On checking the weather, we see a day-old Coastal Flood Warning issued for the District of Columbia which says: “more than a third of Roosevelt Island will be covered by water and back water flooding of Rock Creek in Georgetown will begin.” An unusual occurrence? Not any more. Most low-lying coastal cities, including Washington DC, have begun to experience a new phenomena: High Tide Flooding during quiet weather days, the result of a gradual increase in sea level over the past one hundred and forty year.
Climate experts say that the the rate of sea level rise is speeding up and that the long-term effects could be dire. It’s a challenging subject and we’ve decided to find out more about it, starting today. Our first stop is Washington DC’s tide-gauge station on Pier 5 near the south end of Water Street, one of the many tide-gauge stations operated by NOAA, the National Oceanic and Atmospheric Administration.
Washington DC showing location of NOAA Tide Guage
It’s a cloudy, not-too-hot September day. From Independence Avenue we walk ten blocks south on 4th Street to where it ends at P Street, then eaby a short footpath to the Washington Channel shoreline. The Titanic Memorial (a large granite statue of a man with arms outstretched as if in flight) stands at that point. Pier 5 lies a few hundred yards to the north. We approach it by the waterfront footpath. We can see the tide gauge from the shore but cannot inspect it closely. The DC Police Harbor Patrol have their headquarters on the pier and they refuse to allow unauthorized access. No matter; we’ll look into how tide gauges work later.
NOAA Tide Gauge, Washington DC. Image: NOAA
Knowledge about sea level is based on information generated by a global network of about 2000 tide-level stations. A British organization called the Permanent Service for Mean Sea Level (PSMSL) is responsible for the collection and publication of the data produced by the network.
From: PSMSL website (psmsl.org > data coverage)
There are two trends that give climateologists nightmares: global warming and sea level rise, the second the result of the first. The trend line for the rise in sea level is based on the data generated by the global tide gauge network since 1880. Here’s an example, one of many available on the web.
From: EPA website published 2016
The graph shows that since 1880, sea level has risen by about 9 inches, an average of about 1/16th of an inch per year. However, since 1993, the rate of rise has speeded up to about 1/8th of an inch per year, twice the rate of the long term average. What do the experts say will happen next? Many suggest 1.5 to 3 feet higher by the year 2100. Others, pointing to increasing global warming and the potential for rapid melting of the polar ice sheets, talk about six feet and up by the year 2100, enough to put southern Florida under water and swamp most of the world’s major cities.
Predictions that imply 2100 is the year the rubber hits the road, are not useful. Why? Two reasons: (1) predictions that are safe from being proved wrong within the lifetime of the predictors, are not impressive and easily ignored; (2) the year 2100 is eighty years in the future, much too long a time frame to be of practical use to most people. We need predictions that focus on the near term. We also need a way to keep track of the situation in real time and without having to depend directly on experts for information on which to base personal decisions, such as where to live, for example.
Help is at hand in the form of a paper titled ‘Sea level rise drives increased tidal flooding frequency . . . ‘ published Feb. 3, 2017 in the ‘open access’ journal PLOS ONE. Here’s an excerpt:
“. . . because the general public often perceives climate change as a temporally distant threat, we have chosen to focus on two time frames (15 and 30 years into the future) that are easily comprehensible within a human lifetime.”
In the paper, the authors have predicted the severity of tidal flooding at 52 locations along the U.S. east and gulf coasts by the years 2030 and 2045. They did this by first establishing a correlation between tide-gauge measurements and Coastal Flood Advisories (CFAs) issued by the U.S. National Weather Service. They then show that the number and frequency of CFAs for any given location can substitute for tide-gauge measurnts as a predictor of future flooding severity.
This is great. We, or anyone else with access to the web, can easily keep track of the number and frequency of CFAs affecting coastal property. A daily check on the Coastal Flood Advisory section of the National Weather Service takes little effort. After two or three years we can crunch our numbers and decide for ourselves whether or not sea level rise is a threat to take seriously. We won’t have to depend on media reports about climate change to be in the know.
Here’s an example from the PLOS ONE paper. By 2015, the number of tidal flood events affecting the shore area of Annapolis, Maryland, had risen to about 35 per year. Based on the CFA record for Annapolis, the authors predict that that number will rise to 145 by the year 2030 (only 11 years from now) and to 180 by the year 2045. If those predictions become fact, who is going to put up with streets and shop fronts that get swamped by sea water every second or third day of the year? The report paints a similar near-term future for the waterfront areas of Washington DC and other cities.
Since we intend to keep track of the Coastal Flood Advisories issued for Annapolis, we decide to visit the city to see for ourselves how tidal flooding has affected it so far. Annapolis lies about 30 miles from DC on a different branch of Chesapeake Bay. We retrieve our car from its parking spot and head east out of Washington, aiming to connect with Route 50.
Annapolis MD waterfront showing area affected by intermittent tidal flooding
Washington DC and Annapolis MD in relation to Chesapeake Bay