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Sea Level Rise Hawaii

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3 ft sea level rise scenario along the Honolulu coastlineHonolulu fly-through with 3 ft of sea level rise

 

Sea Level Rise

 “Some scientific conclusions have been so thoroughly examined and tested, and supported by so many independent observations and results, that their likelihood of being found wrong is vanishingly small. Such conclusions are then regarded as settled facts. This is the case for the conclusions that the Earth system is warming and that much of this warming is very likely due to human activities…strong evidence on climate change underscores the need for actions to reduce emissions and begin adapting to impacts.”
America’s Climate Choices, U.S. National Academy of Science, National Research Council, 20111

 

Most Americans, if asked to identify the world’s leading body of scientific expertise, would choose the U.S. National Academies of Science, Engineering, and Medicine. In 2011 the Academies published the 5 volume set “America’s Climate Choices”, a report that concludes climate change is occurring; it is very likely caused by the emission of greenhouse gases from human activities; and it poses significant risks for a range of human and natural systems. These emissions continue to increase, which will result in further change and greater risks. Among these risks are negative impacts related to sea-level rise, which on low-lying coastal plains such as in Hawai’i, pose a range of threats to natural and human assets.

 

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Summary

 

Global warming and the climate change that it is causing are settled facts according to the U.S. National Academy of Sciences1. Strong evidence on climate change underscores the need for actions to reduce greenhouse gas emissions and begin adapting to impacts. It is important for Hawaii to reduce the impacts of sea level rise by incorporating adaptation guidelines into development, trade, agriculture, and conservation activities.
Research currently indicates that global mean sea level may reach approximately 1 ft by mid-century and 2.5 to 6.2 ft by the end of the century, but there are significant unknowns in predicting future sea level. It is recommended that local decision-makers implement sea level rise adaptation programs. Based on present understanding, an appropriate planning target would include a sea level benchmark of 1 ft by mid-century and into the lower end (about 3 ft) of 2.5 to 6.2 ft by the end of the century.
Planners can consider the impact of a worst case scenario of 6.2 ft by end of the century and use this in the design of appropriate projects such as hospitals, waste water treatment, coastal highways, and other cases where public health and safety are at risk. It is prudent to expect that a hurricane will make direct landfall in Hawaii under conditions of higher sea level and that tsunami will continue to arrive at Hawaiian shores; officials can plan and redevelop communities to improve resilience. The Intergovernmental Panel on Climate Change (IPCC) is planning the release of their next assessment report in 20142. Planning guidelines can be re-evaluated in light of the information in this report.

 

Global Mean Sea Level

 

Researchers predict that global mean sea level could rise 32 cm (1 ft) in the next 40 years3; and reach 75 to 190 cm (2.5 to 6.2 ft) over the next century4. However, the low end of this range (0.8 m or 2.6 ft) is considered most plausible by some researchers5 and slightly higher, 1.05 m (3.35 ft), most plausible by others6.
The rate of sea-level rise (SLR) has accelerated since 1990, approximately doubling, with the greatest portion of rise occurring in the southern hemisphere7. However, sea level not only did not rise everywhere, it actually declined in some broad areas. Thus, the pattern of sea level change is complex8. This is due to the fact that winds, ocean currents, continental runoff, salinity, gravity and other factors affect sea level, and those are changing also9.
Using the time it takes for radar to travel to Earth’s surface and back, satellites carrying altimeters can measure the sea surface from space to better than 5 cm (2 in). The TOPEX/Poseidon mission (launched in 1992) and its successors Jason-1 (2001) and Jason-2 (2008) have mapped the sea surface approximately every 10 days for almost 2 decades. These missions have led to major advances in physical oceanography and climate studies.
Altimeter measurements indicate that global mean sea level has risen about 5.4 cm (over 2 in) from 1993 to 2011 at a rate of approximately 3.2 mm/yr10 (over 0.12 in/yr; Figure 1). However, this rise is not uniform across the oceans.


Figure 1. The global mean sea level since January 1993 (3.2 mm/yr or over 0.12 in/yr) is calculated after removing the annual and semi-annual signals. A 2-month  filter is applied to the blue points, while a 6-month filter is used on the red curve. (CREDIT: CLS/Cnes/Legos: http://www.aviso.oceanobs.com/en/news/ocean-indicators/mean-sea-level/)
Figure 1. The global mean sea level since January 1993 (3.2 mm/yr or over 0.12 in/yr) is calculated after removing the annual and semi-annual signals. A 2-month  filter is applied to the blue points, while a 6-month filter is used on the red curve. (CREDIT: CLS/Cnes/Legos: http://www.aviso.oceanobs.com/en/news/ocean-indicators/mean-sea-level/)

A map of altimeter measurements (Figure 2) reveals the localized rate of sea-level change since 1993 on the world’s oceans. Rates are contoured by color: light green indicates regions where sea level has been relatively stable; yellow, orange, and red show areas of SLR; blue and purple indicate areas of sea-level fall. This complex surface pattern largely reflects wind-driven changes in the thickness of the upper layer of the ocean, and to a lesser extent changes in upper ocean heat content driven by surface circulation.

 

Figure 2. Map of sea level change 1993-2010 (mm/yr) as measured by satellite altimetry. (CREDIT: CLS/Cnes/Legos: http://www.aviso.oceanobs.com/en/news/ocean-indicators/mean-sea-level/)
Figure 2. Map of sea level change 1993-2010 (mm/yr) as measured by satellite altimetry. (CREDIT: CLS/Cnes/Legos: http://www.aviso.oceanobs.com/en/news/ocean-indicators/mean-sea-level/)

Most noticeable on the map is the trend of SLR in the western Pacific approaching, and in places exceeding, 10 mm/yr (0.4 in/yr). This pool of rising water has the signature shape of La Niña conditions (times of enhanced trade winds and cooling in the eastern tropical Pacific). The sea-level buildup in the western Pacific coincides with the absence of prolonged El Niño events (times of reduced trade winds), with the last occurring during 1997-98 and a moderate El Niño in 201011.
Another Pacific climate pattern, the Pacific Decadal Oscillation (PDO), is a basin-wide phenomenon consisting of two phases, each historically lasting 20 to 30 years12. In a positive (or warm) phase of the PDO, surface waters in the western Pacific above 20o N latitude tend to be cool, while equatorial waters in the central and eastern Pacific tend to be warm. In a negative (or cool) phase, the opposite pattern develops. Hence, rapid SLR in the western Pacific matches the current negative phase of the PDO.
The degree to which the western Pacific sea level pattern contributes to the global mean rate of SLR observed in satellite altimetry is not known. Also not known is how long the present negative phase of the PDO will prevail, though it is thought to have begun relatively recently13. Some researchers view the PDO pattern of decadal timing as having “broken down” in favor of shorter-duration events14.
Studies15 indicate that winds exert an important control on sea level behavior in the Pacific basin. Researchers16 used a 23-year database of satellite altimeter measurements to investigate global changes in oceanic wind speed and wave height over this period (Figure 3). They discovered a general global trend of increasing wind speed and, to a lesser degree, wave height. The rate of wind speed increase is greater for extreme events compared to the mean condition and indicates the intensity of extreme events is increasing at a faster rate than that of the mean conditions. At the mean and 90th percentile, wind speeds over the majority of the world’s oceans have increased by at least 0.25% to 0.5% per year (a 5% to 10% net increase over the past 20 years). The trend is stronger in the Southern Hemisphere than in the Northern Hemisphere. The only significant exception to this positive trend is the central North Pacific, where there are smaller localized increases in wind speed of approximately 0.25% per year and some areas where there is a weak negative trend.

Figure 3. Global contour plots of mean trend (percent per year); wind speed (top) and wave height (bottom). Points that are statistically significant are shown with dots. (CREDIT: Young et al, 2011)
Figure 3. Global contour plots of mean trend (percent per year); wind speed (top) and wave height (bottom). Points that are statistically significant are shown with dots. (CREDIT: Young et al, 2011)

In summary, it is widely agreed among researchers that the rate of global mean SLR has accelerated (approximately doubling) over the 20th and 21st centuries and reached approximately 3.2 mm/yr. Since the beginning of satellite altimetry (1992) there is no sign of further acceleration. The rate of global mean SLR will have to increase if the predictions of 32 cm (1 ft) by 2050 and 75 to 190 cm (2.5 to 6.2 ft) by the end of the century are to be realized.

 

Sea Level Behavior

 

Planning for, and adapting to the impact of these changes in Hawai‘i requires identifying future sea-level scenarios and their timing, evaluating how the physical environment will change with each scenario, and assessing potential impacts to assets. The planning process should be flexible to accommodate new information and understanding of sea level behavior.
The rate of actual SLR in Hawai’i (approximately 1.5 mm/yr at Honolulu and Nawiliwili) presently lags behind the global average (approximately 3.2 mm/yr) of the past two decades; will this behavior continue? Can it be assumed that future local sea level rates in Hawai’i will also lag the global average? In fact, estimates of future SLR variability diverge on this point. According to the IPCC AR417 models of ocean density and circulation indicate that Hawai’i falls in a zone of slightly reduced sea level change relative to the global mean (Figure 4a). However, IPCC modeling does not take into account the effects of changing ice mass on the main ice sheets Greenland and Antarctica. According to more recent modeling6, Hawaii falls in a zone of slightly higher sea level compared to the global mean when considering the worst case scenario of ice melt (Figure 4b). Additionally, the Pacific pattern of sea level change revealed by altimetry (see Figure 2) may be the result of accelerations in the trade winds9, which blow toward the western tropical Pacific raising sea level to the west of Hawaii.

a)

Figure 4. a) Map of regional sea level change due to ocean density and circulation change relative to the global average (18 to 59 cm) according to IPCC AR4 

 

 

 

 

 

 

 

b)

Figure 4. b) Map of regional variability in sea level relative to global average (1.02 m) when worst case scenario of ice melt is considered according to Slangen et al (2011).

 

 

 

 

 

 

 

 

Figure 4. a) Map of regional sea level change due to ocean density and circulation change relative to the global average (18 to 59 cm) according to IPCC AR418 ; b) Map of regional variability in sea level relative to global average (1.02 m) when worst case scenario of ice melt is considered according to Slangen et al (2011)6. (CREDIT:  a) IPCC and b) Slagen et al. (2011)

Defining the contribution of various sources to global mean sea level is a topic of active research. Global sea level is principally the product to two phenomena:

  1. Melting ice on Antarctica, Greenland, and among alpine glaciers19, and
  2. Thermal expansion of seawater due to surface warming.

Various researchers produce summaries of the components of sea level change20. For example, detailed observations of Antarctic ice reveal net melting at an annually accelerating rate3; the melting rate on Greenland has increased 250% in the past decade21 and it is also accelerating each year3; melting of all of Earths glacial ice19 has added a total of about 12 millimeters to global sea level between 2003 and 2010; there is widespread retreat and thinning of mountain glaciers22; and together these major ice sources presently contribute about 1 to 2 mm/yr (0.04 to 0.08 in/yr) to global sea-level rise.
Thermal expansion is calculated from the amount of heat stored in the upper ocean as revealed by increased water temperature. Until recently, only the contribution from the upper ocean was known23. But observations and modeling of the deep ocean24 indicate that warming below a depth of 700 m (2296 ft) may have contributed 1.1 mm/yr (0.043 in/yr) to the global mean SLR or one-third of the altimeter observed rate of 3.2 mm/yr (0.126 in/yr). Estimates of the sea level contribution due to thermal expansion in the shallow ocean range from 0.05 to 1.1 mm/yr (0.002 to 0.043 in/yr) depending on the source25. Assessments of the components of SLR are in agreement with observed rates of SLR26.

 

Hawai’i Sea Level


Sea level has risen in Hawai’i at approximately 1.5 mm/yr (0.6 in/decade) over the past century27 and probably longer28. Though at first glance this may not seem like a substantial rate, long-term SLR over a century in duration exacerbates hazards such as chronic coastal erosion, impacts from seasonal high waves, coastal inundation due to storm surge and tsunami, and drainage problems due to the convergence of high tide and rainfall run-off. Additionally, this long-term trend has increased the impact of short-term fluctuations in coastal sea level29 leading to more frequent and increasingly severe episodic flooding and erosion along the coast due to extreme tides *.
When considered over a typical planning cycle or the life expectancy of a coastal structure, these processes constitute complex planning challenges31. All of these SLR-related processes are prevalent in Hawai‘i, and their risks to public health, welfare, and safety, as well as to the built environment, will increase in the near and long-term future. However these and other impacts of long-term SLR are not formally taken in account in planning activities. For instance, the Federal Emergency Management Agency (FEMA) requires certain elevation criteria to be met in flood-prone areas (base-flood elevations) in order for a structure to qualify for flood insurance; in the 40 yrs the program has been in operation there has been no adjustment for SLR, despite the fact that sea level has risen over the period.

Satellite altimetry in the region around Hawai‘i (Figure 5) reveals that waters to the north and south are rising at 3 to 4 times the rate of rise locally. This has special meaning for the low-lying atoll islets and sandy shoals of the Papahānaumokuākea Marine National Monument. Rapid sea level rise in this region spells a future characterized by coastal erosion, and heightened damage from storm surge, high wave events, extreme tides, and tsunami inundation31.

Figure 5. Map of sea level change in the region of Hawai‘i. (CREDIT: M.A. Merrifield, University of Hawaii Sea Level Center)
Figure 5. Map of sea level change in the region of Hawai‘i. (CREDIT: M.A. Merrifield, University of Hawaii Sea Level Center)

 

* The term extreme tide refers to high coastal water levels above predictable tidal oscillations. Examples include the superposition of predictable high tide with essentially unpredictable (aperiodic) oceanographic and meteorological processes such as seasonal heating of ocean water, mesoscale eddies, set-up related to high swell, low atmospheric pressure, high onshore winds, and other non-tidal processes that raise the coastal sea level.

 

Sea Level Impacts

 

In Hawai‘i, SLR resulting from global warming is a particular concern. High seasonal waves, hurricanes, and tsunami will penetrate further inland as the water level increases. The coastal groundwater table, which rises and falls with the daily tides, will crop out above ground level creating new wetlands, changing surface drainage, and producing widespread flooding especially when high tide is coincident with heavy rainfall. Coastal erosion will increase, and the characteristics of sediment transport and storage in shallow water will change. Salt water will penetrate into coastal wetlands, streams, and estuarine systems changing their character. In general, low-lying environments, ecosystems, human communities, infrastructure, and other coastal assets will be affected in a number of ways.
The physical effects related to SLR can be categorized. These are:

  1. Inundation – SLR may cause increased wave over-topping, tsunami inundation, and hurricane storm surge with negative impacts to low-lying environments, ecosystems, and developed areas including coastal roads and communities. Studies indicate that the fixed elevation of our low-lying coastal plains grows increasingly vulnerable as sea level rises and 0.6 m (2 ft) of additional SLR may be a point of significantly increased flooding frequency, likely to occur early in the second half of the century4.
  2. Erosion – SLR may lead to changes in coastal sediment transport and storage resulting in erosion of beaches, dunes, bluffs, estuarine shorelines, and tidal wetlands. Fine sediment released by erosion may impact coastal water quality, and combined with ocean acidification and warming cause negative impacts to reefs32. On the islands of Oahu, Maui and Kauai most shorelines are eroding with 70% of all beaches exhibiting erosional trends33. Erosion has led to beach loss, usually in front of seawalls and other types of armoring, such that 9% (13 mi, 21 km) of the original length of beaches on these islands have been completely lost to erosion. Maui beaches are the most erosional of the three islands with 85% of beaches showing chronic erosion, including 11% lost, and an average rate of shoreline change equal to -0.17 ± 0.01 m/yr (-0.55 ± 0.03 ft/yr). Seventy-one percent of Kauai beaches are erosional, including 8% lost, with an average rate of shoreline change equal to -0.11 ± 0.01 m/yr (-0.36 ± 0.03 ft/yr). The majority, or 60%, of Oahu beaches are erosional, including 8% lost, with an average rate of shoreline change equal to -0.06 ± 0.01 m/yr (-0.19 ± 0.03 ft/yr). Studies show that shoreline change is highly spatially variable along Hawaii beaches with pockets of erosion alternating with areas where the beach is stable or even accreting; areas of chronic erosion are identified on all sides of the islands. With chronic coastal erosion already a major problem, continued (and accelerated) sea level rise is likely to pose significant problems in the near future.
  3. Salt Intrusion – SLR may cause salt intrusion into aquatic ecosystems, wetlands, low-land agriculture (taro lo’i and rice), and coastal plain groundwater systems.
  4. Drainage Problems – SLR may raise the groundwater table leading to increased flooding, poor drainage, and storm damage where rainfall and high ocean levels converge.

In Hawai’i, as the ocean continues to rise, natural flooding occurs in low-lying regions during rains because storm sewers back up with saltwater (especially at high tide), coastal erosion accelerates on vulnerable beaches, and critical highways shut down due to marine flooding.
The Mapunapuna industrial district of Honolulu adjacent to the airport is a good example. Until recently, if heavy rains fell during monthly highest tides, portions of the region flooded waist deep because storm drains backed up with high ocean water (Figure 6). The undercarriages of trucks suffered a corrosion problem because floodwaters become salty at high tide. Even when it did not rain, the area flooded with salt water as it surged up the storm drains into the streets and local workers reported seeing baby hammerhead sharks in the 2 foot deep pools. However, in 2011 a series of one-way flow vents were installed on the Mapunapuna storm drain system allowing run-off to exit but preventing high tide from entering.

Figure 6. Low-lying Mapunapuna (in the Honolulu Airport industrial region) experienced saltwater intrusion into the storm drain system. Previously, when it rained, there was no drainage and flooding was common. Now, one-way drainage vents prevent seawater from entering the storm drain system and tidal flooding has been mitigated in most situations. (CREDIT: Photo by D. Oda)
Figure 6. Low-lying Mapunapuna (in the Honolulu Airport industrial region) experienced saltwater intrusion into the storm drain system. Previously, when it rained, there was no drainage and flooding was common. Now, one-way drainage vents prevent seawater from entering the storm drain system and tidal flooding has been mitigated in most situations. (CREDIT:  Photo by D. Oda)

Mapping Vulnerability
Using detailed LiDAR (Light Detection And Ranging) topographic data it is possible to map the vulnerability of low-lying coastal plain assets. Areas lying within 32 cm (1 ft) of modern mean higher high water (MHHW) are especially vulnerable to the impacts of SLR by mid-century while those lying between 0.75 and 1.9 m (2.5 to 6.2 ft) are vulnerable in the latter half of the century (Figures 7 and 8). Although an 80 to 90 cm (3 ft) rise in global mean sea level by 2100 is the most probable34 a worst case scenario of 1.9 m (6.2 ft) rise has been modeled by researchers4.

a)

Figure 7. Waikiki District: a) areas shaded in blue lie at or below 32 cm (1 ft) above modern high tide

 

 

 

 

 

 

 

 

 

 

 

 

 

b)

Figure 7. Waikiki District: b) areas shaded in blue lie at or below 0.9 m (3 ft) above modern high tide

 

 

 

 

 

 

 

 

 

 

 

 


Figure 7. Waikiki District: a) areas shaded in blue lie at or below 32 cm (1 ft) above modern high tide; b) areas shaded in blue lie at or below 0.9 m (3 ft) above modern high tide. These areas are especially vulnerable to sea level rise impacts35. Along the shoreline, impacts will include beach erosion and wave overtopping with increased frequency and magnitude. At inland areas, impacts will include reduced drainage, groundwater rise, and flooding of low lands. (CREDIT: University of Hawaii Coastal Geology Group)

a)

Figure 8. Kakaako District: a) areas shaded in blue lie at or below 32 cm (1 ft) above modern high tide

 

 

 

 

 

 

 

 

 

 

 

 

 

b)

Figure 8. Kakaako District: b) areas shaded in blue lie at or below 0.9 m (3 ft) above modern high tide

 


 

 

 

 

 

 

 

 

 

 

 

Figure 8. Kakaako District: a) areas shaded in blue lie at or below 32 cm (1 ft) above modern high tide; b) areas shaded in blue lie at or below 0.9 m (3 ft) above modern high tide. These areas are especially vulnerable to sea level rise impacts. Along the shoreline, impacts will include beach erosion and wave overtopping with increased frequency and magnitude. At inland areas, impacts will include reduced drainage, groundwater rise, and flooding of low lands. (CREDIT: University of Hawaii Coastal Geology Group)

In figures 7 and 8, innumerable small areas mapped in blue identify the portion of our communities that fall below high tide later in the century. Those lands that are closer to the ocean are highly vulnerable to marine inundation by high waves, storms, tsunami, and extreme tides. Basements may flood, ground floors splashed by wave run-up, and seawater may come out of the storm drains onto most of the streets within 5 to 8 blocks of the ocean from Honolulu to Waikiki.
An important process that accompanies SLR is the greater penetration that high swell events have across low-lying beaches and the adjoining coastal plain. The coastal plain of Hawaii is where most of our housing tracts, roads, economic centers, and infrastructure are located. These coastal plains lie at a fixed elevation above mean sea level. That difference in elevation between mean sea level and the coastal plains is shrinking every day that sea level rises. Modeling of wave overtopping suggests that when high winter swell arrives on an ocean that is 30 to 60 cm (1 to 2 ft) above modern sea level (MHHW), the inundation by the highest wave of the year will reduce from rare events (e.g., 25 year high wave) to annual inundation (e.g., the annual high wave; Figure 9). That is, today inundation is a rare event with a probability of occurring only every 25 yrs or so but when sea level is 2 ft higher, it may occur every year.


Figure 9. A simulation36 of wave overtopping indicates that when sea-level reaches about 2 ft above present relatively rare flooding of low-lying coastal communities will become a frequent event. (Credit: University of Hawaii Coastal Geology Group).

 

Planning Steps

In addition to filling the information gaps that will allow the Hawaii community to formulate a response to SLR, there are planning tools to be considered. The question driving coastal planning is “How can we reduce the vulnerability of human communities and natural ecosystems to the negative impacts of SLR?”
Step 1 is to acknowledge the reality of SLR. This can be achieved by writing SLR into our laws, public awareness efforts, and planning activities. Coastal planning in Hawaii is a shared endeavor between federal, state, and county authorities. Planning is achieved through a system of permitting. If you want to build something in the special management area (SMA) along our coasts, say, a house, you need a permit. Depending on where you want to build it, various levels of government have the opportunity to comment, alter, approve, or disapprove your request. Currently, you are not required to consider the future threat of SLR in where or how you build (or redevelop existing structures). This could change through a collaborative effort involving planners at all levels who jointly agree to changes in the laws governing construction and other permitted activities in regions vulnerable to SLR. Because SLR will produce flooding that is located some distance landward of the shoreline, through a combination of storm drain inundation and groundwater inundation, it would be worthwhile to review SMA boundaries as they may not fully identify lands vulnerable to SLR (see step 4).
Step 2 is to require projects potentially vulnerable to SLR to have elements that mitigate the negative impacts of SLR. In the example of your house, it could be required that it is designed and located in light of the risk. Obviously, if a few feet of SLR are anticipated this century you would want to build your house with features to mitigate negative impacts. By shifting the planning process to a risk-based footing, guidelines could be implemented to improve the safety of your house and reduce negative impacts on the environment. Planning is already on a risk-based footing with regard to tsunami and storm surge inundation, and there is a growing effort to plan for the risk of coastal erosion. But there are no planning requirements in Hawaii with regard to SLR.
Step 3 is to require that development plans contain an environmental assessment that appraises the risk associated with SLR. This will not be particularly challenging. There are several professional tools in place for meeting such a requirement: engineering software for calculating wave overwash, estimates of sea-level rise during the course of this century can be made with reasonable authority, coastal erosion data is publically available or can be produced by consultants, and lidar data are publically available that can be used by consultants working for applicants. Sea-level rise vulnerability assessments can be produced for permit applicants seeking permission to develop the coastal zone. Privately funded SLR assessments can be created immediately, permit by permit, even as public data bases are also developed.
Step 4 is to redefine the special management area (SMA) in light of SLR impacts. The SMA is an official planning zone adjacent to the ocean that varies in width from place to place, typically less than 1500 ft. However, given the rising water table and drainage problems related to sea-level rise, a simple distance from the shoreline is no longer adequate. Low-lying lands many blocks from the coast are vulnerable to drainage problems related to SLR (e.g., Mapunapuna). The SMA could be re-mapped on the basis of these realties. As public information gaps are filled, areas that have high vulnerability can be considered for special management status.
Step 5 is to designate no-build and no-rebuild zones. This would move the coastal community toward improved resiliency (the ability to quickly recover from catastrophic events). But removing private land from the threat of development is difficult, expensive, and ideally requires an owner willing to form a partnership. The most-straightforward approach is to purchase the land, or purchase restrictions on how the land is developed. That is, pay the land owner to not develop. This is called a conservation easement. Purchasing land is expensive – especially in Hawai’i. But tools exist, including: increasing revenues to conservation land funds (already in existence for each county and the state), tax exemptions for conservation uses, gifting programs to transfer title, reverse mortgage purchases, transferable development rights, and others37. A comprehensive review of key land use policy tools for state and local government agencies and officials to facilitate leadership and action in support of sea-level rise adaptation in Hawaii has been published by the Sea Grant College of the University of Hawaii35.
Step 6 is to employ new tools in protecting the coastal environment while at the same time building improved safety into new development. A common scenario is for a wealthy land owner to hire a battalion of experts to secure a permit for some activity, such as building a dream house on the beach. Permit authorities at times have trouble justifying restrictive steps in the face of so much assembled talent bent on proving that the planned activity is benign. One compromise would be to allow a development to proceed but require a deed covenant forbidding any future seawall construction (or any activity that could damage the environment or restrict public access), no redevelopment, and no rebuilding.
Such a step would essentially declare the property a “no-rebuild zone” when it is damaged in the future by coastal processes. Along with this, design elements can be employed to mitigate impacts such as no slab on grade, nourishing the coastal dune with additional sand, and ensuring adequate public access in perpetuity. Planning for no-rebuild is a step toward recovering developed land and creating future beach preserves.
Step 7 Lastly, it is time to “climate-proof” our communities. Allowing the continued development of accreted lands, such as still occurs on some of the last healthy beaches in Hawai’i, makes no sense. In an era of accelerated sea-level rise, this has got to end. Climate proofing can involve steps such as raising road beds when they are due for maintenance, improving culverts and drainage features, adding one-way flow features to culverts to protect developed lands (as in Mapunapuna) and wetlands (as at James Campbell Wildlife Sanctuary, and which would also help protect taro lo’i), re-engineering ports, and planning for the future impacts of sea-level rise on our community infrastructure.
Using the best case and worst case scenarios presented in Vermeer and Rahmstorf (2009) it is possible to project a schedule of global mean SLR (Table 1) which could be adopted in Hawaii in lieu of a local analysis.

Table 1 – Schedule of sea-level rise 2011 to 2100

 

Worst case

Best Case

1 ft

2040

2050

2 ft

2050

2070

3 ft

2070

2090

1 ft – Don’t think that waves will be rolling down the streets to reach the blue areas on the maps shown in Figures 7 and 8. More likely, these lands lying below mean higher high water (MHHW; the average of the highest high tides) in the future will be dry at low tide between rain events. But they will have high water tables, standing pools of rainwater that stubbornly refuse to dry, and backed up storm drains when it rains and tides are high.
Beaches will erode at various rates and different beaches will experience increased erosion at different times. Beach-oriented communities will experience negative impacts in varying degrees. As beaches narrow in some communities, economic benefits of wide beaches will be amplified in communities that still have wide sandy areas for public use. Many areas that formerly had beaches will be lined by large seawalls unless communities make specific plans otherwise. High value streets may have installed drainage gates and one-way flow vents that close the storm drain system to saltwater intrusion on a schedule tied to the tides.
2 ft – Despite the wet conditions most coastal buildings will probably still be inhabited and residents will have to time their movement around the convergence of rainfall and tides, just as they did until recently in Mapunapuna. Certain key intersections and transportation arteries have been retro-fitted with raised roadbeds or low-profile bridges to allow drainage. However, these climate-proofed regions shed runoff onto surrounding lands which also need to mitigate flooding; one approach is purchase key lands with public funds to offset abutting impacts.
Public doubt regarding climate change will likely vanish by this stage and most corporate business plans will include direct and immediate climate mitigation actions based on the best science. Waikiki is in decline as the gateway for tourism. New investment by the visitor industry will probably be focused on any remaining healthy beaches on Oahu such as the Nanakuli, Waianae, Mokuleia, Kailua, and Waimanalo districts.
In McCully and Makiki, residents won’t see seawater; they will see the wetlands of the 19th century re-emerging as the water table rises above ground level in some areas. Under these conditions, when it rains, there will be significant drainage problems. Runoff will raise the water table, seawater will have filled the drainage systems except at the very lowest state of the tide, and standing pools of water will accumulate throughout the region without a place to drain. Street travel will be limited, some lands will return to wetlands, and on some lands there may be permanently standing water. Decisions regarding investment and infrastructure repair will be faced with limited options.
3 ft – Decisions to abandon specific buildings and even entire blocks will be made. Highly valued regions such a central Honolulu and portions of Waikiki may still be traversable because the groundwater table will be pumped by state-sponsored efforts funded by user fees and assessments. If global warming leads to an increase in hurricane number, as currently forecast38, the higher sea level will make the majority of the coastal plain of all islands vulnerable to high levels of damage by storm surge. Beaches may still exist but are likely only in places where coastal development has been specifically formulated to capture sand and promote beach accretion. Most coastal segments where homes still exist in their early 21st century footprints will be protected by seawalls but the wave splash and salt air is likely to make many of these locations run-down and relatively undesirable neighborhoods; additionally, the persistence of standing water in most coastal plain neighborhoods also make these undesirable places to invest.
Unfortunately, as SLR and global warming continue into the second half of the 21st century, the mix of high groundwater, saltwater emerging from the storm drain system, paving and buildings that have not been removed nor maintained, and annual flooding by high waves, makes the makai ¼ mile of the coastal plain an unsafe location for development (Figure 10).

Figure 10 Digital elevation model of southeast Oahu. Areas in red are vulnerable to drainage problems and salt intrusion when sea level rises 0.9 m (3 ft) above modern mean higher high water. (CREDIT: University of Hawaii Coastal Geology Group).
Figure 10 Digital elevation model of southeast Oahu. Areas in red are vulnerable to drainage problems and salt intrusion when sea level rises 0.9 m (3 ft) above modern mean higher high water. (CREDIT: University of Hawaii Coastal Geology Group).

 

Conclusion

 

To avoid this future requires commitment and action. What can you do?

  • Live a low carbon lifestyle as your part to help stabilize warming.

  • Encourage elected officials to implement SLR adaptation programs.

    • Enact a SLR planning guideline of 1 ft by mid-century and the low end (3 ft) of 2.5 to 6.2 by the end of the century.

    • Consider the impact of a worst case SLR scenario of 6.2 ft by end of the century. Use this in the planning of appropriate projects such as hospitals, waste water treatment, coastal highways, and other cases where public health and safety are at risk.

    • Expect that hurricanes and tsunamis will make direct landfall in Hawaii under conditions of higher SLR. Plan and redevelop communities to improve resilience.

    • Invest more funds to purchase key vulnerable lands.

    • Promote a state-wide retreat from our moving shoreline.

  • Order state and county agencies to consider climate change, especially SLR, in their missions and to define fundable projects and programs that lead to adaptation.

Hawai’i must adapt to global warming if we want to avoid the most serious impacts.

 

References

 

1 National Academy Press, America’s Climate Choices: 5 volume set. See the website http://www8.nationalacademies.org/onpinews/newsitem.aspx?RecordID=12781; last accessed 3/9/2012.

2 See the IPCC website for AR5 http://www.ipcc.ch/activities/activities.shtml; last accessed 3/9/2012.

3 Rignot, E., Velicogna, I., van den Broeke, M.R., Monaghan, A., and Lenaerts, J., 2011 “Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise,” Geophysical Research Letters, v. 38, LO5503, doi:10.1029/2011GL046583.

4 Vermeer, M. and Rahmstorf, S., 2009 “Global sea level linked to global temperature,” Proceedings of the National Academy of Sciences, http://www.pnas.org/content/early/2009/12/04/0907765106.full.pdf.

5 Pfeffer, W.T., Harper, J.T., O’Neel, S. (2008) “Kinematic constraints on glacier contributions to 21st century sea level rise” Science, Sept. 5, v. 321, DOI: 10.1126/science.1159099

6 Slangen, A., C. Katsman, R. Van de Wal, L. Vermeersen, and R. Riva (2011) Towards regional projections of twenty-first century sea-level change based on IPCC SRES scenarios. Climate Dynamics. DOI: 10.1007/s00382-011-1057-6

7 Merrifield, M.A., Merrifield, S.T., Mitchum, G.T. 2009 “An anomalous recent acceleration of global sea level rise” Journal of Climate, 22, p. 5772-5781.

8 Global sea level is measured by satellite detection. See the NASA website “Rising waters: new map pinpoints areas of sea level increase”; http://climate.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=16; last accessed 9/15/11.

9 Merrifield, M.A., 2011 A shift in western tropical Pacific sea level trends during the 1990s, Journal of Climate, 24, p. 4126-4138.

10 The University of Colorado Sea Level Research Group provides updates on altimetry records. See the website http://sealevel.colorado.edu/; last accessed 9/15/11.

11 See the Wikipedia website http://en.wikipedia.org/wiki/ENSO; last accessed 9/15/11.

12 See the Wikipedia website http://en.wikipedia.org/wiki/Pacific_decadal_oscillation; last accessed 9/15/11.

13 See the NASA announcement of the start of the present PDO negative phase in 2008 here: http://earthobservatory.nasa.gov/IOTD/view.php?id=8703; last accessed 9/15/11.

14 See the NOAA Northwest Fisheries Science Center description: http://www.nwfsc.noaa.gov/research/divisions/fed/oeip/ca-pdo.cfm; last accessed 9/15/11.

15 Timmerman, A., McGregor, S., Jin, F.-F., 2010 “Wind effects on past and future regional sea level trends in the southern Indo-Pacific” Journal of Climate, 23, p. 4429-4437.

16 Young, I. R., Zieger, S., Babanin, A. V., 2011 “Global trends in wind speed and wave height” Science, 332, 22 April, p. 451-455.

17 Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.). (2007) Contribution of Working Group I to the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 996 pp.

19 Jacob, T., Wahr, J., Pfeffer,W.T., Swenson, S. (2012) Recent contributions of glaciers and ice caps to sea level rise. Nature,; DOI: 10.1038/nature10847

20 Cazenave, A., Llovel, W. (2010) “Contemporary sea level rise” Annual reviews in Marine Science, 2010.2:145-173.

21 Velicogna, I., Wahr, J. (2006) “Acceleration of Greenland ice mass loss in Spring 2004” Nature, 443, 329-331 (21 September 2006) | doi:10.1038/nature05168

22 Meier M.F., Dyurgerov M.B., Rick U.K., O’Neel S., Pfeffer W.T., et al. 2007, “Glaciers dominate eustatic sea-level rise in the 21st century”, Science317:1064–67. See also: Kaser G., Cogley J.G., Dyurgerov M.B., Meier M.F., Ohmura A., 2006, “Mass balance of glaciers and ice caps: Consensus estimates for 1961–2004,” Geophysical Research Letters33:L19501, doi:10.1029/2006GL027511

23 Domingues, C.M., Church, J.A., White, N.J., Gleckler, P.J., Wijffels, S.E., Barker, P.M., Dunn, J.R. (2008) “Improved estimates of upper-ocean warming and multi-decadal sea-level rise” Nature 453, 1090-1093 (19 June 2008) | doi:10.1038/nature07080

24 Song, Y.T., Colbert, F., 2011 “Deep ocean warming assessed from altimeters, gravity recovery and climate experiment, in situ measurements, and a non-Boussinesq ocean general circulation model” Journal of Geophysical research, 116,C02002.

25 Moore, J.C., Jevrejeva, S., Grinsted, A., 2011 “The historical global sea level budget” Annals of Glaciology, 52.59, p. 8-14.

26 Church, J. A., N. J. White, L. F. Konikow, C. M. Domingues, J. G. Cogley, E. Rignot, J. M. Gregory, M. R. van den Broeke, A. J. Monaghan, and I. Velicogna (2011) “Revisiting the Earth's sea-level and energy budgets from 1961 to 2008” Geophys. Res. Lett., 38, L18601, doi:10.1029/2011GL048794.

27 See the Honolulu tide record at the National Oceanographic and Atmospheric Administration website “Sea Levels Online” http://tidesandcurrents.noaa.gov/sltrends/sltrends.html; last accessed 3/9/2012.

28 Jevrejeva, S., Moore, J.C., Grinsted, A., Woodworth, P. L., 2008 “Recent global sea level acceleration started over 200 years ago?” Geophysical Research Letters, 35:LO8715.

29 Firing, Y., Merrifield, M.A., 2004 “Extreme sea Level Events at Hawai’i: Influence of Mesoscale Eddies” Geophysical Research Letters, 31:L24306.

30 Fletcher, C.H., Boyd, R., Neal W.J., Tice, V., 2010 “Living on the Shores of Hawai‘i: Natural Hazards, the Environment, and Our Communities” University of Hawai‘i Press, 371p.

31 See the website of the Northwestern Hawaiian Islands Research Partnership “Mapping Cumulative Human Impacts” http://www.hawaii.edu/himb/nwhi/?page_id=290; last accessed 9/15/11.

33 Storlazzi, C.D., Elias, E., Field, M.E., and Presto, M.K., 2011. "Numerical modeling of the impact of sea-level rise on fringing coral reef hydrodynamics and sediment transport." Coral Reefs, v. 30(1), p. 83-96, DOI: 10.1007/s00338-011-0723-9

33 Fletcher, C.H., Romine, B.M., Genz, A.S., Barbee, M.M., Dyer, M., Anderson, T., Lim, S.C., Vitousek, S., Bochicchio, C., and Richmond, B.M. (in press) National Assessment of Shoreline Change: Historical Shoreline Changes in the Hawaiian Islands, Open-File Report 2011-1051, 123p. See also, Romine, B.M. and Fletcher, C.H. (in press) A summary of historical shoreline changes on beaches of Kauai, Oahu, and Maui; Hawaii. Journal of Coastal Research.

34 See review by Fletcher, C.H. (2010) “Sea level by the end of the 21st century: A review” Shore and Beach, 77.4, p. 1-9. See also Pfeffer, W.T., Harper, J.T., O’Neel, S. (2008) “Kinematic constraints on glacier contributions to 21st century sea level rise” Science, Sept. 5, v. 321, DOI: 10.1126/science.1159099

35 Codiga, D., & Wager, K. (2011). Sea-level rise and coastal land use in Hawai‘i: A policy tool kit for state and local governments. Honolulu: Center for Island Climate Adaptation and Policy. Retrieved from http://icap.seagrant.soest.hawaii.edu/icap-publications

36 Vitousek, S., Fletcher, C.H. and Barbee, M. (2008) “A practical approach to mapping extreme wave inundation: consequences of sea-level rise and coastal erosion” Proceedings: Solution to Coastal Disaster 2008, Oahu, Hawaii, April 13-16, p. 85-96

37 Here is the website of the Hawai’i Legacy Lands Program. A state program that purchases lands for conservation and other public purposes: http://hawaii.gov/dlnr/dofaw/llcp; last viewed 9/16/11.

38 Li, T., M. Kwon, M. Zhao, J.-S. Kug, J.-J. Luo, and W. Yu (2010) “Global warming shifts Pacific tropical cyclone location” Geophysical Research Letters, 37, L21804, doi:10.1029/2010GL045124.

 

 

 

 

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