When sea levels rise, the ocean pushes farther inland, flooding low-lying areas more frequently, contaminating freshwater supplies with salt, eroding coastlines, and threatening the infrastructure that hundreds of millions of people depend on. The process is already well underway. A NASA-led analysis found that in 2024, seas rose at a rate of 5.9 millimeters per year, more than double the rate when satellite measurements began in 1993.
Why Sea Levels Are Rising
Two forces drive the ocean higher. The first is thermal expansion: as ocean water absorbs heat, it physically expands and takes up more space. This single process accounts for about 56% of global sea level rise in recent decades. The second is meltwater from glaciers, ice caps, and the massive ice sheets covering Greenland and Antarctica, which adds new volume to the ocean.
These two drivers reinforce each other. A warmer atmosphere melts more ice, and a warmer ocean expands more. The result is an accelerating trend. Seas aren’t just rising; they’re rising faster with each passing decade.
Flooding Becomes Routine
The most visible consequence is flooding that no longer requires a storm. As the baseline water level creeps higher, ordinary high tides begin spilling into streets, parking lots, and stormwater systems. This is called high-tide flooding or “sunny day” flooding, and it has exploded in frequency across the United States.
Along the Southeast Atlantic coast, high-tide flood days increased by more than 400% compared to the year 2000. Charleston, South Carolina, went from roughly two flood days per year in 2000 to 14 by 2020. The Western Gulf coast saw the most dramatic jump: Bay Waveland, Mississippi, experienced an increase of more than 1,100%, reaching 22 flood days in a single year. Northeast Atlantic and Eastern Gulf coastlines saw increases of 100 to 150%.
These aren’t catastrophic hurricane-driven floods. They’re shallow, repetitive events that corrode infrastructure, close roads during commutes, back up sewage systems, and quietly destroy property values over time. For cities that experience them dozens of times a year, the cumulative damage is enormous.
Freshwater Supplies Turn Salty
Beneath many coastal communities, underground aquifers hold the freshwater people drink and use to irrigate crops. As the ocean rises, saltwater pushes into these aquifers through a process called saltwater intrusion. It has already occurred to some degree in many coastal aquifers across the United States.
The problem compounds when communities pump more groundwater than nature can replenish. Heavy pumping lowers the freshwater level underground, which allows saltwater to creep in even faster. In California’s Sacramento-San Joaquin Delta, reduced freshwater flow from rivers and reservoirs means more salty water pushes inland from the Pacific. The consequences ripple outward, affecting drinking water, agriculture, wildlife habitat, and entire delta ecosystems. Once an aquifer is contaminated with salt, restoring it is extremely slow and expensive.
Coastlines Reshape and Ecosystems Shrink
Rising seas accelerate coastal erosion. Waves reach farther inland, eating away at beaches, bluffs, and barrier islands. Shorelines that took centuries to form can retreat noticeably within a single generation. For communities built close to the water, this means homes, roads, and utilities gradually lose the ground beneath them.
Coastal wetlands, including salt marshes and mangrove forests, face a particular threat. These ecosystems naturally sit at the boundary between land and sea, and they can sometimes migrate inland as water levels rise. But when development, roads, or steep terrain block their path, they have nowhere to go. The wetlands drown in place. This matters beyond ecology: coastal wetlands act as natural buffers that absorb storm surge and wave energy. Losing them leaves the communities behind them more exposed to flooding.
Storms Hit Harder
Sea level rise doesn’t just create new flooding on calm days. It also amplifies the damage from hurricanes, nor’easters, and tropical storms. Storm surge rides on top of the baseline water level, so even a modest rise in average sea level can push surge significantly farther inland than it would have reached a few decades ago.
Changes in ocean circulation patterns may make this worse in certain regions. A slowdown in the Atlantic’s major circulation system could raise sea levels along the North American Atlantic coast beyond the global average, while also strengthening storms and shifting precipitation patterns. The U.S. East Coast is already experiencing sea level rise faster than the global mean, partly due to these regional ocean dynamics.
The Economic Toll
Coastal flooding carries a price tag that grows with every centimeter of rise. One widely cited projection estimated that flood damages for the world’s 136 largest coastal cities could reach $1 trillion annually by 2050 if protective measures aren’t put in place. Even with significant investment in defenses like dikes and levees, annual damages are projected to reach $60 to $63 billion by midcentury.
The costs go beyond direct flood damage. Property values decline in repeatedly flooded areas. Insurance premiums rise or coverage disappears entirely. Tax bases erode as residents and businesses relocate. Infrastructure designed for a stable coastline, including wastewater plants, power stations, ports, and airports, requires expensive retrofitting or relocation. For many smaller coastal communities, these costs will exceed what local budgets can absorb.
How Much Rise to Expect
Projections depend heavily on how much greenhouse gas the world continues to emit. Under the most optimistic scenario, where emissions drop sharply and quickly, the median projection is about 0.38 meters (roughly 15 inches) of total rise by 2100. Under a middle-of-the-road scenario, that climbs to 0.56 meters (about 22 inches). If emissions continue accelerating, the median reaches 0.77 meters (around 2.5 feet), with an upper range exceeding 1 meter.
Low-confidence but plausible scenarios involving rapid ice sheet collapse push the numbers higher still. Under the worst case, rise could reach 1.6 meters (over 5 feet) by 2100. These outcomes are considered unlikely but cannot be ruled out, and they would be catastrophic for virtually every major coastal city on Earth.
What makes these numbers deceptive is that sea level rise doesn’t stop at 2100. Even if emissions ceased tomorrow, thermal expansion and ice melt already set in motion would continue raising oceans for centuries. The choices made in the coming decades determine not just how much rise this generation will see, but what the coastline looks like for the next several hundred years.