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The Blue Line – Chip Fletcher |
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In Hawaii, sea-level rise resulting from global warming is a particular concern. Riding on the rising water are high waves, hurricanes, and tsunami that will be able to penetrate further inland with every fraction of rising tide. In addition, the coastal groundwater table is likely to crop out above ground level and lead to widespread flooding. The physical effects of sea-level rise fall into 5 categories. These are:
- Marine inundation of low-lying developed areas including coastal roads,
- Erosion of beaches and bluffs,
- Salt intrusion into aquifers and surface ecosystems,
- Higher water tables, and
- Increased flooding and storm damage due to heavy rainfall.
Assessing the impact of these on Hawaii requires identifying a likely global sea-level scenario. Global sea level is principally the product to two phenomena: 1) melting ice on Antarctica, Greenland, and among alpine glaciers, and 2) thermal expansion of seawater due to surface warming. The first detailed observations of Antarctic ice reveal net melting; the melting rate on Greenland has increased 250% in the past decade; there is widespread retreat and thinning of mountain glaciers, and together these major ice sources contribute about 2.0 mm/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. While changes in water temperature over past decades have been difficult to measure, studies indicate that thermal expansion increased from an average rate of about 0.36 mm/yr in past decades, to 1.6 mm/yr in the most recent decade. The total contributions to global sea level (~3.6 mm/yr) agree remarkably well with the observed rate of rise (~3.4 mm/yr) as measured by satellites.
The Intergovernmental Panel on Climate Change has predicted future sea-level changes to the year 2100 in the range 18 to 58 cm. However, these projections do not include a component based on ice behavior, and hence, are widely considered to underestimate the potential for flooding. Two studies published in 2007, both by German climate researcher Stefan Rahmstorf and colleagues, indicate a more likely scenario of future climate change and sea-level rise. In one study, Rahmstorf compared projections of future atmospheric warming and sea-level rise made in 1990 by the IPCC to observations in 2006. Results indicate that the climate system, in particular sea level, may be responding to global warming more quickly than models specify. Observed temperature changes are in the upper part of the range projected by the IPCC and sea level has been rising faster than even the extreme scenarios projected by the models. Notably, Rahmstorf found that the rate of sea level rise for the past 20 years is 25% faster than the rate of rise in any 20-year period in the preceding 115 years. In his second paper of 2007, Rahmstorf estimates 21st century sea-level change on the empirical relationship between 20th century temperature changes and sea-level changes. The study establishes a proportionality constant of 3.3 cm of sea-level rise per decade per °C of global temperature warming. When applied to future warming scenarios of the Intergovernmental Panel on Climate Change, this relationship results in a projected sea-level rise in 2100 of 0.5 to 1.4 m above the 1990 level. On the basis of Rahmstorf’s research, and the documented accelerations in melting of both the Greenland and Antarctic ice sheets, it seems highly likely that a sea level of approximately 1 m above present could be reached by the end of the 21st century.
In Hawaii, as the ocean continues to rise, natural flooding occurs in low-lying regions during rains because storm sewers back up with saltwater, coastal erosion accelerates on our precious beaches, and critical highways shut down due to marine flooding. The Mapunapuna industrial district of Honolulu adjacent to the airport is a good example. If heavy rains fall during monthly highest tides portions of the region flood waist deep because storm drains are backed up with high ocean water. The undercarriages of trucks suffer a rust problem because floodwaters become salty at high tide. Even when it does not rain, the area floods with salt water as it surges up the storm drain into the streets and local workers report seeing baby hammerhead sharks in the 2 foot deep pools.
Using sensitive topographic data collected by NOAA and the Army Corps of Engineers, it is possible to map the contour line marking 1 m above present day high tide. This “blue line” identifies the portion of our communities that fall below sea level when seas reach the 1 m mark later in the century. This dramatic map has roughly 30 cm accuracy. Those lands that are closer to the ocean are highly vulnerable to inundation by seawater during high waves, storms, tsunami, and extreme water levels. Hotel basements will flood, ground floors will be splashed by wave run-up, and seawater will come out the storm drains on most of the streets in Waikiki and along Ala Moana Boulevard.
Don't think that waves will be rolling down the streets and reaching the blue line. More likely, lands lying below sea level in the future will be dry at low tide during arid summers. But they will have high water tables, standing pools of rainwater, and backed up storm drains when it rains and tides are high. Beaches will be mostly gone and we'll have built large seawalls lining most of our shores. Despite the wet conditions, most of the buildings will probably still be inhabited and residents will have to time their movement between the tides, just as they do today in Mapunapuna. Back up in the McCully and Makiki areas residents won't see any seawater, they will see the wetlands of the 19th century reemerging as the water table rises above ground level in some areas (not all areas). Under these conditions, when it rains, we will have a real problem. The runoff will raise the water table, the storm drains will be full of seawater except at the very lowest state of the tide, and standing pools of water will accumulate throughout the region without a place to drain. Travel will be limited and many lands will turn to wetlands, there may be some areas of permanently standing water.
What can you do? Live a low carbon lifestyle as your part to help stabilize warming, encourage elected officials to support research into this problem and to set up funds to purchase key vulnerable lands, and promote a state-wide retreat from our moving shoreline.
Rahmstorf, S., 2007, A semi-empirical approach to projecting sea level rise. Science, 315, 368-370.
Rahmstorf, S., et al., 2007, Recent climate observations compared to projections. Science, 316, 709.
Figure 1 The blue line marks the contour of high tide when sea level is 1 m above present. Lands makai of the line are highly vulnerable to coastal hazards. These are targets for redevelopment to increase resiliency to natural hazards. 