Guest essay by Kip Hansen, St Thomas, USVI
Even if research shows that Hurricane Sandy was not “caused” by Global Warming, it is certain that sea level rise caused by anthropogenic global warming increased the resultant destruction from Sandy’s storm surge.
“ … sea levels continue to rise due to global warming. The picture here is very clear. And that means that every single hurricane that hits land will push seawater farther inland when it does so. Or as one scientist told me in the wake of Sandy, “There is 100 percent certainty that sea level rise made this worse. Period.” “ (footnote 1)
“Climate Change Made Sandy Worse. Period.” (footnote 7)
From the New York State Sea Level Rise Task Force Report to the Legislature (2010) — “Sea Level Rise–Causes and Projections : Local sea levels are affected by ocean currents, gravitational forces, prevailing winds, and rise and fall of the land mass. Within the coastal regions of New York State, the land mass is slowly sinking, with the exception of the Hudson estuary north of Kingston. This movement is a result of geological forces and impacts of human activity and development. It affects local, or relative, rates of sea level rise. “
“These projections are supported by empirical data documenting recent sea level rise in New York State. For example, gauges at the New York City Battery indicate that sea level in the 2000s is 4 to 6 inches higher than in the early 1960s” (footnote 8)
“Storm surge in the NYC–LI area can result from tropical storms and extratropical cycles. Hurricanes have directly hit NYC (Scileppi and Donnelly 2007), such as on 3 September 1821 (Ludlum 1963) and 25 August 1893 (National Hurricane Center 2008). The category-3 ( ; 110 kt; 1 kt ’ 0.5 m s 2 1 ) winds during the 1821 event flooded a large portion of southern Manhattan (Ludlum 1963), but at that time the NYC population was only ; 150 000. There have been no other direct hits by major hurricanes (greater than category 2) across NYC–LI since the 1938 ‘‘Long Island Express’’ (National Hurricane Center 2008). Hurricane Gloria (1985) was originally labeled as category 3 at landfall for Long Island but has since been reanalyzed as category 1 (C. Landsea 2008, personal communication). However, it is only a matter of time before another major hurricane will impact the NYC–LI area. “ (This statement proved to be prophetic – Hurricane Sandy hit NJ/NY on 29 October 2012)
“Sea level along New York’s coast has been rising at the rate of almost one foot per century for at least 100 years” (footnote 3)
(footnote 2 – the above four quotes are all from NYS Sea Level Risk Task Force Report to the Legislature 2010)
“The mean sea level trend [at the Battery, Manhattan Island, NY] is 2.77 millimeters/year with a 95% confidence interval of +/- 0.09 mm/yr based on monthly mean sea level data from 1856 to 2006 which is equivalent to a change of 0.91 feet in 100 years.” (footnote 6)
The following NOAA graphic shows the current trend of sea level rise for the Battery at 0-3 mm/yr, based on this same tide gauge.
This is in agreement with the Battery’s tide gauge long-term trend of 2.77 mm/yr both by data from NOAA and from Leatherman 1995.
Overall Global Sea Level Rise (Douglas 1991) gives 1.8 mm/yr for the 100 year trend, 1880 – 1980, based on Tide Gauges. This figure is generally accepted throughout the field, for example, Church and White found 1.7 mm/yr for 1870 – 2004 (footnote 5) [one sees figures ranging from 1.45 mm/yr to 1.8 mm/yr (1870-2004) based on tide gauges, and 2.9 to 3.4 mm/yr, based on the still short satellite data series (1993-2003)]. (footnote 11)
We can now look back now to the causes section of the NYS Sea Level Rise Task Force Report which told us that one factor in sea level change is “rise and fall of the land mass” – which refers both to normal subsidence (in which land sinks for some immediate cause – such as settling of coastal areas created by fill as has been reported recently in Norfolk, VA) and to Glacial Isostatic Adjustment (GIA), which is “Post-glacial rebound (sometimes called continental rebound, glacial isostasy, glacial isostatic adjustment) … the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period” (footnote 9).
The NYSSLRTF Report quoted above stated clearly that “Within the coastal regions of New York State, the land mass is slowly sinking”. But by how much? In his seminal 2007 paper on North American Glacial Isostatic Adjustment, which was based on GPS data, Giovanni Sella states “The uplift rates generally decrease with distance from Hudson Bay and change to subsidence (1 – 2 mm/yr) south of the Great Lakes. “ (footnote 4) In plain English, the coastal areas of New York, including NY City, much of coastal New England and areas to the south are sinking at a rate ranging from 1 to 2 mm/yr due to the effects of GIA. To be clear here, while GIA normally refers to land masses rising, in the area concerned, coastal NY, GIA has caused the local land mass to subside or sink.
Assumptions for Analysis:
None of the numbers — facts and figures — contained in the quotes above are controversial — they represent the mainstream views on sea level rise and GIA subsidence, both qualitatively and quantitatively, in the area of concern, New York State, and certainly apply to New York City and surrounds as affected by Hurricane Sandy.
All quoted figures are referenced in footnotes (linked directly to source materials where possible) and come from open source (non-pay-walled) peer-reviewed journal papers and government agency web sites.
I take it as a given that, for the purposes of this discussion, we can all agree that sea level change is an entirely LOCAL issue. Hurricane Sandy’s storm surge and resulting damages therefrom depend ONLY on local sea levels relative to local land mass. NY City’s tunnels were not flooded by any putative sea level rise in Sydney, Australia or the South Sea Islands. (There is another discussion about Global Sea Levels, but this is not it.)
This is a common sense, rough-estimate hypothesis-testing exercise, not a proof of anything – so we only need reasonably accurate and readily agreed upon approximations. Therefore, for our “back of the envelope” calculations, I will use the following:
a) Time period = 50 years (1960-2010) – because this is the time period used by the NY State SLR Task Force (footnote 2)
b) For relative sea level rise in NY City, I will use NYSSLRTF 2010’s “4 to 6 inches higher than in the early 1960s”. (footnote 2) This is the same (+/-) as the tide gauge data from NOAA for the Battery, Manhattan Island, NY. It is the longest tide gauge time series in the US — beginning in 1856 and running to the present day. (footnote 6) (footnote 10)
d) For subsidence at NY City, I will use the mean of 1.5 mm/yr but give as range extremes both 1 and 2 mm/yr. (footnote 4)
The following section gives the elementary school level mathematics (addition, subtraction, multiplication, and division only – no statistical methods or analysis).
Start with NYSSLRTF’s “at the New York City Battery indicate that sea level in the 2000s is 4 to 6 inches higher than in the early 1960s.” As our other numerical quantities are all in millimeters, as is common in science, we will use these numbers: 4 inches = 101.6 mm (let’s use 100 for ease…it changes nothing). 6 inches = 152.4 mm (let’s use 150, same reason) , the mean is 5 inches which is 127 mm (we’ll use 125). Note that this is well within keeping with NOAAs trend of 2.77 mm/yr for the entire length of the Battery’s tide gauge record.
NYSSLRTF’s stated sea level change at the Battery, 1960-2010, is in mm:
plus 125 mm. (mean of 100 to 150). Note that this is approximately 2.5 mm/yr over the 50 year period. If we were to increase this figure to 138 mm, it would agree precisely with NOAAs 2.77 mm/yr—instead we use the figures from the NYSSLRTF Report for consistency.
For subsidence take 1.5 mm/year (halfway between 1 and 2 mm/yr) times 50 years == 75 mm or 2.95 inches of subsidence. This number has a negative sign, as it is land sinking (as opposed to the sea rising). The Battery has apparently sunk approximately 3 inches since the 1960s.
As the New York State Sea Level Rise Task Force has estimated that “sea level in the 2000s is 4 to 6 inches higher than in the early 1960s” and we find that approximately 75 mm or 2.95 inches of that was due to the land sinking (GIA effects only — nothing included here for subsidence caused by the settling of land created by fill), that leaves us with:
Or, in pictures: (footnote 12)
To cover all the bases, let’s include a grid of both variables, Relative SLR and GIA:
This grid shows that the mean value (highlighted) at about 2” of absolute sea surface rise over our 50 year time period at the Battery.
Then using the long term, pre-AWG 100-year trend for Global Sea Level Rise of 1.7 mm/yr, we would expect, for 50 years (1960-2010): 50 times 1.7 mm = 85 mm (or 3.35 inches) of sea level rise due to this inexorable rise of the sea as the world works its way out of the last ice age
Subtracting the expected geologically-caused sea level rise from the actual sea level rise experienced should result in a remainder that would be the portion of the absolute sea level rise that could be attributed to recent Anthropogenic Global Warming.
Thus we now can calculate:
plus 50 mm attributable to rising sea level minus 85 mm expected from long-term general worldwide sea level trends equals minus 35 mm or 1.37 inches less than expected from the geologically-caused sea level rise trend, leaving less than nothing to be attributed to AGW-induced sea level rise.
All of this in pictures:
Using our grid of two variables:
Note that only under the Highest Relative SLR/Lowest Subsidence Rate scenario does this result even become positive – and then only by 15 mm/0.6 inches.
This brings us the somewhat surprising conclusion that the relative sea level change at the Battery in NYC which can be attributed to Anthropogenic Global Warming (by any of its commonly used names) is probably less than zero and certainly not significant. Rather, sea level change at the Battery, when calculating with the means of the variable ranges, is approximately 35 mm less than would be normally expected if the sea level change at the Battery was simply following the long-term slow and gradual rise seen to have begun worldwide at least 150 years ago (long before AGW is posited to have begun). It is not suspected that AGW caused this deficit.
At the Battery, Manhattan, NYC over the period 1960-2010, we found less than zero mm/inches of sea level rise to be attributed to post-1960 AGW. With admittedly rough calculations, it appears that NY City has seen “4 to 6 inches” – 100 to 150 mm — of sea level change over the last 50 years, which is approximately equivalent to “8 inches in 100 years”, but over 3 inches — 85 mm of the relative change — was due to subsidence (land sinking) as a result of GIA. Subtracting the subsidence from the relative rise leaves only 2 inches — 50 mm –of rise attributable to the sea actually getting higher – which is less than the 3.3 inches — 85 mm — which would be expected from long-term pre-AGW (150 year) worldwide positive sea level rise trends, the trend agreed by all not caused by AGW, but attributed to geological causes, usually to the ocean water warming and expanding after the end of the last Ice Age.
The claim is a “Scientific Urban Legend” – made up of the TRUE and obvious fact that any positive change in local average relative sea level will make any storm surge “worse” – by the simple effect of the relative water level being higher by whatever amount — blended with the FALSE assumption/assertion that some or all of the oft cited 8 inches of Global Sea Level rise over the last 100 years had actually taken place in or around NY City and that this sea level rise had been caused by Anthropogenic Global Warming.
At the Battery, Manhattan, NYC, there has been, so far, no significant sea level rise attributable to AGW. Period. Therefore, it is extremely unlikely that AGW-induced sea level rise contributed in any real world sense to the destruction caused by Hurricane Sandy’s storm surge.
None of the above means that NY City and surrounding areas should quit worrying about sea level rise. The sea is rising, has been rising, and will continue to rise. The coastal areas of NY and NJ are sinking and will continue to sink. Both of these conditions will continue until geological conditions change to stop them—perhaps in millennia. The recent warming of the atmosphere may yet cause discernible additional sea level rise at the Battery. NY/NJ should plan for a continued sea level rise range of 1.7-2.8 mm/yr for the foreseeable future and take positive steps to mitigate the known dangers. So far, it simply has had nothing to do with AGW.
1 – Here Comes the Story of No Hurricanes by Chris Mooney | Fri Sep. 6, 2013 6:55 AM PDT
2 – New York State Sea Level Rise Task Force Report to the Legislature — Dec 31, 2010 http://www.dec.ny.gov/docs/administration_pdf/slrtffinalrep.pdf
3 – NYS Sea Level Rise Task Force Report 2010 cited this to:
Leatherman, S.P., R Chalfont, E. Pendleton, S. Funderbunk and T. McCandless. 1995. Vanishing Lands , Sea Level, Society, and Chesapeake Bay. Univ. of Maryland Laboratory for Coastal Research & US Fish and Wildlife Service Chesapeake Bay Field Office
4 – Observation of glacial isostatic adjustment in ‘‘stable’’ North America with GPS — Sella et al GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L02306, doi:10.1029/2006GL027081, 2007
5 – http://users.coastal.ufl.edu/~arnoldo/ocp6050/homeworks/douglas91.pdf also “A 20th century acceleration in global sea-level rise” Church and White 2006 http://naturescapebroward.com/NaturalResources/ClimateChange/Documents/GRL_Church_White_2006_024826.pdf
6 – data from NOAA http://tidesandcurrents.noaa.gov/sltrends/msltrendstable.htm
7 – “Climate Change Made Sandy Worse. Period.” —By Chris Mooney | Thu Nov. 8, 2012 http://www.motherjones.com/environment/2012/11/climate-change-made-sandy-worse-period The scientist that makes most of the sea level claims is Dr. Ben Strauss who “holds a Ph.D. in Ecology and Evolutionary Biology from Princeton University, an M.S. in Zoology from the University of Washington, and a B.A. in Biology from Yale University” and is a paid employee of Climate Central, a AGW advocacy organization.
8 – NYSSRLTF Report cited this as:
Colle, B.A., K. Rojowsky, and F. Buonaiuto. 2010. New York City storm surges: Climatology and an analysis of the wind and cyclone evolution. Journal of Applied Meteorology and Climatology 49 : 85 – 100. Pub ID# 3772
9 – Definition from the Wiki:
10 – The NOAA reported sea level trends at http://tidesandcurrents.noaa.gov/sltrends/msltrendstable.htm and in the graph at
are relative sea levels – where the sea surface is in relation to a certain point on the land mass. There are no adjustments made, as they would be irrelevant. So, in plain English, the relative sea level change is the sea surface movement (up or down) plus the land mass movement (up or down). This is the sea level change you would see with your eyes if you were there.
11 – Although Church and White arrived at a figure of 1.77 mm/yr, CSIRO cites Church 2011 for uncertainty bars but states that the graph shown indicates “an average rate of rise of about 1.6 mm/yr over the 20th Century” rather than Church’s finding of 1.77 mm/yr. http://www.cmar.csiro.au/sealevel/sl_hist_few_hundred.html
12 – For our cartoon pictures, we assume that the US Geological Survey Team installed a brass reference marker at the 1960 Mean Sea Level position on our imaginary sea wall at the Battery and placed a reference tide gauge marked in inches alongside, all for our convenience.
13 – h/t for the format of this article to snopes.com
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