Global mean sea level has been on the rise since the beginning of the 20th century, but trends in sea level extremes are less clear. By definition, extreme sea level events—like those triggered by storm surges—are relatively rare: Just a few occur per year. Yet they can pose a bigger threat to coastal communities than rising mean sea level alone.
Until now, studies of long-term trends in sea level extremes have mostly linked their changes to mean sea level. Now Marcos et al. have isolated the long-term influence of another major factor: storm surges.
The new findings reveal a relationship between the intensity and frequency of extreme sea level events. The researchers also suggest that large-scale climate change affects long-term trends in these extremes.
The team used hourly sea level measurements from 77 tide gauges and included only data sets that spanned at least 50 years (some spanned more than 150 years). Because 50-year measurements do not exist for many sites, most of the data came from coastal Europe and North America. The scientists corrected for the effects of mean sea level changes and tides and also removed unrealistic and nonclimatic jumps (such as spikes associated with tsunamis). This enabled them to isolate the effects of storm surges and assess long-term changes in sea level extremes that are unrelated to changes in global mean sea level.
The team analyzed changes in both intensity and frequency of extreme sea level events. To do this, they used a state space approach rather than a maximum likelihood estimation, the method used in most previous studies of sea level extremes. The new, more complex approach allows for gaps and uneven sampling in the data set and avoids potentially incorrect assumptions about trends in sea level extremes.
In most locations, the scientists found, increased intensity of sea level extremes goes hand in hand with increased frequency. However, further study is needed to determine whether extreme sea level events associated with storm surges are becoming more frequent.
The researchers also found similarities between long-term trends among geographically clustered tide gauge stations. This suggests that large-scale climate change affects decade-to-decade trends in sea level extremes. The authors emphasize the need for further regional studies in order to pinpoint the physical mechanisms behind these trends. (Journal of Geophysical Research: Oceans, doi:10.1002/2015JC011173, 2015)
—Sarah Stanley, Freelance Writer
Citation: Stanley, S. (2016), Tracking long-term changes in global sea level extremes, Eos, 97, doi:10.1029/2016EO044285. Published on 27 January 2016.