We published a paper in the journal Geophysical Research Letters in May (paper, supporting information), and a very important yet simple result from the paper is that Stefan Talke (Portland State University) recovered historical sea level data from NY Harbor and created this great 1844-2013 dataset for annual maximum storm tide that is twice as long as datasets previously available.
This figure shows the annual maximum storm tides (AMST) in meters above mean sea level — water levels purely driven by storm winds, atmospheric pressure, and tides. Looking at these data one can clearly see that the storm tides have been increasing, and much of the paper is about quantifying this trend and also the variability. Each bar on the plot represents a year’s maximum flood height minus that year’s mean sea level, so sea level rise is not the reason you see increasing AMST.
A sound-byte in the media articles has been the part of the final sentence of the paper’s abstract, which reads “… the annual probability of overtopping the typical Manhattan seawall [has risen] from less than 1% to about 20-25%.” This is unquestionably a powerful result!
This has led to headlines such as this one from Salon:
“Manhattan’s surging flood risk: City could end up underwater every 1 in 4 years” – Salon.com (with a photograph of deep-water flooding in Manhattan during Sandy)
I want to make clear that just because a seawall is overtopped doesn’t mean the city is “underwater”. There are many different flood height thresholds for flooding different neighborhoods, and the 1.75m threshold is just a nominal Manhattan seawall height (from Colle et al. 2008) and even those vary substantially in height. The National Weather Service considers a 1.8m flood height to be the threshold for “moderate flooding” at NY Harbor. But in most places, only a waterfront walkway or park floods with a 1.75m or 1.8m flood height, and neighborhoods are dry until the flood height reaches a higher elevation.
We are trying to work with NYC to quantify what neighborhoods flood at what flood heights, but we already know that at least a few neighborhoods of NYC are susceptible to flooding at low levels of ~2m (about a 10-year flood event). On the other extreme, we also know that about 5% of the city population had floodwaters during Hurricane Sandy.
Stefan and I estimate Sandy to be a roughly 300-year return period flood event (unpublished work), so this is not something I personally expect to occur again frequently. This estimate is based on historical data only, and if storms are changing, then it may underestimate the frequency of such floods. There is some evidence for an increase in North Atlantic storms and their intensity (National Climate Assessment, 2013, Chapter 2), but scientists are still debating whether climate change is responsible and whether the trend will continue into the future.
A less frequent focal point of the media is the great story of discovery of the historical data, explained in this nice video:
The American Geophysical Union did a great job with the video, capturing the wonder and excitement of the data archaeology and research.