Human-Caused Sea-Level Rise Has Made Extreme Coastal Floods Four Times More Common Since 1900, Study Finds

Climate Change Is Transforming Coastal Flood Risks Worldwide

Human-caused sea-level rise has dramatically increased the frequency of extreme coastal flooding around the world, according to new research published in Nature Climate Change. The study concludes that floods once considered exceptionally rare are now occurring far more often due to rising sea levels driven by greenhouse gas emissions.

Researchers found that coastal flood events historically expected to occur only once every 100 years are now approximately four times more likely on average than they were at the beginning of the 20th century. In some locations, the increase in risk is even more pronounced, with extreme flood events occurring up to 12 times more frequently than they did in the past.

The findings provide some of the clearest evidence yet that human-induced climate change has fundamentally altered coastal flood hazards across the globe. By analyzing more than a century of tide-gauge records from over 100 coastal sites, scientists were able to identify the growing influence of rising sea levels and separate human impacts from natural climate variability.

As global temperatures continue to rise and oceans expand, researchers warn that coastal communities may face increasingly severe and frequent flooding in the decades ahead.

Rising Seas Are Changing the Meaning of “Rare” Flood Events

For generations, engineers, urban planners, and policymakers have relied on statistical estimates of flood frequency when designing infrastructure and assessing coastal risks.

A so-called “100-year flood” does not mean a flood occurs exactly once every century. Instead, it refers to an event that has a 1% chance of occurring in any given year. Historically, these floods were considered rare enough that communities could prepare for them through long-term planning and coastal defenses.

However, rising sea levels are changing those probabilities.

According to the study, flood thresholds that were once reached only during the most extreme weather conditions are now exceeded much more often because the baseline ocean level has increased. As a result, even moderate storms and high tides can trigger flooding that would previously have required far more extreme circumstances.

The researchers found that, on average, the likelihood of a 100-year flood event has increased by a factor of four since 1900. In certain coastal regions, the increase is substantially higher.

This means that events once considered extraordinary are becoming increasingly common realities for many communities.

The Role of Human-Caused Climate Change

The study identifies anthropogenic radiative forcing as the primary driver of the observed increase in coastal flood risk.

Anthropogenic radiative forcing refers to the warming effect caused by greenhouse gases released through human activities, including the burning of fossil fuels, industrial processes, and deforestation. These emissions trap heat within Earth’s atmosphere, raising global temperatures and contributing to long-term climate change.

One of the most significant consequences of this warming is sea-level rise.

As temperatures increase, seawater expands through a process known as thermal expansion. At the same time, glaciers and ice sheets around the world are melting at accelerating rates, adding more water to the oceans.

Together, these processes have caused global sea levels to rise significantly over the past century.

The new research demonstrates that this human-driven rise in sea levels has become the dominant factor behind increasing coastal flood risks worldwide.

While natural climate patterns such as El Niño, ocean currents, and atmospheric variability still influence local conditions, the study concludes that human-induced sea-level rise is fundamentally reshaping flood probabilities on a global scale.

Researchers Analyzed More Than a Century of Data

To understand how coastal flood risks have changed over time, scientists examined historical tide-gauge records from more than 100 coastal locations around the world.

The analysis focused on the period from 1900 to 2005, providing researchers with a long-term perspective on how sea levels and flood frequencies have evolved over more than a century.

By combining observational records with climate model simulations, the research team was able to compare real-world changes with scenarios that excluded human influence.

This approach allowed scientists to isolate the impact of greenhouse gas emissions and determine how much of the observed increase in flood frequency could be attributed to anthropogenic climate change.

The results showed a clear and consistent pattern across many regions: rising sea levels caused by human activities have substantially increased the probability of extreme coastal flooding.

The findings also align with complementary research published in Science Advances, further strengthening confidence in the conclusions.

Wellington, New Zealand: A Striking Example

Among the locations highlighted in the study, Wellington, New Zealand, provides one of the most dramatic illustrations of how flood risks have changed.

Historically, a particular coastal flood level in Wellington was considered a once-in-a-century event. Based on past conditions, residents could expect such flooding to occur only once every 100 years on average.

Today, that same flood level occurs approximately twice each year.

This remarkable shift demonstrates how even relatively modest increases in sea level can dramatically alter flood frequencies.

The flood threshold itself has not changed. What has changed is the height of the ocean from which storms and tides begin.

As sea levels continue to rise, many coastal cities around the world may experience similar transformations, where events once viewed as exceptionally rare become regular occurrences.

Why Coastal Flooding Matters

Coastal flooding is among the most costly and disruptive natural hazards affecting communities worldwide.

Even relatively minor flooding can cause significant damage to infrastructure, transportation networks, homes, businesses, and public services. Repeated flooding can also accelerate coastal erosion, contaminate freshwater supplies, and reduce property values.

For densely populated coastal regions, the consequences can be especially severe.

Many of the world’s largest cities—including New York, Miami, London, Shanghai, Tokyo, Mumbai, Jakarta, and Bangkok—are located near coastlines and are increasingly vulnerable to rising seas.

Frequent flooding can strain emergency response systems, increase insurance costs, disrupt economic activity, and place additional pressure on governments responsible for maintaining protective infrastructure.

As sea levels continue to rise, the financial and social costs associated with coastal flooding are expected to increase substantially.

Regional Differences Still Exist

Although the study found a clear global trend, researchers emphasize that flood risks do not increase uniformly everywhere.

Natural climate variability, local geography, land subsidence, ocean currents, and weather patterns all contribute to regional differences in flood frequency and severity.

Some regions experience faster rates of sea-level rise than the global average, while others face additional risks from sinking land caused by groundwater extraction, urban development, or geological processes.

Similarly, storm patterns vary considerably from one coastline to another.

As a result, some communities may experience dramatic increases in flooding while others see more gradual changes.

Nevertheless, the overall conclusion remains consistent across regions: human-caused sea-level rise is significantly increasing coastal flood hazards worldwide.

Future Risks Could Be Even Greater

The study examined changes between 1900 and 2005, but sea-level rise has continued since then and is expected to accelerate throughout the 21st century under many climate scenarios.

According to climate projections, continued greenhouse gas emissions could lead to substantially higher sea levels by the end of the century. This would further increase the frequency and severity of coastal flooding events.

Floods that currently occur once every decade could become annual events in some locations. Likewise, today’s 100-year floods could occur every few years—or even more frequently—in highly vulnerable areas.

Scientists warn that the relationship between sea-level rise and flood frequency is often nonlinear. Small increases in average sea level can lead to disproportionately large increases in the number of extreme flood events.

This means future impacts may escalate more rapidly than many people expect.

Adapting to a Changing Coastline

As flood risks continue to grow, experts stress the importance of adaptation measures designed to protect vulnerable communities.

Potential strategies include:

• Constructing or upgrading seawalls and flood barriers
• Restoring wetlands and natural coastal buffers
• Improving drainage infrastructure
• Revising building codes and land-use planning
• Elevating critical infrastructure
• Developing early warning systems and emergency preparedness plans

In some cases, managed retreat from highly vulnerable coastal areas may become necessary where protective measures are no longer economically or technically feasible.

Adaptation efforts are likely to play an increasingly important role as communities seek to reduce risks associated with rising seas.

A Clear Signal of Human Influence

The new study adds to a growing body of evidence showing that climate change is already affecting people and communities today, not merely posing risks for the distant future.

By examining more than a century of observations, researchers demonstrated that human-driven sea-level rise has significantly increased the frequency of extreme coastal flooding around the world. Events once expected only once every hundred years are now occurring several times more often, fundamentally altering the risk landscape for coastal regions.

The findings highlight how greenhouse gas emissions are reshaping Earth’s coastlines and increasing exposure to flooding for millions of people. While local conditions vary, the global message is clear: rising seas are making extreme coastal floods more common, more costly, and more difficult to ignore.

As climate change continues to drive sea-level rise, understanding and preparing for these growing risks will become an increasingly urgent priority for governments, businesses, and communities worldwide.