If sea levels rise as projected, hundreds of millions of people face flooding, contaminated drinking water, destroyed farmland, and trillions of dollars in damage to coastal infrastructure. This isn’t hypothetical. Global mean sea level has already risen 111 millimeters since satellite measurements began in 1993, and the rate has more than doubled in that time, climbing from about 2.1 mm per year in 1993 to roughly 4.5 mm per year in 2023.
How Much Rise Is Expected
The range depends entirely on how much greenhouse gas the world continues to emit. Under the lowest emissions scenario, global mean sea level rises between 28 and 55 centimeters (roughly 1 to 2 feet) by 2100 compared to recent levels. Under the highest emissions scenario, that jumps to 63 centimeters to just over 1 meter (about 2 to 3.3 feet) by 2100, and nearly 2 meters by 2150.
Those numbers represent global averages. In many places, the water rises faster than the global mean because the land itself is sinking, a process called subsidence. In Galveston, Texas, for example, sea level has risen about 71 centimeters since 1904, a rate nearly four times the global average. The primary driver there has been groundwater pumping, which compresses underground rock and soil layers and causes the surface to drop. Subsidence contributed as much as 85% of Galveston’s total relative sea level rise over the past century. Similar dynamics play out in Jakarta, New Orleans, and parts of the Chesapeake Bay region.
What Drives the Water Higher
Two main forces push sea levels up. The first is thermal expansion: as ocean water absorbs heat, it physically expands and takes up more volume. The second is meltwater from glaciers and ice sheets in Greenland and Antarctica. Glaciers alone account for 25 to 30 percent of the currently observed rise. The Greenland and Antarctic ice sheets contribute a growing share as warming accelerates ice loss at their edges. These contributions are compounding, meaning the rate of rise is accelerating rather than holding steady.
Coastal Flooding Gets Worse
The most immediate and visible consequence is more frequent, more severe flooding. Higher baseline sea levels mean that storm surges, high tides, and heavy rainfall events push water further inland than they used to. A storm that would have caused minor street flooding 30 years ago can now inundate homes, hospitals, and roads. Cities that currently experience a major flood once every few decades could see similar events multiple times per decade as water levels climb.
Without additional adaptation, global coastal flood damages are projected to increase 150-fold between 2010 and 2080 under moderate to high emissions scenarios. By 2100, annual flood costs could reach 10 to 12 trillion dollars per year, representing nearly 3% of total global GDP if warming exceeds 2°C. Even with significant investment in seawalls, levees, and other defenses, annual damages are still estimated at 10 to 75 billion dollars.
Drinking Water Turns Salty
Rising seas don’t just flood the surface. They push saltwater into underground freshwater aquifers that coastal communities depend on for drinking water and irrigation. This process, called saltwater intrusion, is already a serious problem in parts of California, Florida, and island nations across the Pacific. As the ocean creeps higher, the boundary between fresh and salt groundwater shifts inland, and wells that once produced clean water start pumping brackish or salty water instead.
Drought makes the problem worse. When less freshwater flows through rivers and estuaries, saltwater advances further upstream. In California’s Sacramento-San Joaquin Delta, reduced freshwater runoff has allowed more ocean water into a system that supplies drinking water for millions of people and irrigates some of the most productive farmland in the country. Once an aquifer is contaminated with salt, it can take decades to recover, even if the source of intrusion is addressed.
Farmland and Food Production
Coastal agriculture is particularly vulnerable. Rice, one of the most important global food crops, is grown extensively in low-lying coastal areas across South and Southeast Asia. When saltwater reaches rice paddies, the damage is severe: across surveyed coastal farming communities, 60% of farmers reported that salinity caused empty grains (meaning the plant grew but produced no usable rice), 25% saw reduced grain development, and 21% observed stunted plants. The worst losses happen when salt exposure coincides with the reproductive stage of the plant, which can devastate an entire season’s harvest.
This isn’t limited to rice. Any crop irrigated with increasingly salty water or grown in salt-contaminated soil produces lower yields or fails entirely. For subsistence farming communities in Bangladesh, Vietnam, and other low-lying regions, this translates directly into food insecurity and economic hardship.
Critical Infrastructure at Risk
Sewage treatment plants, power stations, and industrial facilities were overwhelmingly built near coastlines and at low elevations. A study examining U.S. coastal hazards identified roughly 5,500 facilities at risk of a major flood event by 2100 under high emissions. That total includes about 2,580 sewage treatment facilities, representing 22% of all coastal sewage plants in the country. Depending on the amount of sea level rise, between 60 and 394 wastewater treatment plants serving 4 to 31 million people could be directly exposed to flooding.
When these facilities flood, the consequences go beyond interrupted service. Floodwaters mix with raw or partially treated sewage and carry toxic substances from industrial sites into surrounding neighborhoods and waterways. The communities most affected tend to be lower-income and historically marginalized populations, who are more likely to live near these facilities and less likely to have resources to evacuate or rebuild.
Who Is Most Exposed
The impacts of sea level rise are wildly uneven. Small island nations like Tuvalu, the Marshall Islands, and the Maldives face existential threats, with some land areas only a meter or two above current sea level. Major coastal megacities, including Mumbai, Shanghai, Lagos, and Miami, have enormous populations concentrated in flood-prone zones. In the United States, the Gulf Coast is especially vulnerable because land subsidence compounds the global rise. Eugene Island, Louisiana, experiences relative sea level rise of nearly 10 mm per year, roughly triple the global satellite-measured rate.
Even within a single city, exposure varies dramatically by neighborhood. Waterfront areas with aging infrastructure and fewer flood protections tend to bear the brunt, while wealthier districts often sit on higher ground or benefit from better drainage systems and protective investments.
What Adaptation Looks Like
Communities are responding with a mix of strategies. Hard defenses like seawalls, storm surge barriers, and raised levees protect some areas but are expensive and can shift flooding to neighboring zones. Nature-based approaches, including restoring wetlands, mangroves, and barrier islands, absorb wave energy and provide buffers. Some cities are elevating roads, redesigning drainage systems, and updating building codes to require higher foundations.
In agricultural areas, farmers are adjusting planting schedules to avoid salt exposure during the most sensitive growth stages. Early transplanting of rice, for instance, has shown the most promise for reducing salinity damage by shifting the vulnerable reproductive phase to a period when salt concentrations are lower.
For the most exposed communities, managed retreat, the deliberate relocation of people and infrastructure away from the coast, is increasingly part of the conversation. It’s politically difficult and emotionally wrenching, but in some places the math is straightforward: the cost of repeatedly rebuilding after floods eventually exceeds the cost of moving. Several U.S. communities have already begun buyout programs for repeatedly flooded properties, and some island nations are negotiating land purchases in neighboring countries as a long-term contingency.