’’ said Andrea Dutton, a climate scientist at the University of Florida and one of the world’s leading experts on rising seas. “It’s a slow, gradual attack, but it threatens the safety and security of the United States.’’

Because the land is sinking as the ocean rises, Norfolk and the metropolitan region surrounding it, known as Hampton Roads, are among the worst-hit parts of the United States. That local factor means, in essence, that the region is a few decades ahead in feeling the effects of sea-level rise, and illustrates what people along the rest of the American coast can expect.

The biggest problems involve frequent flooding of homes and roads. As the sea rises, hundreds of tidal creeks and marshes that thread through the region are bringing saltwater to people’s doorsteps.

Several studies have concluded that Naval Station Norfolk, the world’s largest naval base, is profoundly threatened by rising seas, as are other coastal bases. The Pentagon has managed to build floodgates and other protective measures at some facilities. But attempts by the military to develop broader climate change plans have met fierce resistance in Congress.

That was the case this summer, when an effort by the Pentagon to appoint officers to take charge of climate resilience led to a House vote prohibiting taxpayer money from being spent on the plan.

Many people in Congress, almost all of them Republicans, express doubt about climate science, with some of them promulgating conspiracy theories claiming that researchers have invented the issue to justify greater governmental control over people’s lives.

The Obama administration has been pushing federal agencies, including the Pentagon, to take more aggressive steps. But without action in Congress, experts say these efforts fall far short of what is required.