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This blog is expressly directed to readers who do not have strong training or backgrounds in science, with the intent of helping them grasp the underpinnings of this important issue. I'm going to present an ongoing series of posts that will develop various aspects of the science of global warming, its causes and possible methods for minimizing its advance and overcoming at least partially its detrimental effects.

Each post will begin with a capsule summary. It will then proceed with captioned sections to amplify and justify the statements and conclusions of the summary. I'll present images and tables where helpful to develop a point, since "a picture is worth a thousand words".

Friday, December 14, 2012

Storm Surges and Sea Level Rise

Summary.  Hurricane Sandy inflicted heavy damage on the northeastern U. S. states of New Jersey, New York and Connecticut on October 29, 2012.  Much of the damage arose from the storm surge of unprecedented intensity that accompanied the storm.


This post summarizes several recent scientific articles characterizing melting of polar ice, historical sea level rise over recent decades, and model calculations projecting future sea level rise.  Melting of polar ice, higher sea levels, and stronger storm surges have occurred in recent years, in conjunction with the long-term increase in global average temperature.  Model projections incorporating various scenarios that continue to emit carbon dioxide predict that sea levels will continue rising to high levels for the next 290 years.

The nations of the world will continue paying the damages caused by extreme events such as Hurricane Sandy, with expenses passed on as higher tax rates and higher insurance premiums, among others.  As an alternative to spending resources on such remediation, humanity should undertake investment in technologies that limit greenhouse gas emissions, and indeed should deploy industrial scale technologies that deplete carbon dioxide already emitted from the atmosphere.

 

Introduction.  Hurricane Sandy struck the state of New Jersey and the New York metropolitan area on Monday October 29, 2012.  It caused damage estimated at upwards of US$50 billion, much of it due to storm surges that impacted wide stretches of shoreline in New Jersey, the heart of New York City, and eastward along the states of New York and Connecticut. 

The ravages of the storm are likely due to factors related to global warming, such as increases in the moisture content of air over warm ocean waters, rising sea levels, and a blocking high pressure system that forced the path of the storm westward toward land instead of northeastward following the coastline.

U. S. President Obama is submitting a request for about US$50 billion to the Congress for emergency funding to help recovery efforts from the storm.  This amount is based on estimates of the physical damage suffered from the storm and costs for new infrastructure to minimize future storm threats.  It is less than the amount of US$80 billion sought by the affected states, New Jersey, New York and Connecticut, which includes estimates for complete restoration of property and lost economic activity.

Much of the damage from Hurricane Sandy arose from an ocean storm surge.  This was made worse by the documented increase in sea levels in recent decades attributed to global warming.  The world-wide average sea level rise is shown in the graphic below.
 
Global average sea level trend from 1870 to 2000, referenced to a zero value given as the average for the period from 1961 to 1990, in mm (50 mm is approximately 2.0 in.).
Source: Intergovernmental Panel on Climate Change, 4th Assessment Report, 2007; http://www.ipcc.ch/publications_and_data/ar4/syr/en/figure-spm-1.html. 
 
Sea level rise has not attracted as much of the public’s attention as have other phenomena related to global warming, such as extreme weather events more generally.  Purely by coincidence several articles in scientific journals appeared in recent weeks related to sea level rise.  This post reviews some of them.  They were all submitted by their authors to the respective journals some months before Hurricane Sandy hit, so they cannot be considered to have been stimulated by this event. 
 
Historical Record of Sea Level Rise
Polar Ice Melting.  A team of 47 climate scientists from 26 institutions in eight countries, assembled as the Ice Sheet Mass Balance Exercise, reviewed and collated existing data on loss of ice mass in Greenland and Antarctica. 

 
Details: Their report (A. Shepherd and coworkers, Science, Vol. 338, pp. 1183-1189, 2012 ) assessed previous data sets obtained over 19 years by satellite using the methods of radar altimetry (elevation measurement), laser altimetry, radar interferometry between two satellites and gravimetry (measuring changes in the force of gravity due to lost ice mass), 32 years of model calculations of surface mass balance, and other models of changes in glacier properties.  There was a need for this because the earlier reports from one technique or another were never considered together.  It was not clear whether the results were or were not consistent.

 
After demonstrating that the differing methods produced consistent results, Shepherd and coworkers obtained results for Greenland as a whole, and for three different regions in Antarctica, as summarized in the following table:
 
 
Source: Shepherd and coworkers, Science, Vol. 338, pp. 1183-1189, 2012; http://www.sciencemag.org/content/338/6111/1183.full.
 
About two-thirds of the change originates in Greenland and West Antarctica.  The rates of mass loss become more pronounced at the end of this time span.  This can be seen in the following graphic:
Cumulative traces of ice mass (left vertical axis) and the equivalent contribution to sea level rise in mm (right vertical axis; 5 mm is about 0.2 in.).
Source: Shepherd and coworkers, Science, Vol. 338, pp. 1183-1189, 2012; http://www.sciencemag.org/content/338/6111/1183.full. 
 
The significance of the report by Shepherd and coworkers is emphasized in an accompanying news comment (R. A. Kerr, Science, Vol. 338, p. 1138, 2012),  as providing a single set of results that all agreed to.  The report firmly establishes the large and accelerating rate of loss of ice mass especially from Greenland and West Antarctica.  Kerr points out that the loss reported represents about 20% of the contribution to sea level rise, the remainder originating from melting mountain glaciers and the expansion of the water of the oceans as its temperature increases.  All these effects are due to global warming.
 
Mechanisms of Polar Ice Melting.  In a review I. Joughin and coworkers (Science, Vol. 338, pp. 1172-1176, 2012) discuss the present state of understanding of the factors involved in ice sheet loss in Greenland and Antarctica. 
 
 
Details:  The principal source of melting is the heat content of ocean waters bathing the ice shelf (Antarctica) or the outlet glacier (Greenland).  The mechanisms involved are complex, and differ in the two cases.  The Antarctic ice shelf floats extensively over ocean waters, which circulate according to circumpolar ocean currents, with changes in density arising as fresh water from melted ice enters the ocean, and from tidal mixing at the surface interface with the ice.  Recent warming of the underlying ocean currents leads to more rapid melting of the ice shelf from its lower surface.
 
 
Greenland glaciers, on the other hand, migrate directly over land surfaces without floating on the ocean, so the flow of currents differs greatly.  These glaciers calve icebergs, and are bathed in the North Atlantic Circulation.  Surface melting occurs in Greenland, but not in Antarctica, and the liquid descends through glacial crevices to the ice-land interface, thus changing the salinity of the ocean at the glacial front. 
 
 
Over the past two decades, Greenland glaciers have flowed 50% faster than before, likely owing to the effect of global warming on providing warmer water at the ice-ocean boundary.  The authors conclude that much still remains to learn about these phenomena.  Incorporating the present knowledge into general circulation models in order to predict future melting rates will be successful if the models operate with high spatial resolution.  It appears that such models for melting “indicate the potential for far more extreme changes within this century than had been anticipated”.
 
Atlantic storm surges.  A. Grinsted and coworkers (Proceedings of the (U. S.) National Academy of Sciences, Vol. 109, pp. 19601-19605, 2012), based in China, Sweden, Finland and the United Kingdom, studied storm surges along the Gulf of Mexico and the U. S. Atlantic coast over the period 1923 to 2008 by analyzing tide gauge readings from six locations along these shorelines.  (1923 was chosen for the start because it is the year of a strong storm surge; active surges continued during the 1930’s.) 
 
 
Details:  The small number of locations is justified because surges extend large distances from storm centers (their cutoff was 250 km (155 mi,)) and last several days.  The authors constructed a new surge index which accounts for the potential energy contained in elevated water levels, and adjusts the data by removing an annual background tidal level for each location.  The results are correlated with whether an event falls in a cold year or a warm year, where deviations from the median temperature for the interval studied here govern whether an event is classified as a cold or warm year event.  The temperature data are global average annual temperatures.  They cross from being generally cold to being strikingly warm at about 1978; the years in the period from 2000 on are generally 0.4-0.7ºC (0.7-1.3ºF) above the median.  This temperature trend is already quite well known.  The authors then generate a graph of the surge index for all surge events, plotted against the frequency of their occurrences.
 
 
Surge events were segregated according to whether they occurred in a cold year or a warm year.  Strikingly, events with high surge indexes (i.e., having the highest energy at landfall) occur twice as frequently in warm years as in cold years.  Additionally the authors find that warm years generate more storm surge events than do cold years. 
 
This suggests that global warming leads to more intense storm surges, understandably since warmer air can hold higher amounts of water vapor, leading to stronger winds.  Wind strength is an important factor in creating the energy contained in a storm surge.
 
In a commentary on the work of Grinsted and coworkers, G. J. Holland (Proceedings of the (U.S.) National Academy of Sciences, Vol. 109, pp. 19513–19514, 2012) notes that an advantage of the surge index created by these workers is its inherent assessment of storm intensity, and propensity for damage at landfall, incorporating separate factors such as storm speed, wind speed and overall size.
 
Historical Sea Level Trends.  A. Sallenger and coworkers (Nature Climate Change, Vol. 2, pp. 884–888, 2012) report a hotspot in recent sea level rise along the northeast coast of the U. S. between Cape Hatteras, North Carolina, and Boston, a distance of about 1000 km (620 mi.).  This includes the New Jersey-New York-Connecticut shoreline subjected to the storm surge of Hurricane Sandy. 
 
This study analyzed tide gauge data from 1894 to the present.  Most analysis focused on time windows of 60 years, 50 years and 40 years all ending in 2009.  They find that as the time window narrows and becomes weighted more to recent decades, this hotspot becomes more intense.  The 60 year window shows tide gauges in this region with annual rates of sea level rise in the range (this writer interpreted color-coded data points) of about 1 to 3 mm per year, whereas the 40 year window shows that this rate has increased to 3 to 5 mm per year in most cases.  In contrast to this hotspot, the gauge data from further south than Cape Hatteras and further north than Boston show mostly no sea level rise, indicating that the regional nature of the hotspot appears to be real.  The authors relate that their demonstration of a sea level rise hotspot along the northeast coast is consistent with several model predictions by other workers of such a hotspot.
 
S. Rahmstorf, one of the coworkers with M. Schaeffer in work described in detail below, warned on Nov. 28, 2012 that sea levels have been increasing in recent decades even faster than predicted earlier by the Intergovernmental Panel on Climate Change. 
 
Large Future Sea Level Rise Due to Further Planetary Warming.  M. Schaeffer and coworkers (Nature Climate Change, Vol. 2, pp. 867–870, 2012) modeled sea level rise projected into the future based on a range of greenhouse gas/temperature rise scenarios. 
 
 
Details:  At the United Nations Framework Convention on Climate Change annual conference held in Cancun, Mexico in December 2010 the nations of the world pledged to restrain further emissions of greenhouse gases (GHGs) such that the long-term global average temperature increase would not be greater than 2ºC (3.6ºF) above the level that prevailed before the industrial revolution began.  This limit corresponds to an atmospheric concentration for carbon dioxide or its equivalent GHGs of about 450 ppm (parts per million).  Sea level rise model projections were calibrated by correctly reproducing sea level data starting from the year 1000 up to 2006.  Projections overlapped by starting as early as 1860, extending to the year 2300.
 
 
Large sea level rises are foreseen for 2100, continuing to even higher sea levels by 2300.  In the year 2100, an emissions scenario that maintains the 2ºC limit is predicted to generate a sea level rise of 75-80 cm (29.5-31.5 in.) above the level of 2000, while a scenario with no abatement of emissions generates a rise of about 1 m (39.4 in.) and a radical scenario in which all emissions cease after 2016 provides a rise of about 60 cm (23.6 in.) by 2100. 
 
The oceans contain a great deal of thermal and climate inertia since the ability of liquid water to store and release heat is about 1000 times greater than for air, and various ocean depths circulate to exchange heat content only over very long time frames.  For these reasons sea level rise trends that are apparent in projections at the year 2100 continue along similar trajectories further into the future.  Schaeffer and coworkers extended their projections to the year 2300.  The scenario maintaining the 2ºC limit is projected to generate a sea level rise of 2.7 m (8.9 ft.) above the level of 2000.  A relatively unconstrained scenario (similar to no abatement) is predicted to produce a further sea level rise of about 3.5 m (11.5 ft.).  Even in the third scenario, reducing emissions to zero in 2016, sea level is projected to continue rising to 1.25 m (4.1 ft.) by 2300.
 
Schaeffer and coworkers show by their sea level projections that drastic extents of sea level rise are locked in place already at this time, regardless of which emissions policy is undertaken; only the degree of rise is subject to vary.  They conclude that sea level rise can be constrained “within a few centuries” only by implementing worldwide industrial scale processes to lower the concentration of atmospheric carbon dioxide.  This has not been commonly discussed to date; Schaeffer and coworkers suggest such reductions, for example, by combining a switch to bioenergy (which permits approaching zero net emissions) coupled with use of technology for carbon dioxide capture and geological storage in energy generating facilities.  This combination would result in a cumulative negative flux of carbon dioxide, lowering its concentration in the atmosphere.
Schaeffer and coworkers importantly conclude “A key aspect of [slowing sea level rise] … is the long response time of sea level that is physically expected from the slow response of large ice sheets and the deep ocean to climate change, [which is] also found in [the geologic climate record]. This … means that about half of the twenty-first century [sea level rise] is already committed from past emissions. It further means that mitigation measures, even [radical reductions], have practically no effect on sea level over the coming 50 years and only a moderate effect on sea level by 2100.  [Such measures, however, can have] … a major effect on magnitude of [sea level rise] in the centuries thereafter.”
 
Analysis
 
This post summarizes several recent scientific articles, most of which (except for the Rahmstorf release) were transmitted to the respective journals several months before Hurricane Sandy impacted the northeast U. S. coast.  Thus their publication is not a response to that event.  Shepherd and coworkers, and the comment by Kerr, documented the regions in Greenland and Antarctica that have undergone the most loss of ice mass, generating liquid water that contributes to sea level rise.  Joughin and coworkers review physical mechanisms that come into play in providing the heat that results in melting of ice mass.  Grinsted and coworkers, and the comment by Holland, traced historical tide gauge data, showing that storm surge frequency and intensity have been increasing in recent years and preferentially arise in years of warm global average temperatures.  Sallenger and coworkers analyzed tide gauge data along the northeast Atlantic coast of the U. S. and showed a recent trend of increased sea level rise, and rate of rise, as a hotspot in this region, which is not present along adjacent coastlines.
 
Schaeffer and coworkers, projecting sea level rise trends into the future using several different scenarios for the emission of heat-trapping greenhouse gases, predict that pronounced increases in sea level will occur by 2100.  Furthermore, the trends creating them will continue beyond that time, generating even stronger sea level increases by 2300.  They conclude that worldwide efforts must be undertaken not only to slow the rate of new emissions, but in fact to use combinations of technologies that result in a net depletion of greenhouse gases from the atmosphere.
 
Hurricane Sandy struck the northeastern U. S. in October 2012, causing profound damage, much of it due to Sandy’s ocean storm surge.  This post summarizes that sea levels are rising, and storm surges are becoming more intense, in correlation with increased global warming. 
 
President Obama’s US$50 billion request for unbudgeted emergency relief to help restore the northeast is particularly difficult to consider now, in December 2012, coming as it does during intense fiscal negotiations seeking to balance reducing outlays and increasing revenues.  It is believed the request will not include compensating offsets to spending elsewhere.  This means that any emergency aid passed into law is added to the U. S. national debt, requiring that it be paid back at some later time by increasing taxes and/or cutting spending.  Likewise, insurance companies have been hit hard by anticipated claims arising from the storm.  Their benefit payments will have to be made up by increasing future premiums for weather-related claims.  More generally, because of the high probability that global warming contributes to the damage caused by storms such as Hurricane Sandy, it is expected that future extreme weather events caused or worsened by global warming will inflict continued large financial consequences on the nations of the world for their remediation.
 
In recognition of this clear understanding, Schaeffer and coworkers have called for large scale remediation involving the deployment of new technologies for decarbonizing energy production, including the implementation of carbon capture and storage.  This blog has taken a comparable position many times over the past year or more.  The consideration of the harms brought about by intensifying sea level rise, summarized in this post, creates a clarion call for robust action by all nations of the world to act as soon as possible.  Investment in mitigating technologies will reduce the need for emergency government expenditures as responses to extreme climate events.
 
© 2012 Henry Auer


2 comments:

  1. Hi Henry,
    I loved reading this piece! Well written! :)

    jason
    Student pods

    ReplyDelete