‘It affects your daily life suddenly’: Sea level researcher explains why once-in-a-century floods could become the new normal

Extreme coastal flooding events are occurring far more frequently in our warming world than ever before, threatening countless communities that will only become more vulnerable as climate change intensifies.

A recent study found that once-in-a-century flooding events now strike on our planet about once a decade. The study, published June 10 in the journal Nature Climate Change, also found that human-driven climate change has quadrupled the frequency of coastal sea level extremes since 1900.

The findings were released alongside another study by a separate team, published in the journal Science Advances, that found human-caused sea level rise was clearly measurable at 97% of sites sampled around the world, and was responsible for around 58% of the observed daily extreme water level exceedances between 2000 and 2018. In other words, human activities are the primary drivers behind sea level extremes — which include storm surges, high tides and rising sea level anomalies.

A photo of Sönke Dangendorf on a boat with the ocean behind.

Dangendorf is a David and Jane Flowerree associate professor in the Department of River-Coastal Science and Engineering at Tulane University.

Sönke Dangendorf: The absolute basis for doing these kinds of investigations is to work with observations, but observations are unfortunately sparse. We have a few more than 100 tide gauges [distributed worldwide] that offer long, century-scale records of sea level change since the early 20th century. We see that sea levels are changing at those locations, but we cannot tell a whole lot about what is happening elsewhere. So, what we can do is use the same climate models that we use to produce future projections of sea level change.

If you compare observations with the models, then the models need to be able to reproduce what the observations have shown. That was the first step we did, and indeed we proved in this study that the climate models are capable of reproducing observed climate. The nice thing with climate models then is that if you are able to reproduce what has been observed, you can start doing experiments. For instance, you can keep greenhouse gases constant in the modeling and then assess the influence of natural climate variability on the changes that we have observed. That’s what we did in the study.

SD: A couple of centimeters of sea level rise doesn’t sound like a whole lot in theory, but if you live along the coast, it hits you much harder than you might think. Just an example: I used to live along the U.S. East Coast in Norfolk, Virginia. The community was built along the water, and they used to be fine in the 1950s and 1960s; they wouldn’t see flooding at all, or maybe just once every five or six years. But due to sea level rise, what happens now is that high tide leads to flooding. You see flooded streets, and that affects your commute, so people increasingly cannot make it to work.

With respect to what we assessed in these more extreme events, people may absorb a once-in-a-100-year event once in their lifetime. They may experience something like a Storm Sandy, and they may be able to recover from that, but if you think about recovering from that basically every eight years — the frequency of these events nowadays compared to 1900 — that’s of course way more difficult and not feasible for many people.

The reaction to changes in climate is often delayed, so we have sea level rise that we are already committed to. For instance, if you look into projections, they all agree with respect to what happens until around 2060, independently of how much greenhouse gases are emitted. There’s no way around that. We could stop emitting greenhouse gases today, and we would still experience it. So, we need to adapt to that sea level rise, but if we mitigate climate change, if we stop emitting greenhouse gases, then we can avoid dangerous sea level rise, and that’s very important.

The changes are happening much quicker in low-latitude locations. The reason is that in the tropics you typically have a less variable climate. You don’t have these massive swings all the time. For instance, in the North Sea, we have a storm season, and we have huge tide ranges. We are already adapted to pretty massive changes of multiple meters, so a couple of centimeters of sea level rise do not play out that hard, but in the tropics, where it’s more calm, it makes a huge difference.

As an analogy, think about two hurdle runners. You have one hurdle runner that’s compared to the tropics; he jumps very consistently but always below the hurdle, so he never makes it over. Then you have another hurdle runner that’s the North Sea type of guy. His jumps are very variable. Sometimes he’s very high and makes it over that hurdle, but very often he also jumps very low. Now think about lowering that hurdle, which is comparable to increasing sea levels. Suddenly, that hurdle runner who is very consistent makes it over all of the time, while the guy from the North Sea may jump over the hurdle a couple more times, but he also still has the very extreme lows. So, the constant jumper is way more sensitive to these changes.