A pedestrian walks through floodwater on Union Street in Old Town, Alexandria, Va., in October. Washington Post photo by Salwan Georges

The shorelines of the United States are projected to face an additional foot of rising seas over the next three decades, intensifying the threat of flooding and erosion to coastal communities across the country, according to a report released Tuesday by the National Oceanic and Atmospheric Administration.

Human-caused climate change, driven mostly by the burning of fossil fuels, has accelerated global sea-level rise to the fastest rate in more than 3,000 years. The report by NOAA and other federal agencies – updating a study from 2017 – predicts that ocean levels along U.S. coasts will increase as much by 2050 as they did over the past century.

This amount of water battering the coasts “will create a profound increase in the frequency of coastal flooding, even in the absence of storms or heavy rainfall,” NOAA said.

“We’re unfortunately headed for a flood regime shift,” said William Sweet, an oceanographer at the NOAA National Ocean Service and the nation’s top scientist on sea-level rise. “There will be water in the streets unless action is taken in more and more communities.”

Drawing on data from tidal gauges and satellite imagery, as well as cutting-edge models from the most recent United Nations report on climate change, the NOAA analysis gives decade-by-decade projections for sea-level rise for all U.S. states and territories over the next 100 years. Advances in ice sheet modeling and better observational data allowed the authors to give more definitive near-term projections than ever before, Sweet said.

Even if the world takes swift action to curb carbon emissions, he said, the trajectory for sea-level rise “is more or less set over the next 30 years.”

Kristina Dahl, a principal climate scientist with the Union of Concerned Scientists, said research she and colleagues have done suggest that 10 to 12 inches of sea-level rise by 2050 would put roughly 140,000 homes at risk of “chronic inundation,” or flooding every other week on average.

Already, she said, high-tide flooding in places such as Charleston, S.C., has quadrupled in frequency since the 1970s. Other communities, from Louisiana to New Jersey to the Eastern Shore of Maryland, have wrestled with flooding that has become more common and costly.

“They’re already having to make difficult decisions or major investments to cope with the flooding they are seeing,” said Dahl, who was not involved in Tuesday’s report. While other coastal communities have avoided major impacts, “they will have to start grappling with these same kind of issues.”

Looking ahead to the end of the century, the amount of planet-warming pollution people release into the atmosphere could mean the difference between sea levels stabilizing at about two feet above the historic average or surging by almost two feet, NOAA reports.

“This new data on sea rise is the latest reconfirmation that our climate crisis – as the president has said – is blinking ‘code red,’ ” Gina McCarthy, the White House’s national climate adviser, said in a statement. “We must redouble our efforts to cut the greenhouse gases that cause climate change while, at the same time, help our coastal communities become more resilient in the face of rising seas.”

In years past, powerful storms pushing water up shorelines were the primary drivers of flooding along coasts. But as surging seas raise the level of high tides, communities from the Gulf Coast and the Pacific Northwest to the beaches of Hawaii and the barrier islands of North Carolina increasingly suffer from “sunny day” floods, when saltwater bubbles up from storm drains and spills into streets without a drop of rain.

A report released last year by Sweet and his Ocean Service colleagues found that U.S. coastlines experience twice as much high tide flooding as they did just 20 years ago.

By the middle of the century, the new NOAA analysis finds, minor high tide flooding events could happen more than ten times a year. More significant and destructive events, which currently have about a 4 percent chance of happening in a given year, could occur twice a decade.

This means coastal communities must start planning for regular inundation, Sweet said, especially in places where coastal development and sinking land compound the risks of sea level rise.

Storm and wastewater systems may need to be upgraded to cope with the influx of saltwater. Homes and important infrastructure located within the new upper bounds of high tides might have to relocate.

“What we are trying to communicate to folks is these are real life impacts that will influence their day-to-day decision-making,” Sweet said.

While Tuesday’s report makes clear that a certain amount of sea level rise is essentially guaranteed based on the world’s past emissions, Dahl said that reality only underscores the importance of reducing future greenhouse gas pollution.

“The fact that there’s this locked in sea level rise is not a reason to throw up our hands and say there’s nothing we can do about this, because there absolutely is,” she said. “This decade we’re in right now is one of the most consequential decades for our climate future.”

That is because only by acting rapidly to reduce global emissions can the world limit Earth’s warming to 1.5 degrees Celsius (2.7 Fahrenheit) compared to preindustrial levels – the most ambitious goal of the Paris climate agreement.

If nations fail to hit that target, she said, it risks destabilizing the Antarctic ice sheet and creating conditions for catastrophic amounts of sea level rise in the future.

“It’s really critical that we start bringing our emissions down quickly and steeply,” Dahl said, “so that we don’t get anywhere near those tipping points.”

The reports finds that if warming exceeds 2 degrees Celsius above preindustrial levels, there’s at least a 50 percent chance of sea level rise exceeding 1.6 feet on average globally by 2100. With 3 to 5 degrees Celsius warming, that likelihood balloons to 80 to 99 percent. The sea level rise in the U.S. would be near or higher than the global average.

Reide Corbett, dean of Integrated Coastal Programs at East Carolina University and executive director at Coastal Studies Institute on North Carolina’s Outer Banks, said the NOAA report is important because it so clearly and definitively details a real-world threat posed by climate change.

“We need to make people understand this isn’t just a bunch of scientists talking and arm waving. This is really happening, and it’s happening on time scales that matter to the individual,” said Corbett, who was not involved in the study.

“If we’re talking about 2050, for someone buying a house today that’s within the range of their mortgage.”