How do human activities affect sea levels and increase the risk of flooding?

Rising sea levels are one of the most visible effects of climate change, increasing the risk of coastal flooding. Until now, it has been difficult to determine exactly how much of this risk stems directly from human activity and how much is unrelated to it. The authors of a study published in *Science Advances* set out to investigate this by analyzing data from hundreds of tide gauge stations located around the world.

How was the study conducted?

The main objective of the research was to determine the extent to which anthropogenic effects of climate change are responsible for sea-level rise and the increasing frequency of extreme water levels (EWL), which heighten the risk of coastal flooding.

A team of researchers led by Daniel M. Gilford of Climate Central in Princeton, USA, analyzed data from 519 sea-level stations around the world. For 279 locations, the records were detailed enough to allow for an assessment of changes in the number of days with extremely high water levels.

The researchers used two independent approaches. The first was based on an analysis of the sea-level change balance, taking into account the thermal expansion of the oceans, the melting of mountain glaciers, and the mass loss of the Greenland and Antarctic ice sheets. The second approach used semi-empirical models linking global temperature to the rate of sea-level rise.

As a result, it was possible to estimate what proportion of the observed changes would not have occurred without the greenhouse gases emitted as a result of human activity.

Most of the rise in sea levels is the result of human activity

The results show that human activity is the dominant factor driving global sea-level rise. Depending on the scenario considered, the researchers estimated that from 1900 to 2018, human activity accounted for a rise in global sea levels of 119–194 mm. This accounts for 57 to 93 percent of the total observed rise, which amounted to approximately 209 mm.

An alternative modeling method indicated that, on average, 69 percent of sea-level rise can be attributed to anthropogenic factors, with a range of 45 to 94 percent. The authors emphasize that these figures are higher than many previous estimates, primarily due to a more careful assessment of the impact of glacial melting on ocean water levels.

The changes are visible almost everywhere

One of the most important findings of the study is the prevalence of the phenomenon under analysis. Even under the most conservative scenario, a positive impact of human activity on sea level was observed at 97 percent of the monitoring stations analyzed.

The greatest anthropogenic rise in sea level was observed in the southern parts of the Indian and Pacific Oceans. The smallest increases, however, occurred at high latitudes in the Northern Hemisphere.

Importantly, the study showed that not all local trends in sea-level rise can automatically be attributed solely to climate change. In some regions, geological and oceanographic processes that influence local conditions also play a significant role.

What causes sea levels to rise?

The significance of individual factors varies by region of the world. The melting of the Greenland ice sheet plays a major role in sea-level rise, which has a particularly strong impact on the South Atlantic and the North Pacific.

In turn, the thermal expansion of the oceans—that is, the increase in water volume as temperature rises—accounts, on average, for 32 percent of the total anthropogenic rise in global sea level. Its impact is particularly significant along the coasts of South America and at high latitudes in the Northern Hemisphere.

Researchers also point to exceptions. In Sitka, Alaska, sea levels are falling as a result of processes related to the melting of nearby glaciers. This shows that human activity can lead to both rising and falling sea levels at the local level.

Extreme water levels are becoming the new normal

The most alarming findings concern extreme water levels. It is these levels that are directly linked to the risk of flooding.

According to one of the scenarios tested, human activity between 2000 and 2018 was responsible, on average, for 38–50 additional instances of extreme water level thresholds being exceeded per decade at the analyzed location. This means that, on average, each station experienced 4.6 additional days per year with extreme water levels due solely to anthropogenic sea-level rise.

On a global scale, as many as 60 percent of all instances where extreme water level thresholds were exceeded between 2000 and 2018 could be attributed to human activity. The model results yielded a very similar outcome—56 percent of such events were caused by anthropogenic sea-level rise.

In the areas most affected by this phenomenon, the number of additional exceedances reached as high as 8.9 days per year.

Human impact is growing rapidly

An analysis of changes over time reveals a clear trend. Since the 1950s, the contribution of human activity to the occurrence of extreme water levels has been steadily increasing. According to model analyses, in the 1950s, less than 15 percent of such events could be attributed to human-induced climate change. In the 2010s, this percentage exceeded 45 percent.

Even more telling is the fact that the average number of days with anthropogenically induced extreme water levels between 2000 and 2018 was as much as 2.8 times higher than during the period from 1970 to 1989.

In many parts of the world—including along the U.S. East Coast and in the southeastern Pacific—the number of such events was as much as 5 to 10 times higher than it had been several decades earlier.

What do the test results mean?

The authors reach an unequivocal conclusion: anthropogenic sea-level rise is now the primary factor responsible for the extreme water levels observed along the world’s coastlines.

By combining the results of both methods, the researchers estimate that human activity was responsible for at least 58 percent of all instances in which extreme water level thresholds were exceeded over the past two decades.

According to the authors, the impact of climate change on the risk of coastal flooding is now clear and easy to detect. The increasing number of extreme water levels poses an ever-more-frequent threat to coastal residents, local infrastructure, and the economy.

The study also provides tools for assessing whether specific floods or damage would have occurred in the absence of human influence on the climate. This may be important not only for coastal protection planning, but also for future analyses of losses and liability related to the effects of climate change.

---

Source:

Daniel M. Gilford et al., “Human-caused sea level rise drives 21st-century worldwide water level extremes.”Sci. Adv. 12,eadz3595(2026).DOI:10.1126/sciadv.adz3595