Sea Level Rise On The Eastern Seaboard

By Paul Homewood

 

Tallbloke ran a post a couple of weeks ago about this Reuters report on rising sea levels off the East Coast of America.

 

http://uk.reuters.com/article/2014/07/10/uk-usa-sealevel-flood-exclusive-idUKKBN0FF20720140710

 

Reuters) – Coastal flooding along the densely populated Eastern Seaboard of the United States has surged in recent years, a Reuters analysis has found.

During the past four decades, the number of days a year that tidal waters reached or exceeded National Oceanic and Atmospheric Administration flood thresholds more than tripled in many places, the analysis found. At flood threshold, water can begin to pool on streets. As it rises farther, it can close roads, damage property and overwhelm drainage systems.

Since 2001, water has reached flood levels an average of 20 days or more a year in Annapolis, Maryland; Wilmington, North Carolina; Washington, D.C.; Atlantic City, New Jersey; Sandy Hook, New Jersey; and Charleston, South Carolina. Before 1971, none of those locations averaged more than five days a year. Annapolis had the highest average number of days a year above flood thresholds since 2001, at 34.

The analysis was undertaken as part of a broader examination of rising sea levels Reuters plans to publish later this year.

As many Americans question the causes and even the reality of climate change, increased flooding is already posing a major challenge for local governments in much of the United States.

    “Chronic flooding is a problem our coastal managers are dealing with every day,” said Mary Munson, executive director of the Coastal States Organization, a Washington nonprofit representing 35 states and territories. “Flooding causes the quality of life in these communities to decrease along with the property values, while the flood insurance rates go up.”

In Charleston, for example, a six-lane thoroughfare regularly becomes impassable when high tides block rainwater from emptying into the Atlantic Ocean, restricting access for half of the city to three hospitals, four schools and police headquarters. The city, which has more than 120,000 residents, has $200 million in flood-control projects underway.

Laura Cabiness, director of public service for Charleston, said street flooding has always been a problem in the low-lying city. But more recently, she said, “it’s deeper than usual and higher than usual, and the tide has remained higher longer.”

For its analysis, Reuters collected more than 25 million hourly tide-gauge readings from nearly 70 sites on the Atlantic, Gulf and Pacific coasts and compared them to NOAA flood thresholds.

Reuters then narrowed the analysis to include only the 25 gauges with data spanning at least 50 years. Nineteen gauges were on the Eastern Seaboard, three on the West Coast, and three on the Gulf Coast. Comparing the years prior to 1971 to the years since 2001, the average number of days a year that readings exceeded flood thresholds had increased at all gauges except two: those in St. Petersburg, Florida, and Woods Hole, Massachusetts.

The trend roughly tracks the global rise in sea levels. The oceans have risen an average of 8 inches in the past century, according to the 2014 National Climate Assessment. Levels have increased as much as twice that in areas of the Atlantic and Gulf coasts where the ground is sinking because of subsidence – a process whereby natural geological forces or the extraction of underground water, oil or gas cause the ground to sink.

The most dramatic increases in annual flood-level days occurred at 10 gauges from New York City to the Georgia-South Carolina border, a stretch of coast where subsidence accounts for as much as half the rise in sea level in some locations, according to U.S. Geological Survey studies.

 

 Now, to be fair, they do mention the problem of subsidence and sinking land, but the impression is nevertheless clearly given that:

1) Climate change is the real problem

2) Things are getting worse.

 

As is often the case with these matters, though, things are never quite what we are led to believe. As they have used the example of Charleston, let’s take a look at what the tide gauges are actually telling us.

 

 

The first table is from NOAA, and shows sea levels rising by 3.15mm/yr. However, it is apparent that there has been a steady rise since 1921, with no evidence of any acceleration.

 

http://tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?stnid=8665530

 

Indeed, as the next table shows, sea levels were actually rising faster up to the mid 1970’s. (Remember, the plots below are “mid points”).

 

http://tidesandcurrents.noaa.gov/sltrends/50yr.shtml?stnid=8665530

 

Subsidence

The US Atlantic coast is an area that is sinking, due to geological reasons. Melting of the huge ice sheets to the north west, as the ice age ended, has allowed the land there to gradually rebound. In turn, areas to the south east are being gradually tilted downwards.

In their reconstruction of sea levels from 1880 to 2009, Church & White have given estimates of these isostatic effects, which at Charleston are 0.81mm/yr.

 

 

 

So, we can say that the absolute sea level rise, or the eustatic level which takes out the effect of sinking land, has been 2.34mm/yr since 1921. (This takes no account, either, of land subsidence due to local factors, such as water extraction. This is well recognised as a problem at many locations on the east coast).

 

Using the tide gauge data from PSMSL, we can see that, since 2000, the rate of increase has been even smaller, rising just 34mm in 13 years – a rate of 2.61mm/yr. Taking out the effect of the land sinking, this equates to 1.80mm/yr, in line with estimates of global sea level rise during the 20thC.

 

Chesapeake Bay.

Research by John Boon et al, in 2010, also found no evidence of accelerating sea level rise, when analysing ten tide gauges around Chesapeake Bay, just to the north.

 

Temporal comparisons at five bay stations over two periods, 1944-1975 and 1976-2007, suggest that, while RSL continues to rise at some of the highest rates found along the U.S. Atlantic coast, there is presently no evidence of a statistically significant increase marking an acceleration in RSL rise at any of the five bay stations.

Present evidence suggests an ASL rise rate of about 1.8 mm/yr in Chesapeake Bay over the 1976-2007 period.

[ASL = Absolute Sea Level.   RSL = Relative Sea Level]

 

[Note that the 1944-75 period marked a slower rate of increase at Charleston. The most rapid rate of rise was during the 1930’s & 40’s]

 

As for subsidence, the authors find:

 

Subsidence, or the downward movement of the earth’s crust relative to the earth’s center, is particularly evident in the mid-Atlantic section of the U.S. east coast. Engelhart et al. (2009)

used a geological database of late Holocene sea level indices to estimate subsidence rates of <0.8 mm/yr in Maine increasing to 1.7 mm/yr in Delaware before returning to rates <0.9 mm/yr in the Carolinas.

 

This supports the Church & White numbers for Charleston, and also suggests much greater levels of subsidence in the mid-Atlantic states.

 

 

Summary

• It is clear that there is no evidence at all of accelerating rates of sea level rise in these areas. Indeed, the rate of rise has slowed since the first half of the 20thC.
• There is nothing to suggest that recent sea level rise is anything other than part of a natural process, which began when the LIA ended.
• It is also apparent that subsidence is a major factor, particularly in the mid-Atlantic states.