Sea level extremes now hit about once a decade, study finds, quadrupled by human forcing since 1900

Once-in-a-century coastal floods are no longer once-in-a-century. A study published June 10 in the journal Nature Climate Change found that these “once-in-a-century” flooding events now strike on Earth about once a decade, and that human-driven climate change has quadrupled the frequency of coastal sea level extremes since 1900.

Sönke Dangendorf, lead author of the Nature Climate Change study, explains why this matters so much for day-to-day reality. The increase is not just about higher water levels in abstract terms. He describes a concrete example from the U.S. East Coast in Norfolk, Virginia: in the 1950s and 1960s, the community built along the water “wouldn’t see flooding at all, or maybe just once every five or six years.” Now, “high tide leads to flooding,” with flooded streets that directly disrupt commutes, and rising insurance costs that can accumulate to levels “as much as a major hurricane making landfall.”

That shift from occasional to recurring is the through-line across the research. The Nature Climate Change work reports an average global 12-fold increase on the ground in what used to be a once-in-100-year event in 1900. Then the study’s attribution findings get even more forceful: a second team’s analysis, published in Science Advances, found human-caused sea level rise was clearly measurable at 97% of sites sampled around the world. That same study estimated that human causes were responsible for around 58% of the observed daily extreme water level exceedances between 2000 and 2018.

In plain English: the ocean extremes executives and planners worry about are not being driven solely by natural weather variability. The companion studies argue human activity is the primary driver behind sea level extremes, which include storm surges, high tides, and rising sea level anomalies. Dangendorf also explains how researchers separate human effects from natural forces when observations are limited. There are only “a few more than 100 tide gauges” worldwide with long, century-scale records since the early 20th century. The key technique is using the same climate models that support future sea level projections, then checking whether the models can reproduce what the tide gauge records show. If the models can replicate observed climate, they can be used for experiments that hold greenhouse gases constant to assess how much natural variability explains the changes.

For coastal communities, the operational reality is brutal: even small rises can shift when water overtops defenses. Dangendorf emphasizes that “a couple of centimeters of sea level rise doesn’t sound like a whole lot in theory,” but for people living on the coast, it “hits you much harder than you might think.” He frames it as a matter of thresholds. If your “daily life suddenly” includes high-tide flooding, then the event is no longer a one-off disruption you can plan around. It becomes a recurring shock to transport, work attendance, local budgets, and household finances.

This is where board-level and capital allocation implications show up. Dangendorf notes that people may absorb a once-in-a-100-year event once in their lifetime, recover from something like Storm Sandy, and move on. But if the frequency becomes “every eight years” compared to 1900, recovery gets much harder and often becomes “not feasible for many people.” Translate that into executive risk language and you get a pattern: higher frequency reduces the time to rebuild, increases the probability of simultaneous impacts on multiple assets, and strains insurance markets precisely when claims repeat too often. Even if a single event does not look like a repeatable revenue disaster, the compounding effect on insurance costs and recovery timelines can turn coastal exposure into a chronic balance-sheet issue.

The research also signals that adaptation has to be paired with mitigation, because some sea level rise is already “committed.” Dangendorf points out that projections agree on outcomes until around 2060 “independently of how much greenhouse gases are emitted,” which means there is “no way around that.” Stopping emissions today would not erase near-term impacts, but it can avoid dangerous levels later. He frames the “silver lining” this way: since humans are the dominant forcing factor behind the changes, “if we react, we can do something about it.”

Finally, the forecast becomes startling when you connect it to where risk concentrates. According to the latest IPCC report cited by Dangendorf, what used to be a once-in-a-100-year event in the present day will occur annually at 19% to 31% of tide gauge locations by 2050. That means at “about a quarter of the locations,” communities could experience these once-in-a-100-year events every year. Dangendorf also notes the changes are happening faster in low-latitude locations because those regions typically have less variable climate. For context, he contrasts North Sea conditions, where storm seasons and large tide ranges mean people are already adapted to changes of multiple meters, with the tropics, where it is more calm and a couple of centimeters can dramatically lower the odds of extremes missing the “hurdle.” His hurdle runner analogy captures the management takeaway: when conditions are consistent, lowering the hurdle (higher sea levels) makes it easier for extreme outcomes to occur regularly, not occasionally.